1. Art and Diseño

COLLABORATIVE TACTICS FOR NESTSITE SELECTION BY
PAIRS OF BLUE FOOTED BOOBIES
by
JUDY STAMPS 1,2), MIRIAM CALDERÓN-DE ANDA 3) , CARMEN PEREZ 3) and
HUGH DRUMMOND 3,4)
California, Davis, Davis, California, 95616;
3 Instituto de Ecología, Universidad Nacional Autónoma de México, Ciudad Universitaria,
04510, México, D.F.)
(1 Evolution and Ecology, University of
(Acc. 13-IX-2002)
Summary
The dual concern model suggests that pairs of animals can use four different behavioural
strategies to resolve disputes that arise when making joint decisions. Based on their reproductive biology, we predicted that mated pairs of blue footed boobies would use one of these
strategies, collaboration, when deciding on a location for their nest. Many of the behaviour
tactics diagnostic of collaboration were observed in boobies. For instance, nestsite selection
was accompanied by extensive exchanges of a specialized communication signal (nestpointing), rates of nestpointing at a given site were strongly related to the likelihood that a pair
would select that site for their nest, couples in which the male and female ‘disagreed’ about
the merits of an initial site went on to investigate additional sites together (‘expanding the
pie’), individuals pointed at maximal rates at a site only after their partner had already pointed
at that same site (‘feeling out procedures’), and both sexes appeared to have ‘veto power’ over
potential nestsites, in the sense that a site was virtually never accepted for the nest if one of
the two partners failed to point at that site prior to clutch initiation. These results support
the hypothesis that mated pairs of blue footed boobies may use collaborative tactics when
selecting a nestsite; descriptive accounts suggest that similar tactics may occur in other birds
in which mated pairs jointly decide on the location for their nest.
2)
Corresponding author’s e-mail address: [email protected]
Financial assistance was provided by a grant from UCMexus to JS, and grants to HD
through Consejo Nacional de Ciencia y Tecnología (grants D112-903581, 4722-N9407), the
UNAM, and the National Geographic Society (grants 4535-91 and 4536-91). We are very
grateful to Neil Willits for statistical assistance, Cristina Rodríguez for help with Ž eldwork,
and to the Mexican Navy and the Ž shermen of San Blas and Boca de Camichín for logistical
help.
4)
© Koninklijke Brill NV, Leiden, 2002
Behaviour 139, 1383-1412
Also available online -
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Keywords: decision-making, nestsite selection, dispute resolution, collaboration.
Introduction
Animal behaviourists frequently observe situations in which pairs of animals
appear to make joint decisions about space. Nestsite selection in birds
provides a striking example of this behaviour. Anecdotal and descriptive
accounts from a wide range of species suggest that mated pairs engage in
an extended series of displays and social interactions at potential nestsites,
a process which eventually results in the selection of one of those sites for
the nest (Tinbergen, 1953; Scott, 1972; Nethersole-Thompson & NethersoleThompson, 1979; Ratcliffe, 1980; Furness, 1987; Newton, 1986; Marzluff &
Balda, 1992; Tarof & Ratcliffe, 2000). Other examples of joint decisions
about space occur in territorial species, when neighbouring pairs use a series
of aggressive interactions to determine the location of the boundary between
their respective territories (e.g. Stamps & Krishnan, 1998), or species in
which pairs of animals travel around together, and jointly decide on the
direction of travel at any given moment (e.g. butter y Ž sh, Reese, 1975).
Interestingly, although many animal behaviourists are familiar with situations in which animal dyads appear to make joint decisions about space,
little is known about the behavioural basis of this process. Some empiricists
have described how groups of animals make joint decisions about space, but
such groups typically consist of many individuals. Examples include studies
of the behaviour that troops or herds of mammals use to decide on direction
of travel for the day (Sigg & Stolba, 1981; Prins, 1996; Boinski & Garber,
2000), or studies of the behaviour by which swarms of honeybees decide on
a new nestsite (Visscher & Camazine, 1999; Seeley & Buhrman, 2001). Theoreticians have considered one situation in which pairs of animals make joint
decisions about space, in suggesting that prospective territory neighbours use
aggressive behaviour patterns to negotiate a location for their mutual territory
boundary (Adams, 1998; Stamps & Krishnan, 2001). Otherwise, animal behaviourists seem to have largely ignored the behavioural processes animals
use to make joint decisions about space.
Perhaps one reason for the lack of interest in this topic is the lack of a
framework for predicting the types of strategies and tactics that dyads might
use to make joint decisions about space. For instance, it makes intuitive sense
JOINT DECISIONS ABOUT NESTSITES
1385
that territorial animals might use aggression to negotiate the location of a
boundary between their territories, but it seems less likely that mated pairs
of birds would use comparable behaviour to select a place to raise their
young. Indeed, descriptive accounts of nestsite selection in birds suggest
that mated pairs do not employ aggressive displays or interactions in this
situation. Instead, ornithologists describe specialized displays that pairs use
to attract one another to potential nestsites, repeated inspections of potential
sites by individuals and pairs, and extended series of social interactions that
somehow result in the selection of one site for the nest (e.g. see Ratcliffe,
1980). However, though these accounts indicate that avian pairs may use
complicated behavior when selecting a nestsite, they offer no insights into
general categories of behaviour that these animals might use to make these
decisions.
Fortunately, scholars working in other disciplines have considered the
types of behaviour that pairs of individuals use when making joint decisions,
including decisions about space (e.g. Greenhalgh & Chapman, 1995; Pruitt,
1998). In particular, the dual-concern model developed by social psychologists predicts the types of behaviour that human dyads are expected to use
when making joint decisions about a variety of topics, including space use
(e.g. how married couples select a new home or vacation venue, Pruitt & Rubin, 1986; Pruitt & Carnevale, 1993; Rubin et al., 1994). Empirical studies
of humans support the predictions of the dual concern model (De Dreu et al.,
2000), implying that this model might provide a useful conceptual framework for studying the behavior that other types of animals use to make joint
decisions about space.
The current paper uses nestsite selection in blue footed boobies, Sula
nebouxii, to show how the dual concern model can be used to study the
behavioural processes that pairs of animals employ to make joint decisions
about space. Because our goal is to both introduce the dual concern model,
and to use it to study nestsite selection in birds, the current paper is
necessarily more complicated than contributions that focus exclusively on
either conceptual or empirical issues. The paper is divided into six sections.
First, we brie y introduce the dual-concern model, and summarize its
predictions with respect to the types of behaviour patterns animals might
use to make important joint decisions about space. Then, we introduce the
study species, and indicate why nestsite selection in blue footed boobies
might provide a good system for testing some of the predictions of the
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STAMPS, CALDERÓN-DE ANDA, PEREZ & DRUMMOND
dual concern model. Next, we detail the methods we used to study nestsite
selection in the boobies. The results on nestsite selection in boobies are
divided into two sections. The Ž rst of these focuses on a conspicuous display
(nestpointing) that is closely associated with the process of nestsite selection
in this species, and shows that rates of nestpointing at a given site are related
to the likelihood that a pair will select that site for their nest. Based on these
results, we then show how nestpointing can be used to ask whether pairs of
boobies exhibit speciŽ c types of behavioural tactics predicted by the dual
concern model. Finally, we summarize some of the major conclusions from
this study, and suggest that the dual concern model may provide a useful
framework for studying the processes by which other species make joint
decisions about space.
The dual concern model
When pairs of animals make joint decisions, the two parties may initially
disagree about the best option for their joint decision. Initial disputes over
preferred options can occur for a variety of reasons. For instance, two
partners may have strongly overlapping interests in the outcome of the joint
decision, but initially prefer different options because neither is able to
accurately assess the relative merits of different options. Thus, a pair of birds
might both beneŽ t by selecting the nestsite which will optimize  edgling
production and adult survivorship, but initially disagree about the location
that is most likely to achieve these objectives. Note that in this situation,
a dispute over preferred options does not re ect an underlying con ict of
interest; instead, it re ects different assessments by two parties of the relative
quality of alternative options for the joint decision. Alternately, two parties
might have a con ict of interest with respect to the outcome of the Ž nal
decision, such that an outcome favorable for one party comes at the expense
of the other party. Thus, when territory neighbours compete for divisible
space, a boundary location which yields a larger territory for one individual
necessarily results in a smaller territory size for its neighbour (Stamps &
Krishnan, 2001). However, regardless of the reasons for the disagreement
over preferred options, this disagreement must somehow be resolved, if two
parties are to arrive at a joint decision with one another.
A model borrowed from social psychology suggests that animal dyads
can use at least four different strategies to resolve disputes when making joint
JOINT DECISIONS ABOUT NESTSITES
1387
decisions (Pruitt & Rubin, 1986; Pruitt & Carnevale, 1993). The dual concern
model predicts that the behaviour an individual will employ when making a
joint decision depends on two factors: the importance of the decision to the
actor, and the extent to which the actor is concerned about the outcome of the
decision for the other party. Rephrased in terms more familiar to behavioural
ecologists, the dual concern model predicts that strategic choice for dispute
resolution depends on the potential impact of the decision on the actor’s
Ž tness, and on the potential impact of the decision for the other party on
the actor’s Ž tness (Fig. 1).
An interesting and novel prediction of the dual concern model is that pairs
of animals with broadly overlapping interests should use a particular set of
behaviour patterns, here termed ‘collaborative tactics’ to resolve disputes
when making joint decisions that are important to both of them. One situation
in which collaboration is expected is when pairs of animals with extensive
biparental care select a nestsite, since in this situation, offspring production
for each individual is likely to be affected by the effect of the nestsite on
the parental care delivered to the offspring by the other party. For instance,
selecting a nestsite highly aversive to the partner might reduce an individual’s
reproductive success later in the nestcycle, if the partner’s efŽ ciency at
Fig. 1. Four strategies for dispute-resolutionduring joint decision-making. Strategic choice
depends on the potential effect of the decision on the actor’s Ž tness, and the extent to which
the actor’s Ž tness is affected by the outcome of the decision for the other party. Collaborative
tactics are expected when a decision is important to the actor (high potential direct effect of
the decision on actor’s Ž tness) and when actor and other have broadly overlapping interests in
the outcome of the joint decision (a positive outcome for the other party is likely to enhance
the actor’s Ž tness). See text.
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STAMPS, CALDERÓN-DE ANDA, PEREZ & DRUMMOND
incubation, nest protection, or provisioning were compromised as a result
of its reluctance to spend extended periods of time at the nestsite.
Collaborative tactics are expected when two parties have overlapping
interests with respect to most, but not necessarily all, of the features of the
various options that they might adopt for their joint decision. For instance,
both partners in a mated pair of birds have an interest in selecting a site with
high intrinsic quality (e.g. with characteristics likely to enhance offspring
growth and survivorship), and each partner has an interest in choosing a site
with characteristics that are likely to enhance their mate’s ability to provide
high quality care to the offspring. On the other hand, male and female
interests might diverge with respect to other characteristics of potential
nestsites, e.g., a female might beneŽ t by selecting a site that was located
close to a neighboring male with whom she might engage in extra-pair
copulations. The issue with respect to collaboration is not whether male
and female interests in a joint decision are entirely congruent (they virtually
never are), but instead whether males and females have broadly overlapping
interests with respect to the option that will eventually be selected for their
joint decision. In the situation outlined above, this is likely to be the case.
That is, a female would not be expected to prefer a low quality nestsite
just because it was located close to a potential extra-pair partner, although
proximity to an attractive neighbor might in uence her ranking of otherwise
comparable sites. Conversely, most of the characteristics of nestsites that
enhance a female’s Ž tness would also, directly or indirectly, enhance the
Ž tness of her mate. Hence, in this situation, we might expect partners to
use collaborative tactics to resolve disputes over nestsite selection, e.g., after
investigating several sites of comparable quality, the male might veto a site
that was located too close to the attractive neighbor (see below, and Table 1).
Based on the literature on decision-making in humans, collaborative
tactics in animals are expected to exhibit a number of properties. Since
collaboration is expected when an individual beneŽ ts by decisions that
are favorable for its partner, this strategy is expected to involve extensive
exchanges of communicative signals, with which each party reveals its level
of preference for different options for the joint decision (Pruitt & Rubin,
1986; Pruitt, 1998). Collaboration may include ‘feeling out procedures’,
in which one party initially indicates its preferences using low intensity
signals, and then increases the level of signal produced in association with
its preferred option after determining that this option is also acceptable to the
JOINT DECISIONS ABOUT NESTSITES
1389
other party (Pruitt, 1981). One advantage of feeling out procedures is that an
individual does not waste time or energy indicating a strong preference for
an option that is unacceptable to the other party, and hence unlikely to be
adopted for the Ž nal decision. Another collaborative tactic is ‘expanding the
pie’, in which two parties who disagree about an initial set of options go on to
locate, explore and discuss a new set of options (Pruitt & Carnevale, 1993). In
collaboration, both parties are expected to have ‘veto power’ over decisions,
since neither individual is likely to beneŽ t if it obtains an option that is
highly aversive to the other party. Finally, in terms of outcomes, collaboration
is unlikely to produce decisions which are unfavorable for one of the two
parties (win-lose solutions). Instead, it is likely to generate ‘compromise’
solutions (in which each party obtains an outcome that is better than its
least-preferred option, but worse than its most-preferred option), or ‘winwin’ solutions, which are better for both parties than any of the options that
were initially under consideration (Pruitt & Rubin, 1986; Pruitt, 1998).
In contrast, when two parties with broadly non-overlapping interests make
joint decisions that are important to both of them, the dual concern model
predicts individuals will use contentious behaviour to resolve disputes that
arise while making the decision. Many contentious tactics are already familiar to animal behaviourists; examples include aggression, manipulation, deception, persuasion, or harassment (reviews in Huntingford & Turner, 1987;
Archer, 1988; Gadagkar, 1997; Kölliker & Richner, 2001). Contentious tactics include any behaviour patterns which increase the chances that a pair will
select an option favorable to the actor, without regard to, or at the expense of,
the outcome for the other party. For instance, when pairs of animals dispute
the location of a boundary between their respective territories, they typically
use aggressive behaviour to arrive at a joint decision about the boundary location (Stamps, 1994; Stamps & Krishnan, 1998, 2001). The dual concern
model predicts the use of contentious tactics in this situation, because the
location of the boundary is likely to affect the size and quality of the territories of both individuals (e.g. Eason, 1992; Adams, 1998), and because an
individual’s Ž tness is unlikely to be enhanced if its neighbour obtains a better
territory.
Collaborative tactics are similar to contentious tactics in some respects:
both are expected to involve extensive social interactions and exchanges of
specialized communication signals, and both can generate compromise solutions (Table 1). Fortunately, however, there are several ways to discriminate
1390
STAMPS, CALDERÓN-DE ANDA, PEREZ & DRUMMOND
collaborative from contentious tactics. First, feeling out procedures are predicted under collaboration, but not under contention. Contentious tactics often involve increases in signal intensity over time, as part of a phenomenon
termed ‘escalation’ (see review in Archer & Huntingford, 1994). However,
escalation differs from feeling out procedures in one important respect: in
escalation, an individual increases the intensity of signals associated with
its preferred option after determining that the other party prefers another
option, whereas in feeling out procedures, an individual increases the level
of signaling associated with its preferred option after determining that the
other party prefers that same option. Another difference between collaboration and contention is that the latter often involves the use of behaviour patterns that improve the outcome for the actor by in icting costs or punishment
on the other party (Clutton-Brock & Parker, 1995; Gowaty, 1997; Stamps &
Krishnan, 1999, 2001; Bisazza et al., 2001; Schlupp et al., 2001). In contrast,
collaboration is predicted in situations in which two parties have broadly
overlapping interests, which limits the extent to which one party is likely to
in ict costs on the other while making the joint decision. Instead, since one
goal of collaborative behaviour is to obtain accurate information about the
other party’s level of preference for various options, collaboration is likely
to include behaviour by which one party ‘invites’ the other to consider options preferred by the actor (with no penalty should the other party fail to
accept that invitation), or behaviour by which an individual encourages the
other party to freely express its own preferences for various options (Pruitt
& Carnevale, 1993). Third, collaboration is expected to involve the mutual
exploration, investigation, and discussion of new options (see expanding the
pie, above), whereas contention is not. Fourth, each party in collaboration is
expected to have veto power over the Ž nal decision, whereas in contention,
the pair can adopt an option that is aversive to one of the two parties. Finally,
in terms of outcomes, contention is more likely than collaboration to produce
‘win-lose’ decisions, whereas collaboration is more likely than contention to
produce ‘win-win’ decisions. The similarities and differences between contention and collaboration are summarized in Table 1.
Even this brief review of the dual concern model indicates why this approach might prove useful for studying joint decision-making in animals.
First, the model suggests that extensive exchanges of specialized communication signals are expected whenever pairs of animals make joint decisions
that are important to both of them, i.e. this type of behaviour is consistent
1391
JOINT DECISIONS ABOUT NESTSITES
TABLE 1. Comparison of contentious and collaborative tactics for joint
decision-making
A. Behaviour patterns
1. Extensive exchanges of communication signals
2. Feeling out procedures
3. Escalation
4. Behaviour that in icts costs on the other party
5. Expanding the pie
6. Veto power
B. Likely Outcomes
1. Win-lose
2. Compromise
3. Win-win
Contention
Collaboration
x
x
x
x
x
x
x
x
x
x
x
with both collaborative and contentious tactics. This point is worth emphasizing, because it is sometimes assumed that communication in cooperative
contexts typically involves brief exchanges of simple, low-cost signals (‘conspiratorial whispers’, Krebs & Davies, 1984; Maynard Smith, 1991, 1994).
However, the dual concern model suggests that even if two parties have very
broadly overlapping interests, they may need to exchange extensive amounts
of information about their levels of preference for various options, in order
to make the joint decision that is best for both of them (see also Seeley &
Buhrman, 2001, for a related example involving nestsite selection in honeybees). Second, the dual concern model predicts behaviour patterns that have
not, to our knowledge, been previously described by empiricists studying
joint decisions about space by animal dyads, i.e. expanding the pie, feeling
out procedures, or veto power. As we illustrate below for the boobies, one
can devise tests to reveal the behavioural tactics that pairs of animals use to
select a nestsite, and hence ask whether those animals might be using collaborative tactics to make this type of joint decision.
On the other hand, the dual concern model also has some obvious
deŽ ciencies. For instance, it is descriptive rather than mathematical, it does
not specify which tactics (or mixture of tactics) an individual should employ
when the outcome for the other party has an intermediate effect on its
own Ž tness, nor does it specify the behaviour or outcomes expected if two
members of a dyad employ different strategies when making a joint decision.
However, we feel that it would be premature to reŽ ne and formalize the
1392
STAMPS, CALDERÓN-DE ANDA, PEREZ & DRUMMOND
dual concern model, before Ž rst determining if it has any relevance to the
behaviour that animals use to make joint decisions. Hence, the primary,
and relatively modest, goal of the current study is to ask whether pairs of
blue footed boobies exhibit collaborative behaviour patterns when selecting
a nestsite.
Reproductive biology of blue footed boobies
Blue footed boobies are colonial seabirds, which nest on the ground within
a small (7 to 20 m2 ) territory, which is situated near the territories of neighbouring pairs (Gonzalez & Osorno, 1987). The nest itself consists of a shallow scrape on the ground, located inside of the territory boundaries (Nelson,
1978). Preliminary observations indicate that nestsite selection in boobies
occurs after a pair has formed and established a territory (Stamps, unpubl.
data). As is detailed below, pairs typically investigate several different potential nestsites within their territory, and prepare several of these sites for
possible use, before selecting one of them for the nest. Because potential
nestsites are located within the territory, preemption of sites by other pairs is
typically not a problem for boobies; as a result, pairs can investigate a number of different sites, over an extended period of time, before choosing one of
them for the nest. Both sexes share in all parental duties associated with the
nest, including preparation of potential nestsites, incubation, brooding, protection of eggs and young from predators and aggressive conspeciŽ cs, provisioning of nestlings, and provisioning of  edglings (Nelson, 1978; Guerra
& Drummond, 1995). Both parents spend extensive amounts of time in offspring production: approximately 170 days elapse between laying of the Ž rst
egg and independence of one to three chicks (Nelson, 1978), and for the Ž rst
two months of this period, both parents spend many hours each day on or
near their nest.
The reproductive biology of these birds suggests that they might use
collaborative tactics when selecting one of several potential sites for their
nest. First, the choice of a nestsite is likely to be important to both partners,
given the amount of time, and the extensive amount of parental care, that
both individuals invest at the nest over the course of the reproductive season.
To the extent that attributes of the nestsite affect the ability of parents to
provide to high quality care to their young, or affect parental condition or
survivorship, it seems reasonable to assume that the choice of a nestsite
JOINT DECISIONS ABOUT NESTSITES
1393
would be important for both sexes in boobies. Second, it seems reasonable
to assume that male and female boobies might attend to the preferences of
their partners when selecting a nestsite. Not only do both parents provide
extensive amounts of care at the nest within each breeding season, but
individuals often re-pair with the same partner in successive years (Stamps
& Drummond, unpubl. data; see also below). As a result, an individual
whose partner obtained a highly preferred nest location might enjoy higher
reproductive success or survivorship during the current season, or might
stand a better chance of retaining that mate in subsequent seasons, than an
individual whose partner was unenthusiastic about the nest location. Hence,
to the extent that a booby’s current or future reproductive success is affected
by its partner’s level of satisfaction with the nestsite, one might expect booby
pairs to use collaborative behaviour to select a nestsite.
Methods
Nestsite selection in boobies was studied on Isla Isabel, Nayarít, Mexico from January 1997
to February 1998. All of the  edglings and most adults in this colony were banded using
numbered metal tags for the previous 12 years. As a result, virtually all of the adults in the
current study could be identiŽ ed as individuals based on their band numbers (which were
visible at distances of up to 5 m via binoculars). We determined whether each banded adult
had bred in previous years, and for the experienced breeders, the location (to within 0.5 m)
of the nest and identity of the mate during the previous breeding season.
Preliminary observations suggested that two previously undescribed displays, nestscraping and nestpointing, were closely associated with nestsite selection in this species. In nestscraping (abbreviated NSC), boobies remove twigs, rocks and soil from potential nestsites
using the bill and the feet. Nestpointing (NP) occurs when a standing individual points its
bill at the substrate, with the tip approximately 15 cm from the ground, and moves the bill
laterally and vertically, often while vocalizing. NP and NSC were only observed when an
individual was standing at a location that was physically appropriate for a nest. NSC and NP
were performed by the male alone (male NSC, male NP), the female alone (female NSC,
female NP), or by both partners simultaneously, as they stood side by side at the same location (mutual NSC, mutual NP). NP and NSC were Ž rst observed after an individual or a pair
established a territory, and neither of these behaviour patterns was observed after the onset of
egglaying and incubation.
In the current study, we were only interested in sites that were sufŽ ciently attractive to
elicit repeated visits by the members of a pair. Thus, for the purposes of this study, a ‘site’
is deŽ ned as a location at which we observed either NSC or NP by either (or both) partners
on at least three different days between territory establishment and the onset of egglaying or
incubation.
Two sets of birds were used for the current study. Focal data were collected on pairs that
nested during the 1996-97 breeding season, with observations conducted from January 29
1394
STAMPS, CALDERÓN-DE ANDA, PEREZ & DRUMMOND
to April 10, 1997. Our goal here was to study the process of nestsite selection in Ž rst-time
breeders, so we monitored solitary territorial males with no prior breeding experience, and
watched for the arrival in the study area of couples composed of two Ž rst-time breeders.
Since we were interested in the process of nestsite selection, we only included pairs in the
Focal sample if they remained together from the day the female joined a solitary male on
his territory (or the couple arrived in the study area), until the onset of incubation; a total of
8 pairs satisŽ ed these criteria. We used onset of incubation as the endpoint of observations
because although most (7/8) pairs laid eggs, one female disappeared without laying any eggs.
In this pair, the male incubated a rock at one of the sites the pair had investigated prior to the
female’s disappearance. Most (14/16) of the individuals in the Focal sample were Ž rst-time
breeders; the two exceptions were 2 experienced females who each paired with a solitary,
territorial male.
For each Focal pair, we scheduled one hour of observation each morning (between 6:00
and 12:00) and one hour in the afternoon (between 14:00 and 16:00) on a rotating schedule to
control for behavioural variation as a function of time of day; observations were taken during
these scheduled periods whenever one or both members of the pair were present on their
territory. On average, pairs were observed for 26.96 (SD D §16.4) hours, virtually every day
from pair formation to the onset of incubation. Since we were primarily interested in social
interactions between mated pairs, unless otherwise indicated, analyses were based on periods
when both partners were together on the territory at the same time.
During the focal observations, we noted the location of every bout of individual or mutual
NSC and NP to the nearest 1 m on a scale map of the region encompassing the pair’s territory.
Each ‘bout’ began when the actor (s) began NP or NSC at a site, and ended when the birds
paused for at least 30 sec, began performing another behaviour at the same site, or moved
away from the site. Rates of NSC and NP refer to the number of bouts observed during
periods when the male and female were together on the territory, divided by the number of
hours that both partners were present together on the territory.
Survey data were collected for pairs who nested during the 1997-98 breeding season, with
behavioural observations conducted from November 20, 1997 to February 15, 1998. The
Survey sample consisted of 40 pairs who laid their Ž rst egg before February 15, 1998, and
who were either present on their territories at the beginning of the study (Nov 20, 1997) or
who formed a pair, and established a territory in the study area after that date. These birds
were observed from four blinds, from each of which an area of approximately 80 m2 could
be monitored. Every day, trained observers in each of the four blinds recorded the behaviour
of pairs around each blind from 6:30 to 8:45, and from 15:00 to 17:15, the periods when the
birds were most active in the colony. Each period was divided into 45 minute intervals; during
each interval, observers used 1-0 sampling to record the presence and location of male NP,
female NP and mutual NP, male NSC, female NSC and mutual NSC; the location of each of
these displays was noted to the nearest 1 m on a scale map of each of the four areas. Rates
of behaviour patterns for Survey birds re ect the proportion of 45-minute intervals in which
we observed individuals or pairs engaging in each of these behaviour patterns. Because each
observer monitored a number of pairs simultaneously, it was not possible to keep track of the
time each individual arrived or left the study area. Hence it was not possible to restrict these
analyses to periods in which one or both members of each pair were present on their territory.
Of the two behaviour patterns that booby partners might use to communicate nestsite
preferences, we focused on NP for several reasons. First, NP has no direct effect on a site,
whereas NSC changes the physical characteristics of a site, in addition to any possible
JOINT DECISIONS ABOUT NESTSITES
1395
communicative functions this behaviour might have. Second, NP virtually always occurs
when an individual and its partner are together on the territory, whereas NSC is often
performed when an individual is alone on the territory. For instance, in the current study, we
observed focal individuals when they were alone on their territories for a total of 60.0 hours
(xN D 7:49 hrs/pair), but only recorded one bout of NP, by a male, during this entire period.
In contrast, during the same period, solitary females performed NSC at an average rate of
1.84 bouts/hr, and solitary males performed NSC at an average rate of 0.55 bouts/hr. In fact,
solitary territorial males typically prepared one site using NSC before acquiring a mate, and
then displayed at this site when courting potential partners (see also Nelson, 1978). Finally,
analyses of nestsite selection based on NSC produced results which were qualitatively similar
to those based on NP. Hence, in the interest of simplicity and brevity, the current study focuses
on nestpointing behaviour.
Statistical analyses were parametric when the assumptions of normality and equal
variance were satisŽ ed; otherwise, non-parametric tests were used. Tests were two-tailed
unless otherwise indicated; multiple tests of signiŽ cance were conducted using Hochberg’s
procedure (Hochberg, 1988).
One new statistical method was developed for this study. One of our goals was to
determine whether nestpointing behaviour at a given site was related to the likelihood that
that site would be accepted for the nest. This lead to the hypothesis that pairs might select
the site at which one of the two individuals had nestpointed at its maximal rate during the
prelaying period; the null hypothesis was that pairs selected a site without regard to that
individual’s nestpointing rates during the prelaying period. For each pair, we computed the
likelihood that a pair would select the site they did, under the null hypothesis. For instance,
if pair D had 5 sites, the female nestpointed at her maximal rate at site c, and she laid the
Ž rst egg at site c, then under the null hypothesis, there was a 1/5 (or p D 0:2) chance of
obtaining this result. Since the number of sites varied among the pairs, each pair had its
own likelihood of accepting the site where an individual had pointed at its maximal rate.
Results from different pairs were treated as separate ‘experiments’, and were combined using
Fisher’s test (Fisher, 1954; see also Sokal & Rohlf, 1995). This test generates a statistic, S,
the signiŽ cance of which is traditionally tested under the assumption that it converges on a
chi-square distribution (Fisher, 1954, section 21.1). However, this assumption is invalid in
the current situation, since in each pair, probabilities could take on only one of two possible
values (e.g. either pair D selected the site with maximal female NP (p D 0:2) or they did not
(p D 0:8)). Hence, our statistical consultant, N. Willits, wrote a program in SAS to compute
the exact probability of obtaining any value of S, based on the number of pairs in the study,
and the number of sites for each of those pairs. Since this test might be useful for studying
other situations in which individuals select one item from a set of options, and in which
the number of options in the set varies among individuals, copies of this SAS program are
available on request.
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STAMPS, CALDERÓN-DE ANDA, PEREZ & DRUMMOND
Results
Nestpointing and nestsite selection
Selection of nestsites by experienced breeders
A key assumption of this study is that each pair selects a new nestsite every
year, since this is the situation in which the behaviour of a pair during the
prelaying period would be most strongly related to their choice of a nestsite
for the current breeding season. In order to determine whether experienced
breeders reuse nestsites from previous years, we compared the location of the
current year’s nest with the location of the same individual’s nest the previous
year. Seventy-two of the 80 individuals in the Survey sample were banded,
and all of these banded birds had nested in the study area the previous
year. On average, during the 1997-98 season, these 72 experienced breeders
selected a nestsite that was 6.1 (§ 8.4) m away from the location of the
nest they had used in 1996-97. Twenty-six of these pairs retained the same
mate, and nested in the same region of the colony, in 1996-97 and in 1997-98.
However, even among these pairs, only 3 selected a nest that was near (< 1 m)
the nest they had used the previous year. In the Focal sample, two of the
Ž rst-time breeder males attracted experienced females as mates, but neither
of these experienced females selected a nestsite near her nest of the previous
year (13.6 m, 25.4 m). The fact that virtually all of the experienced breeders
in our study selected a new nestsite is important, because it suggests that even
individuals who retained both a territory and a mate from the previous year
made a new decision about the location of their nest each breeding season.
Nestpointing
Most booby pairs investigated a number of different sites, before selecting
one of them for their nest. The Survey pairs investigated an average of 3.5
(SD D §2.4, range D 1-9) sites before selecting one of them; the Ž gures
were nearly identical for the 8 pairs in the Focal sample (xN D 3:7 § 2:6 sites,
range D 2-8).
From the perspective of a human observer, at least, NP was quite useful for
predicting which of a pair’s sites would eventually be selected for their nest.
For instance, the Survey birds pointed at the site eventually chosen as the
nest during an average of 44% (SD D 24%) of the days during the prelaying
period. Over the same period, those same individuals pointed a much smaller
JOINT DECISIONS ABOUT NESTSITES
1397
proportion of the time (on average, on 8% (§ 8%) of the days) at other sites
in their territory (Wilcoxon paired test, z D 4:29, p < 0:001). Similarly,
for the Focal birds, NP rates averaged 5.96 (SD D 7.52) bouts/hr at the site
that was eventually selected for the nest, but averaged only 0.43 (SD D 0.42)
bouts/hr at other sites in the territory (Wilcoxon paired test, exact probability,
p D 0:016). Hence, high rates of NP at a site during the weeks prior to clutch
initiation provided human observers with an excellent indication that that site
would be selected for the nest.
Nestpointing at a particular site over a series of consecutive days provided
another indication that that site would eventually be chosen for the nest. We
quantiŽ ed this phenomenon by setting a ‘NP criterion’, deŽ ned as a period
of 7 consecutive days, during which male NP, female NP or mutual NP were
observed at a given site on at least 6 of those days. Bayes theorem was used
to determine the extent to which NP to criterion at a site allowed us to predict
the site at which a pair would lay their Ž rst egg (see Bradbury & Vehrencamp,
1998, page 402). Since pairs in the Survey investigated an average of 3.5 sites
prior to selecting one of them for their nest, in the absence of any additional
information, we would assume that any given site would have one chance in
3.5 (or p D 0:29) of being selected for the nest. NP to criterion occurred at 32
of the 40 sites that were selected as nests, but NP to criterion almost never
occurred (4 cases out of 100 sites) at sites that were not selected as nests.
Substituting these values into Bayes theorem, we Ž nd that if we observed
NP to criterion at a site, the likelihood of that site being selected for the nest
increased from 0.29 to 0.89.
Taken together, these results indicate that NP during the period between
territory establishment and the Ž rst egg was strongly related to the likelihood
that a site would be selected for a nest, with high rates of NP, and consistent
NP over time, at the site that was eventually chosen for the nest. These results
are consistent with the hypothesis that NP behaviour is related to the process
of nestsite selection in this species. However, NP might also have additional
functions. For instance, if female NP rates varied as a function of female
reproductive status, males might use female NP rates to estimate the date
when the Ž rst egg was due, in order to be physiologically prepared to begin
incubation as soon as the Ž rst egg was laid (Ligon, 1999). Alternatively,
mutual NP rates might convey information about the reproductive status of
a pair, and help improve reproductive synchrony with other pairs in their
immediate neighbourhood (Murphy & Schauer, 1996).
Female
0:054 § 0:048
0:066 § 0:064
0.154
Male
0:064 § 0:053
0:049 § 0:057
0.038
Mutual
0:085 § 0:073
0:167 § 0:144
0.001*
Male
0:091 § 0:088
0:141 § 0:155
0.096
Female
0:032 § 0:035
0:057 § 0:072
0.114
Mutual
0:149 § 0:098
0:251 § 0:154
0.0004*
Nestscraping (xN § SD)
Rates indicate proportion of observation intervals per day in which males, females or both engaged in each behaviour (N D 40 pairs).
* SigniŽ cant at ® < 0:005 (Hochberg’s procedure).
Period I
Period II
Wilcoxon matched-pairs test, p value
Nestpointing (xN § SD)
TABLE 2. Rates of nestpointing and nestscraping of blue footed boobies, as a function of time to Ž rst egg
1398
STAMPS, CALDERÓN-DE ANDA, PEREZ & DRUMMOND
JOINT DECISIONS ABOUT NESTSITES
1399
If NP behaviour were correlated with the reproductive state of individuals
or pairs, we might expect NP rates to systematically change (increase or
decrease) as a function of time remaining until the laying of the Ž rst egg.
Most of the Survey pairs began NP more than a month (xN D 48:1, §13.7
days) before laying their Ž rst egg. As a result, we were able to compare
rates of male NP, female NP and mutual NP early in the prelaying period
(Period I: initiation of observations to 15 days prior to the Ž rst egg) with
rates of those same behaviour patterns for those same individuals during the
two week period prior to the laying of the Ž rst egg (Period II: 14 day period
before the Ž rst egg was laid). For comparison, we also computed rates of
male, female and mutual NSC for these same periods. Rates of NP and
NSC by individuals were similar for Periods I and II, but rates of mutual
NP and mutual NSC were signiŽ cantly higher in Period II than in Period
I (Table 2, Wilcoxon matched pairs test, including pair-wise p values, and
results signiŽ cant at ® < 0:005, based on Hochberg’s procedure). Hence,
neither male nor female NP rates appeared to change as a function of time
remaining to clutch initiation. On the other hand, these results suggest that
mutual display rates (mutual NP or NSC) may be affected by a different set
of factors than individual display rates (male or female NP or NSC), since
mutual NP and mutual NSC both signiŽ cantly increased as time to the Ž rst
egg approached.
Sex differences in NP
In most of the pairs in this study, the male performed the Ž rst bouts of NP
in the territory. In the Survey sample, the male pointed Ž rst in 27 pairs, the
female Ž rst in only 9 pairs; in the other four pairs, both sexes began pointing
on the same day (Sign test, p < 0:006). In 6 of the Focal pairs, solitary males
prepared one potential nestsite in their territory prior to pair formation; all of
these males performed NP at this Ž rst site while courting the female who
eventually became their mate. Of the two Focal couples that were already
paired when they Ž rst arrived in the study area, the female pointed Ž rst in
one pair, and in the second pair, both partners began pointing on the same
day.
In the Survey sample, 21 pairs eventually accepted the site where we
Ž rst observed NP by one or both partners, another 16 pairs rejected the
Ž rst site with NP; the remaining 3 pairs were not included in this analysis,
because they pointed at several different sites on the Ž rst day of NP. Even
1400
STAMPS, CALDERÓN-DE ANDA, PEREZ & DRUMMOND
though males were much more likely than females to initiate NP (see above),
the likelihood that the Ž rst site with NP would be accepted as the nest
was unrelated to the sex of the partner who initiated NP at that site (44%
acceptance for Ž rst sites indicated by the male, 50% acceptance for Ž rst sites
indicated by the female). Similarly, the amount of time that elapsed between
the Ž rst observation of NP and the Ž rst egg was comparable for pairs that
accepted their Ž rst site (46:5 § 13:6 day), and for pairs that rejected their
Ž rst site (52:4 § 11:8 days). In a logistic regression which included both of
these variables, the likelihood that a Ž rst site would be accepted for the nest
was unrelated to the number of days between Ž rst NP and Ž rst egg (Wald
statistic, p D 0:28) or to the sex of the partner who Ž rst pointed at that site
(p D 0:72) (Model Chi-square D 2.01, 2 df, p D 0:37, based on 35 pairs
in which one of the two partners pointed at one site on the Ž rst day of NP).
These results suggest that Ž rst sites ‘suggested’ by the male were as likely to
be accepted as Ž rst sites suggested by the female, and pairs which began NP
nearly two months prior to clutch initiation were just as likely to accept their
Ž rst site as pairs that began pointing only a few weeks before laying their
Ž rst egg.
Regardless of any behaviour that precedes egglaying, female boobies
necessarily make the Ž nal decision about the location where those eggs will
be laid. Accordingly, we asked whether there was any indication that females
pointed at higher rates than males during the two week period immediately
before the laying of the Ž rst egg. For each Survey pair, we noted the number
of intervals with female NP and male NP during the two week period
before the Ž rst egg, and found that females pointed at signiŽ cantly higher
rates than their mates during this period (females: xN D 0:068 (§ 0.064)
intervals/day, males: xN D 0:048 (§ 0.58) intervals/day, Wilcoxon signed
ranks test, p D 0:037). In the Focal pairs, we noted the maximal rate of
NP observed for each individual at any site, and then asked whether females
had higher maximal NP rates than males. In all 8 pairs, the maximal NP rate
observed at any site in the territory was higher for females than for their
mates (females: xN D 5:03 bouts/hr, males: xN D 2:92 bouts/hr; Wilcoxon test,
p D 0:012). Hence, there were several indications that females pointed at
higher rates than their mates during the period preceding egglaying.
Taken together, the results in this section are consistent with the hypothesis that NP behaviour re ects the preferences of individuals and pairs for
particular nestsites. Accordingly, for the remainder of this study, we assumed
JOINT DECISIONS ABOUT NESTSITES
1401
that NP at a particular site by a male or a female indicates interest by that
individual in that site, and that intra-individual variation in rates of NP at different potential nestsites re ects variation in that individual’s level of preference for those sites. These assumptions allowed us to precede to the next
portion of this study, whose goal is to ask whether boobies use collaborative
tactics to make joint decisions about the location for their nest.
Tactics for nestsite selection in boobies
Nestpointing, dispute resolution, and expanding the pie
The dual concern model suggests that pairs will use speciŽ c types of
collaborative tactics (e.g. expanding the pie) if the members of a pair do
not immediately agree about the merits of their Ž rst option. Hence, in order
to study tactics for dispute-resolution in boobies, we need some way to
estimate the degree to which partners agreed (or disagreed) about the merits
of potential sites for their nest.
One possible way to evaluate agreement about a Ž rst site is to count the
number of days that elapsed between the day we observed one partner Ž rst
pointing at that site, and the day that the other partner was Ž rst observed to
point at that same site. For example, two partners who were both interested in
the same site might both begin NP there on the same day, whereas a lengthy
delay between initiation of NP at a site by one party and the beginning of
NP at that site by the other party might re ect low interest in that site by the
second party. Accordingly, for each pair, we computed ‘nestpoint lag’ for
each Ž rst site where we observed NP by either partner, where nestpoint lag
is the difference in days between Ž rst NP observed at the Ž rst site by one
individual and Ž rst NP observed at the Ž rst site by its partner. Note that a
low value of nestpoint lag is assumed to re ect a high level of ‘agreement’
between two partners about the merits of that site, whereas a high value of
this variable is assumed to re ect disagreement about the merits of the site.
As predicted, boobies in the Survey sample were more likely to select
Ž rst sites for their nest when both partners ‘agreed’ about those sites (low
values of nestpoint lag) than when the two parties did not (high values of
nestpoint lag) (Fig. 2). We used logistic regression to predict whether or
not the Ž rst site with NP by either party would be accepted for a nest, as a
function of three variables: sex of the partner who pointed Ž rst at that site,
number of days between Ž rst NP at the site and Ž rst egg, and nestpoint lag.
1402
STAMPS, CALDERÓN-DE ANDA, PEREZ & DRUMMOND
Fig. 2. Likelihood that a pair of boobies would accept their Ž rst site, as a function of
nestpoint lag at that site. For each pair (dot), the number of days between the Ž rst nestpointing
(NP) observed at the initial site by one partner and the Ž rst NP observed at that same site by
the other partner was used to estimate the extent to which the two parties ‘agreed’ about the
merits of the initial nestsite. Pairs with a short lag from Ž rst NP to Ž rst NP at their initial site
were more likely to select that site for their nest than were pairs with a lengthy delay between
Ž rst NP to Ž rst NP at their initial site. Bars indicate mean values of nestpoint lag for pairs that
accepted or rejected their initial site (see text).
Of these three variables, only nestpoint lag was signiŽ cantly related to the
likelihood that a site would be selected for a nest (Wald statistic D 6.6, 1 df,
p D 0:01; Model Chi-square D 15.9, 3 df, p D 0:0021; N D 35 pairs). A
second logistic regression model which included only nestpoint lag as the
independent variable was equally successful at predicting whether or not a
site would be selected for a nest (Wald statistic D 8.0, 1 df, p D 0:0047;
Model Chi-square D 16.7, 1 df, p < 0:0001, N D 37 pairs). Of the 21 pairs
that eventually selected the Ž rst site for their nest, the average nestpoint lag
for the Ž rst site was 7.9 (§ 9.0) days, whereas for the 16 pairs who rejected
their Ž rst site, the average nestpoint lag for the Ž rst site was 33.6 (§ 23) days
(see Fig. 2).
Also as predicted, pairs who disagreed about the merits of their Ž rst site
were likely to go on to investigate additional sites, i.e. they ‘expanded the
pie’. The total number of sites per pair was strongly related to nestpoint lag
(r D 0:73, N D 36, p < 0:0001), with comparably strong correlations
between nestpoint lag and total number of sites for pairs in which the female
Ž rst pointed at the Ž rst site (r D 0:78, N D 8, p D 0:02) and pairs in which
the male Ž rst pointed at the Ž rst site (r D 0:76, N D 26, p < 0:0001)
(Fig. 3).
JOINT DECISIONS ABOUT NESTSITES
1403
Fig. 3. Expanding the pie. Number of days between Ž rst NP to Ž rst NP at the Ž rst site was
used as an index of the extent to which two parties agreed about the merits of that initial
nestsite (see Fig. 2). Pairs with a long delay between Ž rst NP to Ž rst NP at their initial site
investigated more sites during the prelaying period than did pairs in which both parties began
pointing at the initial site within a few days of one another.
Pairs that visited more than one site typically traveled back and forth
between potential sites before choosing one of them for the nest, but most
pairs did not select the last site they investigated. To study the order in
which each couple investigated their potential sites, we noted the Ž rst day
we observed either party Ž rst pointing at each site, and then arranged the
Ž rst NP days for their sites in chronological order. Of 27 Survey pairs that
investigated more than one site and that Ž rst pointed at those sites on different
days, only 7 pairs selected their last site for the nest. Eleven other pairs
eventually chose the Ž rst site at which they had pointed, while the remaining
9 pairs selected a site that was of intermediate rank with respect to Ž rst NP
date. Similarly, of the 8 Focal pairs, 3 Focal pairs selected their last site, one
pair selected their Ž rst site, and the remaining 4 pairs selected a site with
intermediate rank.
Feeling out procedures
Feeling out procedures are a collaborative tactic by which an individual
determines whether an option is acceptable to its partner before signaling at
a high rate in association with that option. In boobies, feeling out procedures
imply that an individual would only exhibit a high rate of NP at a particular
site after its partner had started to point at that same site. For this analysis,
we used the Focal data to determine the site at which we observed each
1404
STAMPS, CALDERÓN-DE ANDA, PEREZ & DRUMMOND
individual NP at its maximal rate during the prelaying period. If boobies
exhibit behaviour comparable to feeling out procedures, maximal NP at a
site by one party should occur after its partner started to point at that same
site.
Of the 16 individuals in the Focal sample, 11 pointed at their maximal
rate at a site at least one day after we Ž rst observed their partner NP at that
same site, 3 pointed at maximal rate at a site at least one day before their
partner Ž rst pointed at that site; in the two remaining cases, maximal NP
rate by an individual and Ž rst NP by the other partner was observed during
the same focal period (Sign test, one-tailed, p D 0:03). Hence, there was a
signiŽ cant tendency for individuals to NP at their maximal rate at a site after
their partner had begun to NP at that same site.
We also tested an alternative hypothesis, that high rates of NP at a site
indicate escalation (a contentious tactic). For instance, exchanges of NP
might represent a ‘shouting match’, by which two partners produce elevated
NP rates at different sites, in an attempt to induce their partner to accept
their preferred site for the nest. Under this hypothesis, we would expect each
individual to NP at its maximal rate at a site after its partner had NP at a
high rate at another site. For this analysis, we noted the maximal NP rate per
site for each individual, and then noted all of the sites and focal periods at
which that individual NP at > 50% of its maximal rate. If high rates of NP
indicate escalation, then an individual should point at a maximal rate at a site
after its partner pointed at a ‘high’ rate (> 50% of its maximal NP rate) at a
different site. Of the 16 birds in the Focal sample, 7 satisŽ ed these criteria,
but the other 9 did not. Hence, we found little support for the hypothesis that
boobies pointed at maximal rates at a site after receiving signals indicating
that their partner might be interested in another site.
Veto power
In collaboration, two parties are unlikely to adopt an option that is unacceptable to one of the two parties (see Introduction). In boobies, this implies that
a site will be selected for a nest after both individuals have indicated that a
site is acceptable, by pointing at that site. We tested this prediction for the
Survey pairs by focusing on the 31 pairs who pointed at more than one site
before laying their Ž rst egg. For each of these pairs, we determined the proportion of sites at which both male NP and female NP were observed at any
time during the prelaying period. On average, the Survey pairs with multiple
JOINT DECISIONS ABOUT NESTSITES
1405
sites exhibited both male NP and female NP at 50.1 (§ 24)% of their sites.
Hence, if these birds selected nestsites at random with respect to male and
female NP, half of the birds in this sample would have been expected to have
selected a site at which both parties had pointed prior to egglaying. Instead,
29 of the 31 pairs selected a site at which both male NP and female NP were
observed prior to egglaying (p < 0:0001, one-tailed, binomial test). Similarly, the Focal pairs exhibited both male and female NP at an average of
70% of their sites, but all 8 of these pairs eventually selected a site at which
both parties had pointed prior to incubation (p D 0:058, one-tailed, binomial test). Taken together, these results support the prediction that booby
pairs were more likely to select a site that was acceptable to both of them
than to select a site that failed to elicit NP from both individuals.
Final decisions about nestsite selection
The data thus far indicate that boobies were most likely to select sites that
were acceptable to both partners, i.e. sites that stimulated both male and
female NP during the prelaying period. However, many pairs had more
than one site which elicited NP by both of them. On average, Survey pairs
had 1:68 § 1:0 sites with both male and female NP, while Focal pairs had
3:10 § 1:64 sites with NP by both sexes. When confronted with two or more
sites which elicited NP from both parties, how did pairs select one of these
for their nest?
Given the fact that females nestpointed at higher rates than their mates in
the two weeks immediately preceding egglaying (see above), we hypothesized that pairs would select the site that was most preferred by the female.
In that case, under the assumption that NP rate re ects level of preference
for a site, we predicted that pairs would select the site at which we observed the maximal rate of NP by the female during the prelaying period.
In fact, all 7 of the Focal females who laid eggs did so at the site where they
had pointed at their maximal rate prior to egglaying. We used a variation of
Fisher’s combined probability test to determine the probability of this occurring by chance, given the number of sites per pair, and the null hypothesis
that females select a site independently of their NP rate during the period
prior to egglaying (see Methods). The value of Fisher’s statistic S D 15:48
for this test, with an exact probability of p D 0:003; result signiŽ cant at
® < 0:01, based on 3 tests (see also below). These results are consistent with
1406
STAMPS, CALDERÓN-DE ANDA, PEREZ & DRUMMOND
the hypothesis that pairs eventually select the site that was most attractive to
the female during the prelaying period.
We used a similar approach to ask about relationships between male
behaviour and nestsite selection in this species. First, we asked whether
maximal rates of NP in males were related to Ž nal nestsite selection by the
pair. In contrast to the situation with females, only 3 of 7 males pointed
at their maximal rate at the site that was ultimately selected for the nest
(S D 6:99, exact probability D 0.76). Second, we asked whether male
incubation behaviour was related to female NP behaviour during the period
prior to egglaying. This question was suggested by Focal pair G, in which the
female disappeared before laying any eggs (see Methods). Before the female
disappeared, this pair investigated 6 different sites; the female pointed at her
maximal rate (5 bouts/hr) at one site, while the male pointed at his maximal
rate (2 bouts/hr) at another site. After the female disappeared, the male pulled
a rock into one of their sites and then performed incubation behaviour while
sitting on this rock; the site where the male incubated the rock was the site
where the female had pointed at her maximal rate before disappearing. The
males in the 7 other focal pairs incubated at the sites where their mate had
nestpointed at her maximal rate prior to egglaying. Hence, all of the males
in the study incubated at the site at which the female had nestpointed at her
maximal rate in the period prior to the onset of male incubation (S D 19:07,
exact p D 0:0005; result signiŽ cant at ® < 0:01).
Discussion and conclusions
The results of this study are consistent with the hypotheses that pairs of blue
footed boobies employ collaborative tactics when selecting a nestsite. First,
collaboration is expected to involve extensive amounts of communication,
where the level of signaling associated with a given location re ects the
signaler’s level of preference for that location. Boobies typically investigated
several sites before selecting one of them for their nest, and produced a
previously undescribed display (nestpointing, NP) when they were standing
at a site potentially suitable for a nest. Both sexes produced NP displays
at high rates during the weeks in which they were investigating potential
nestsites, and NP rates at a given site were strongly related to the likelihood
that a pair would select that particular site for their nest. Hence, boobies
JOINT DECISIONS ABOUT NESTSITES
1407
clearly engaged in extensive amounts of communication when selecting a
nest, and the NP display seems a likely vehicle by which partners might
indicate their preferences for sites to one another.
Of course, the inferences in this study were based on correlations between nestpointing rates and nestsite selection, so we must consider other
hypotheses that might produce relationships between nestpointing behaviour
and nestsite selection. For instance, a spurious positive relationship between
NP rate and nestsite selection might occur if NP rates increase as time to Ž rst
egg approaches, and (coincidentally) if pairs search for potential nestsites sequentially and accept the last site they visit. However, there was no support
for this alternative hypothesis, because rates of male and female NP did not
change as a function of time to Ž rst egg, and because most of the pairs that
investigated more than one site did not select the last site for their nest.
Collaboration in humans may involve ‘feeling out procedures’, by which
one party attempts to determine the other’s level of preference for an
option before indicating a high level of preference for that same option
(see Introduction). If boobies exhibit feeling out procedures, maximal rates
of nestpointing at a site by one party should occur after its partner began
pointing at that same site, a prediction that was conŽ rmed in the current
study. A second collaborative tactic observed in boobies was ‘expanding the
pie’; as predicted, pairs in which both partners began nestpointing at their
Ž rst site within a few days of one another investigated fewer sites during
the prelaying period than pairs in which there was a long delay between
Ž rst pointing at the Ž rst site by one party, and Ž rst pointing at that same
site by their partner. Also, as expected under expanding the pie, pairs which
‘disagreed’ about their Ž rst site usually rejected this site, and ended up
accepting one of their new sites for the nest. Finally, the prediction that
each party would have veto power over particular nestsites was conŽ rmed
in the current study. Sites that failed to elicit nestpointing from both parties
were much less likely to be selected than expected under the hypothesis that
pointing by both partners was not a prerequisite for nestsite selection. Since
females made the Ž nal decision about where to lay the Ž rst egg, the power
of the veto was especially important for the males.
In contrast, we found no support for the hypothesis that contentious tactics
play a role in nestsite selection in this species. For instance, NP did not
seem to re ect attempts by individuals to persuade a reluctant partner to
accept their preferred site via escalation of signal intensity, nor was there any
1408
STAMPS, CALDERÓN-DE ANDA, PEREZ & DRUMMOND
indication that individuals in icted costs on their partner during the process
of nestsite selection. Indeed, the process of nestsite selection was remarkable
for the lack of constraints on the behaviour of the other party. For instance,
individuals were free to ignore a partner who was vigorously pointing at a
particular site, and in many cases, they did so. We could not envision any way
that one bird could coerce its partner into pointing at a site, yet, as was noted
above, NP by both individuals seemed to be a prerequisite for the selection
of a site for the nest.
Our results also illustrate another important point about collaboration,
namely that both members of a dyad need not perform the same type of
behaviour for this process to produce a joint decision. We noted several sex
differences in behaviour during the nestsite selection process: males were
more likely than females to initiate pointing at potential nestsites, females
pointed at higher rates than males during the two week period prior to
egglaying, and maximal NP by the female (but not the male) predicted the
site that would eventually be selected for the clutch. However, despite these
differences between the sexes, input from both partners was required for
nestsite selection. For instance, sites ‘suggested’ by females (i.e. the female
was the Ž rst bird we observed pointing at that site) were no more likely to be
accepted for the nest than sites suggested by males. And even though females
made the Ž nal choice of nestsite in this species, they appear to have made
their selection from a ‘short list’ of potential sites that were also acceptable
to their mate.
To our knowledge, this is the Ž rst detailed behavioural study of the
process by which pairs of animals make a joint decisions about a nestsite.
However, descriptive accounts suggest that joint selection of nestsites involving lengthy exchanges of specialized communication signals occurs in
other species of birds (Tinbergen, 1953; Scott, 1972; Nethersole-Thompson
& Nethersole-Thompson, 1979; Furness, 1987; Newton, 1986; Marzluff &
Balda, 1992; Tarof & Ratcliffe, 2000). Many of these descriptions of nestsite
selection sound quite similar to the process described here for the boobies.
Thus, in Peregrine falcons, Falco peregrinus, males are more likely than females to show new nestsites to their partner, both sexes produce specialized
‘ledge displays’ at potential nestsites, ledge displays may be given by the
male, by the female, or by both simultaneously, and females seem to invest
more time than males in nestsite selection behaviour as time to Ž rst egg approaches (Ratcliffe, 1980). In addition, indirect evidence suggests that other
JOINT DECISIONS ABOUT NESTSITES
1409
species of birds may consider multiple sites before selecting one of them
for the nest. Thus, O’Connell et al. (1997) reported that a majority of lesser
black-backed gulls (Larus fuscus) constructed more than one nest in their
territories, before selecting one of these nests for the clutch. As was the case
in the boobies, gull pairs with multiple nests were as likely to select the Ž rst
as the last of these nests for the eggs, and never built additional nests once
they began laying in one of them (O’Connell et al., 1997).
Another potential insight from this study is that male and female birds
who jointly select a nestsite are expected to engage in extensive amounts of
communication during the prelaying period. In fact, monogamous species of
birds are notable for the extent to which they display to one another after pair
formation (Wachtmeister, 2001). In a recent review on the topic, Wachtmeister (2001) suggested that extensive exchanges of displays in monogamous
species represent attempts by each partner to manipulate the other (e.g. into
providing more parental care to the young). This argument seems to be based
on the assumption that extensive exchanges of communication signals are diagnostic of contentious tactics. However, as we have emphasized in this paper, extensive exchanges of communication signals are also expected under
collaboration. Given the broadly overlapping interests of males and females
in many monogamous species, and the number of important joint decisions
that pairs may need to make prior to clutch initiation (e.g. where to locate
territory boundaries, where to place the nest, when to initiate egglaying), it
is possible that some of the extensive communication that occurs during this
period might re ect collaborative rather than contentious behaviour.
The results of the current study are encouraging, in that they suggest that
a simple descriptive model derived from studies of joint decision-making
in humans may be able to provide useful insights into the processes that
other species use to make joint decisions about a nestsite. We were heartened
by the number of novel predictions about behaviour patterns that were
generated by the dual-concern model, not to mention the number of those
predictions that were supported by analyses of nestsite selection in boobies.
More generally, we hope that our brief review of the dual concern model,
and our preliminary attempt to study collaborative behaviour in the context
of nestsite selection in birds, may encourage other workers to consider the
range of strategies and tactics that animal dyads might use to make important
joint decisions about space. Given the lack of detailed empirical studies on
this topic, there is clearly plenty of scope for additional research in this area.
1410
STAMPS, CALDERÓN-DE ANDA, PEREZ & DRUMMOND
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