Behavioral Ecology Vol. 12 No. 3: 275-282
© 2001 International Society for Behavioral Ecology
Large barnacle goose males can overcome the social costs of natal dispersal
Department of Animal Ecology, Evolutionary Biology Centre, Uppsala University, Norbyvägen 18D, S-752 36 Uppsala, Sweden
Address correspondence to H.P. van der Jeugd, who is now at the Department of Zoology, Edward Grey Institute of Field Ornithology, University of Oxford, Oxford OX1 3PS, UK. E-mail: henk.vanderjeugd{at}zoo.ox.ac.uk .
Received 25 August 1999; revised 14 January 2000; accepted 5 July 2000.
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ABSTRACT |
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The decision to disperse is influenced by a trade-off between the costs and benefits of moving to a new area, and the optimal dispersal tactic is likely to differ among different individuals. I studied natal dispersal of barnacle geese in relation to fledging characteristics such as body weight and body size. I classified birds as having dispersed when they were known to be alive at the age of 5 years, the maximum age of first breeding, but had not recruited into their natal colony. I assumed that those birds were breeding elsewhere. Because survival was registered on the wintering grounds, survival estimates were not dependent on the size of the study area during breeding. The probability of successful dispersal increased with increasing body weight and tarsus length in males, but not in females. Heavy or large males also won more aggressive interactions with territory owners when returning to their natal colony as 1 year olds, and were therefore probably at a competitive advantage when settling. Individuals that settled in an unfamiliar environment suffered more attacks by territory owners, and as a result, they started reproduction at a later age than birds remaining in the natal colony. Surprisingly, dispersing males did not seem to benefit from their choice to disperse in terms of enhanced reproductive success. The views that dispersal in birds generally results from poor-quality individuals being expelled from their natal area and that low return rates of heavy individuals result from stabilizing selection need to be revised.
Key words: barnacle goose, body size, Branta leucopsis, natal dispersal, philopatry, prospecting.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Whenever fitness varies between different patches, individuals are expected to disperse, leaving unfavorable sites and settling in more favorable ones (Gadgil, 1971
;
McPeek and Holt, 1992).
Additional benefits of dispersal from the natal area might be avoidance of
high levels of inbreeding (Greenwood,
1980; Pusey, 1987)
or avoidance of local resource competition
(Cooch et al., 1993). However,
there might be costs associated with dispersal, such as the direct costs of
moving or costs of settling in an unfamiliar environment
(Bengtsson, 1978;
Pärt,
1994; Stamps,
1987). Settling in an unfamiliar environment might be costly
because of loss of site dominance (Heg et
al., 2000), loss of benefits arising because of the presence of
kin (Hamilton, 1964), lack of
knowledge about good foraging sites
(Bradley and Wooller, 1991;
Greenwood, 1980;
Lessells, 1985;
Rohwer and Anderson, 1988) or
nest sites
(Pärt,
1994), or reduced mating success
(Bensch et al., 1998;
Pärt,
1994). Furthermore, dispersing individuals might not be well
adapted to the new environment (Dhondt et
al., 1990; Dias and Blondel,
1996; Verhulst,
1995).
The balance between these costs and benefits shapes optimal dispersal
tactics and dispersal distances. However, this balance is likely to differ
among classes of individuals. For example, sex-specific dispersal strategies
are common and are believed mainly to originate from larger benefits of
staying for one of the sexes (Greenwood,
1980). Indeed, some studies have shown that the dispersing sex
seemed to benefit from dispersal, whereas the philopatric sex did not
(Bensch et al., 1998;
Greenwood et al., 1979;
Nilsson, 1989;
Pärt,
1994). But relative costs and benefits associated with dispersal
are also likely to differ among individuals differing in phenotype. For
example, some individuals might be constrained in such a way that the optimal
choice, be it dispersal or philopatry, is outside their set of
possibilities.
A number of studies have tried to identify the phenotypic correlates of
dispersal in birds and mammals and have produced a wide variety of results.
Some studies reported that natal dispersal was associated with small body
size, late hatching date, or poor condition
(Dhondt and Huble, 1968;
Greenwood et al., 1979;
Neergaard, 1999;
Nilsson, 1989). Others,
however, found the opposite (Juliard et
al., 1996; Verhulst et al.,
1997). These contradictory results may occur because the quality
of the natal habitat relative to the quality of alternative habitat can
interact with phenotype (Neergaard,
1999; Verhulst et al.,
1997) or because of different scales at which studies are
performed. For example, negative relationships between individual quality and
dispersal may be found when only short-distance dispersers are considered,
which may have been philopatrics that did not make it to their natal area,
whereas positive relationships may arise when analyzing long-distance
dispersers that purposefully set out to find better options.
Colonial-breeding birds are highly suitable for the study of dispersal
because colonies are clearly separated, often conspicuous, and comparatively
easily monitored, so that dispersers are identified unambiguously
(Pradel, 1996). In long-lived
colonial species with delayed reproduction, young birds are often present in
colonies in their prebreeding years. This behavior has been suggested to be an
information-gathering process enhancing the individual's probability of
breeding successfully in the future
(Boulinier et al., 1996;
Danchin et al., 1998;
Porter, 1988;
Reed and Oring, 1992;
Schjørring et al.,
1999). Young birds might also be present as floaters establishing
site dominance at preferred sites and waiting for a breeding opportunity to
arise (Smith, 1984).
Potentially, birds might be doing both at the same time; active prospectors
might gather information and establish site dominance through their persistent
presence in a particular area, enabling them to eventually settle in that area
(Ens et al., 1995;
Heg et al., 2000; Zack and
Stutchburry, 1992). Older, established birds might behave aggressively toward
prebreeders to prevent them from future settlement, but hostility toward such
birds might be lower when they are in a familiar environment
(Stamps, 1987) or when kin are
present. Thus, costs of settlement are likely to be higher for individuals
that are not in their natal area, and the study of prebreeders born at
different sites might therefore reveal some of the social costs of
dispersal.
I studied natal dispersal of barnacle geese born in the largest colony in the Baltic region. The aims of this study are to (1) identify phenotypic correlates of natal dispersal, (2) analyze potential social costs of settling in an unfamiliar environment by studying behavior of prebreeders, and (3) quantify costs and benefits of dispersal by comparing reproductive success of dispersers with that of philopatric birds. Potential costs of dispersal are more likely to show during the prebreeding phase, since reproductive success can only be quantified for successful dispersers, while a large part of the cost of dispersal might be hidden in failed dispersal attempts.
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METHODS |
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Study population
This study was mainly conducted in the oldest and largest colony of the Baltic barnacle goose Branta leucopsis population, situated on the three islands of Laus holmar off the east coast of Gotland, Sweden (57°17' N, 18°45' E) between 1984 and 1998 (Figure 1; colony 1). Colony 1 was naturally established in 1971 (Larsson et al., 1988
) and has increased rapidly since. In 1998 colony 1
consisted of 2220 breeding pairs. Several other breeding colonies have also
been naturally established in the Baltic area
(Larsson and van der Jeugd,
1998). One of these colonies, the second largest Baltic colony on
K
reholm off the east coast of
Öland, Sweden (56°58' N,
16°54' E; Figure 1;
colony 2) was studied from 1986 to 1996, although less intensively. The
Baltic-breeding barnacle geese migrate in autumn to wintering areas in the
Netherlands and Germany, where they mix with birds from the Russian population
(Ganter et al., 1999). In
colony 1, reproductive success of individuals has decreased as density
increased (Larsson and Forslund,
1994; Larsson and van der
Jeugd, 1998; van der Jeugd,
1999), while reproductive success in the newly established
colonies 2-6 is, on average, six to eight times higher than in colony 1
(Larsson and van der Jeugd,
1998; Figure 2).
Natal dispersal from colony 1 is male biased, and male natal dispersal rate
increased during the study period (van der
Jeugd, 1999).
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Birds breeding in colony 1 mainly use two brood-rearing areas where they
take their small young a few days after hatching. One of these areas is
situated on the largest of the three breeding islands making up colony 1
(i.e., in the breeding colony; site A in
Figure 1). The other area is a
peninsula approximately 7 km from site A (site B in
Figure 1). Juvenile survival
was higher at site B then at site A at least during the first years of the
study (Forslund, 1993). In
addition, some birds use a second peninsula approximately 15 km from site A
(site C in Figure 1), which is
a brood-rearing area shared by birds breeding in colonies 1 and 3. In most
cases, I did not know whether young reared at site C were born in colony 1 or
colony 3.
Body size measurements, recruitment, and reproductive success
Juvenile barnacle geese were captured, marked, and measured in the middle
of July each year, 1-2 weeks before fledging, following Larsson et al.
(1998). Differences in body
weight and tarsus length between juveniles at the time of capture can
partially be explained by differences in age and by genetic differences among
individuals (Larsson and Forslund,
1991,
1992), but mostly they reflect
environmental and social conditions that the goslings experienced after
hatching (van der Jeugd and Larsson,
1998; Larsson et al.,
1998). Because juveniles are sexually dimorphic, I standardized
male measurements to female equivalents
(Larsson et al., 1998).
Juveniles that are small at capture show compensatory growth, but usually do
not reach the adult size of birds which were larger at capture
(Larsson and Forslund,
1991).
An individual was classified as having recruited into colony 1 when it had
undertaken a breeding attempt in which at least a nest with eggs was produced.
The probability of observing a breeding attempt in colony 1 was very high in
all years, around 90% (van der Jeugd,
1999). Birds were defined as having recruited into colony 2 when
they were recaptured in colony 2 at least once when at least 5 years of age.
Age at first reproduction has increased considerably in colony 1 over the
study period, but 97% of all recruits that were found in colony 1 were
observed breeding for the first time when between 2 and 5 years of age
(van der Jeugd, 1999).
Therefore, when analyzing age at first reproduction, data on individuals born
between 1984 and 1993 are used (i.e., birds that recruited into colony 1
between 1986 and 1998). I calculated relative age at first reproduction by
subtracting cohort means from individual values. In total, 649 individuals
born in colony 1 were classified as local recruits and could be used in the
analyses (Figure 2). Of these
birds, 63% were females.
I measured reproductive performance of barnacle geese breeding in colony 1
by observing marked individuals in nesting and brood-rearing areas. Clutch
size, hatching date, number of fledged young, natal brood size, and birth date
of young were measured following van der Jeugd and Larsson
(1998) and Larsson et al.
(1998). I calculated relative
values for body size measurements and reproductive parameters by subtracting
yearly means from individual values.
Classification of dispersers
In total, 59 individuals that had been born in colony 1 had dispersed from
colony 1 and were found breeding in the Baltic colonies 2-6
(Figure 2). Of these birds, 63%
were males. However, since it is likely that many dispersers were over-looked,
for example, because they dispersed to one of the Arctic colonies or another
colony outside the study area, I analyzed 48,628 winter resightings of 1302
individuals marked as juveniles in the study colony between 1984 and 1993,
assuming that all individuals that were observed alive during their fifth
winter or later, but had not recruited into colony 1, had dispersed. This
yielded a sample of 231 dispersers, of which 68% were males. To rule out any
confounding effects of survival when comparing local recruits with dispersers,
I only used local recruits that also had survived up to their fifth winter or
later. I then analyzed the effects of sex, tarsus length, and body weight at
fledging, brood-rearing site (A or B), birth date, natal brood size, and
whether birds were observed in colony 1 when 1 year old or not on natal
dispersal probability.
Because data on reproductive success of dispersers could only rarely be
obtained at their breeding locations, I used observations of the number of
young that accompanied the adult birds on the wintering grounds to compare
reproductive success of dispersers and philopatrics born in colony 1. This
method underestimates the number of fledglings because families tend to break
up during winter (Owen et al.,
1988) and because young birds are easily overlooked in large
flocks of wintering geese. These problems should, however, apply both to
philopatric birds as well as to dispersers.
Prebreeding 1-year-old birds
Birds born in colony 1 were regularly observed in colony 1 and other
colonies during their first summer (i.e., when they were 1 year old). At this
age, barnacle geese do not breed (van der
Jeugd, 1999). Behavioral observations of 1-year-old birds were
made during 1995, 1996, and 1997 at site A (i.e., in the breeding colony and,
for comparison, at site B and in flocks of nonbreeders on agricultural fields
in the direct vicinity of colony 1). Marked 1 year olds observed in colony 1
were either born in colony 1 and reared at site A or B. A small number of
marked 1 year olds was born in colony 1 or 3, and reared at site C, or born in
colony 5, approximately 80 km north of colony 1. The behavior of 1-year-old
birds at the main breeding island (site A) fitted the descriptions of
prospecting (Boulinier and Danchin,
1997), and I therefore treated all 1 year olds that had been
observed at site A at least once as having been prospecting in colony 1.
Marked 1-year-old birds were followed for 10 min, and during this period the number of agonistic interactions plus their outcome were recorded. Interactions were classified into wins (opponent is displaced), draws (no reaction or displacement), and losses (focal bird is displaced), and were mainly observed between 1-year-old birds and older birds defending a nesting territory or leading a brood of newly hatched young. Interactions were rarely initiated by 1 year olds. I defined attack rate as the number of attacks the focal bird suffered during the 10-min observation period. Win scores were calculated as the number of wins divided by all interactions.
Statistical analyses
I used linear logistic models to model natal dispersal probability using a
binary response variable (0 recruited into the natal colony, 1 dispersed). I
used Proc CATMOD of SAS software (Collet,
1991; SAS Institute,
1993), with a logit link function in which both continuous as well
as class variables could be used as explanatory variables. The goodness of fit
of all models was tested comparing the likelihood of the current model with
that of a fully saturated model using -2log (Lc/Lf), the
deviance of the current model. I used the Akaikes Information Criterion (AIC)
to evaluate the significance of omitting or introducing variables into the
model, until the most parsimonious model was found
(Akaike, 1973;
Collet, 1991). Significance of
variables in the final model was tested using Wald's tests. The data set
contained 880 individuals. Data on birth data and natal brood size were only
known for 394 of the 880 individuals, and, since they had no effect on natal
dispersal probability (
2 = 0.03, df = 1, ns; 2
= 0.57, df = 1, ns, respectively), they were removed before analyzing the full
data set with all 880 individuals. Year of birth was included in the model to
control for differences in weight at fledging and dispersal rate among
cohorts. Variables were removed hierarchically starting with nonsignificant
interactions.
I analyzed the number of attacks 1-year-old birds suffered during 10-min
observation periods using an ANOVA model using type III sums of squares
(SAS Institute, 1993) with
sex, body weight, and tarsus length at fledging, site (A, B, flocks), and
brood-rearing site of origin (A, B, C, or colony 5) as explanatory
variables.
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RESULTS |
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Phenotypic correlates of dispersers from colony 1
Birds that were not observed prospecting in colony 1 when 1 year old dispersed more often than birds that had been observed prospecting in colony 1, although this effect was stronger in males than in females (Table 1). Males that had large body weights at fledging had a higher probability of dispersing, but such an effect was absent in females (Table 1, Figure 3). Tarsus length at fledging yielded a similar result in males (
21 = 8.06 p <.005) because it was
strongly correlated with body weight at fledging (rp =
0.75, n = 935, p <.0001). Although birds reared at site B
are generally larger and heavier than birds reared at site A
(Larsson and Forslund, 1991),
the effect of body weight on dispersal rate was not caused by birds reared at
site B dispersing more often. There were interactions between weight and
prospecting (21 = 3.04, p <.1; not
retained in the final model) and between tarsus length and prospecting
(21 = 6.04, p <.01) because birds that
had recruited but were not observed prospecting during their first summer had
lower body weights and tarsus lengths at fledging. Using the smaller sample of
dispersers observed in one of the other Baltic colonies produced qualitatively
similar results, although the effect of body weight at fledging in males was
not significant (21 = 1.97, p =.15).
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Only 15 birds born in colony 2 were found to have immigrated into colony 1, of which 14 were males. These males had somewhat larger weights and longer tarsi at fledging than males that had recruited into colony 2, but the differences were not statistically significant (relative weight: t = -0.78, df = 27; relative tarsus length: t = -1.03, df = 27).
Behavior of 1-year-old birds
I most often observed 1-year-old birds at site A, but also regularly at
site B and in flocks of nonbreeders on agricultural fields in the vicinity of
colony 1. In total, 296 10-min behavioral observations of 155 different
1-year-old individuals, 71 males and 84 females, were collected during the
breeding seasons of 1995, 1996, and 1997. One-year-old birds were attacked
frequently, mainly by older birds that were defending a nesting territory or
leading a brood. Attack rates differed significantly between sites, being much
higher at site A than at site B or in nonbreeding flocks
(Table 2,
Figure 4). Males that had been
reared at site A suffered fewer attacks when prospecting at site A than birds
originating from other areas. Attack rates at site A increased with increasing
distance between site A and the site at which a prospecting male was reared
(Table 2,
Figure 4). No such effects were
apparent in females. These results were confirmed by within-individual
comparisons between sites A and B. Males reared at site B were more often
attacked when at site A than when at site B (difference = 1.41, t =
2.66, p <.05), but birds reared at site A were not (difference =
-0.13, t = -0.08, ns).
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Males that were heavier or had larger tarsi at fledging tended to suffer higher attack rates (Table 2). At the same time, 1-year-olds with win scores higher than 0 had significantly longer tarsi at fledging (0:78.8 mm, >0: 80.9 mm; F155,1 = 4.66, p <.05) and tended to have larger body weights at fledging (0:1067 g, >0: 1145 g, F153,1 = 2.62, p <.1).
Because males reared at site A seemed to have an advantage over males reared at site B (and males from other, more distant areas) when prospecting in site A, I analyzed whether this also had repercussions on the time it took them to recruit into the breeding population. Both males and females reared at site B that did not prospect during their first summer took almost 1 year longer to recruit than birds reared at site A. They could, however, lower their age at first reproduction by prospecting as a 1 year old, although they still recruited later than males reared at site A. Among males reared at site A there was no difference in age at first reproduction with respect to prospecting (ANOVA: brood-rearing site, F1,705 = 4.02, p <.05; prospecting, F1,705 = 11.29, p <.001; site x prospecting: F1,705 = 6.62, p =.01). No birds reared at site C or in colony 5 had recruited into colony 1 before the end of the study.
Reproductive success of dispersers
Among birds born in colony 1, dispersers were accompanied by, on average,
0.30 young on the wintering grounds, philopatrics by 0.43 young (t
test: t = 0.67, df = 171, ns). There were no differences in age at
first reproduction, clutch size, hatching date, or number of fledged young
produced between males that were born in colony 2 and males that had
immigrated into colony 1 (Table
3). Thus, I was not able to find any costs or benefits associated
with dispersal in terms of reproduction.
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DISCUSSION |
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Phenotypic correlates of dispersal
This study showed that dispersing males were heavier and had longer tarsi at fledging than philopatric males and that such effects were absent in females. Body weight and tarsus length at fledging reflect, to a large extent, differences in conditions experienced during growth (Larsson et al., 1998
), and
birds that are heavier or have longer tarsi at fledging have higher
post-fledging survival, for example (van
der Jeugd and Larsson, 1998). This study also showed that such
birds had higher win scores when prospecting as 1 year olds. Thus, heavy and
large males might disperse successfully more often because of a competitive
advantage when establishing a territory. This is important, as there seem to
be social costs associated with dispersal in terms of the number of attacks
dispersing males suffer from territory owners when prospecting in an
unfamiliar environment, leading to a greater age at first reproduction. That
dispersers are larger or in better condition has been reported in insects
(Anholt, 1990;
Lawrence, 1987) and especially
in mammals (Clutton-Brock et al.,
1982; LeBoeuf and Reiter,
1988;
Wahlström,
1994;
Wahlström
and Liberg, 1995; Woodroffe et
al., 1995). But to my knowledge, Verhulst et al.
(1997) conducted the only
study that found that successful dispersers were larger than philopatrics in
birds. In contrast, many bird studies found opposite results
(Dhondt and Huble, 1968;
Greenwood et al., 1979;
Neergaard, 1999;
Nilsson, 1989), or found no
effect at all
(Korpimäki
and Lagerström, 1988;
Newton and Marquiss, 1983;
Pärt,
1990). The heterogeneity of these results seems to stem from two
factors.
First, when habitat quality differs between patches and the most successful
competitors occupy the best habitat
(Fretwell, 1972;
Fretwell and Lucas, 1970), the
quality of the natal habitat relative to the quality of the alternative
habitats determines whether high-quality individuals will be philopatric or
disperse. For example, Neergaard
(1999) found that rock pipits
Anthus petrosus dispersed less often from high-quality habitat and
that dispersers were late-hatched birds. Similarly, Verhulst et al.
(1997) found that larger great
tits Parus major dispersed successfully from low- to high-quality
habitat more often, while there was no such effect among birds dispersing in
the other direction. The pattern I found might thus only apply to dispersal
from colony 1 to other Baltic colonies, while no effect of phenotype, or a
negative one, might have been found when dispersal in the other direction was
considered. The absence of a significant body weight difference between
immigrants from colony 2 and local recruits in colony 2 might be seen as
evidence for such an interaction, but the low sample size prevents any firm
conclusions.
Second, the scale at which a study is performed potentially plays a role.
In studies in which dispersal was associated with poor individual quality,
individuals may have tried to achieve philopatry, and short-distance dispersal
probably resulted from philopatric individuals being prevented from settling
in the direct vicinity of their birth site because of strong competition, or
from failure to find their way home
(Pärt,
1991). Studies that found the opposite might have studied
long-distance dispersers that set out to find better options, but could only
be successful in doing so when they were of superior quality. Indeed, the
study by Verhulst et al.
(1997) dealt with dispersal
distances that are far greater than in most other passerine bird studies. When
the whole scale of dispersal distances is considered, a bimodal pattern might
emerge, with dispersal being associated with both poor and good quality, while
intermediate individuals are philopatric (see
Nilsson, 1989). Juliard et al.
(1996) suggested that low
return rates of the heaviest chicks, as found in several bird studies (e.g.,
Tinbergen and Boerlijst,
1990), are probably not a result of stabilizing selection
(Gebhardt-Henrich and van Noordwijk,
1991), but instead result from heavier or larger individuals
dispersing more often. As the results of the present study, together with
those of Verhulst et al.
(1997) show, evidence is now
accumulating that this indeed is the case.
Benefits of dispersal
Natal dispersal in barnacle geese is strongly male biased
(van der Jeugd, 1999), but the
present study also showed that only certain males were able to disperse
successfully from a large colony with low breeding success. One would thus
expect successful male dispersers to benefit from dispersal. Potentially,
dispersal is beneficial because reproductive success is higher in newly
established, and therefore smaller, Baltic colonies than in colony 1
(Larsson and van der Jeugd,
1998; Figure 2).
Yet I was not able to show any direct benefits of natal dispersal in terms of
number of young produced. This might have been caused by the rather indirect
method of measuring reproductive success I had to rely on, or by the fact that
many birds disperse to unknown sites, with unknown reproductive success. But
it is also possible that benefits of dispersal first arise in the second
generation because any philopatric offspring immigrants produce should not be
affected by the costs of dispersal their parents had to pay, and thus might be
expected to enjoy a reproductive success, on average, equal to other birds in
that colony. Furthermore, because reproductive success varies with colony size
(Larsson and van der Jeugd,
1998), variance of reproductive success should be higher among
dispersers than among philopatrics. The benefits of dispersal could thus lie
in the possibility of some birds "hitting the jackpot" by
successfully recruiting into a newly established colony with large potential
for growth, or by successfully establishing a new colony at a high-quality
site. A more general problem is that the benefits of dispersal are difficult
to measure because we cannot know what the fitness of a disperser would have
been had it stayed at home, unless experimental manipulation of dispersal is
invoked, which does not seem feasible.
Only a few bird studies have been able to show direct benefits of dispersal
in terms of enhanced reproductive success of dispersers. Greenwood et al.
(1979) and Nilsson
(1989) both found that female
great and marsh tits that dispersed further produced more offspring, but did
not find any effect in males, while immigrant male collared flyctachers
(Pärt,
1994) and great reed warblers
(Bensch et al., 1998) had lower
fitness because of reduced mating success. The dispersing sex thus seemed to
benefit from dispersal, whereas the philopatric sex did not in those studies.
However, dispersal was disadvantageous in female collared flycatchers
(Pärt,
1991,
1994) and female sparrowhawks
Accipiter nisus (Newton,
1986; Newton and Marquiss,
1983), despite females being the dispersing sex.
Behavior of 1-year-old birds
Nonbreeding, 1-year-old birds were frequently observed in colony 1, as well
as in other Baltic colonies. Two hypotheses explain the presence of young,
nonbreeding individuals at breeding sites. First, young birds might be
gathering information, enhancing their probability of breeding successfully in
the future—the prospecting hypothesis
(Boulinier et al., 1996;
Danchin et al., 1998;
Porter, 1988;
Reed and Oring, 1992;
Schjørring et al.,
1999). Second, young birds might be present as floaters,
establishing site dominance and waiting for a breeding opportunity to
arise—the floater hypothesis
(Boulinier et al., 1996; Smith,
1978,
1984; Zach and Stuchtbury,
1992). The behavior of the 1-year-old birds I observed fitted the descriptions
of prospecting behavior (Porter,
1988; Reed and Oring,
1992; Schjørring et
al., 1999). They arrived relatively late in the breeding season,
at a moment when most public information about variation in reproductive
success among sites is available (Boulinier
and Danchin, 1997), they were generally inquisitive, and females
were regularly observed to inspect, or even squat on, used nests. However,
prospectors were frequently attacked by territory owners, suggesting that
older birds try to prevent them from future settlement. Only birds strong
enough to compete with other nonbreeders and with territory owners could
maintain themselves in the colony, as birds that were not observed prospecting
had lower body weights and shorter tarsi at fledging and recruited later in
life. Some 1-year-old birds limited their efforts to parts of site A, and
large 1-year-old males initiated, and sometimes won, interactions with
territory owners, suggesting that birds were establishing site dominance.
Large 1-year-olds also tended to be attacked more frequently by territory
owners, probably because they were more provocative in their behavior.
Provocation is probably exactly what they tried to achieve to assess their
chances against potential opponents
(Grafen, 1987).
Simultaneously, by channeling their attacks toward the most provocative
1-year-old males, older males probably target the most likely future
competitors
(Wahlström,
1994). Summarizing, I hypothesize that nonbreeding, 1-year-old
barnacle geese are present in colonies to (1) gather information about site
quality and the degree of competition and (2) establish local dominance at
preferred sites and compete for vacancies. In long-lived species, territory
acquisition is probably a long-term process, in which information gathering
and acquiring site dominance yield across-year benefits eventually resulting
in the acquisition of territories (Forbes
and Kaiser, 1994; Zack and
Stutchbury, 1992).
Costs of dispersal
One-year-old males suffered more attacks when in an unfamiliar environment,
and attack frequency increased with the distance between site A and the site
at which a bird was reared. That resident birds respond more intensely to
strangers is a widespread phenomenon (review in
Ydenberg et al., 1988), and
can potentially be explained by the "dear enemy phenomenon"
according to which familiar individuals should fight less because payoffs are
known and boundaries are already established
(Krebs, 1982;
Stamps, 1987;
Ydenberg et al., 1988).
Moreover, unfamiliar individuals have been suggested to make more tactical
errors (Stamps, 1987), or, if
familiar birds tend to be related, kin favoritism could potentially explain
the differential response to familiar and unfamiliar individuals
(Emlen, 1994;
Hamilton, 1964). Barnacle
geese families are very mobile even before the young fledge and occasionally
visit other brood-rearing areas than their natal one (H. P. van der Jeugd,
personal observation). Thus, the degree of familiarity with an area probably
declines with increasing distance from that area to the natal area, which
might explain why attack rate gradually increased with increasing distance,
instead of always being higher for nonphilopatric birds.
Unfamiliar birds not only suffered more attacks when prospecting, but also
recruited at a later age. Thus, dispersing birds that attempt to settle in an
unfamiliar environment seem to be at a disadvantage, which might be an
important cost of dispersal in barnacle geese. My observations also suggest
that choice of brood rearing area is influenced by a trade-off between the
benefits of higher juvenile survival at site B
(Forslund, 1993) and the costs
of producing offspring with a lower degree of familiarity with site A.
Conclusions
There are many potential costs associated with dispersal, and I showed that
social costs during settlement in an unfamiliar environment might be one of
these. As a result, heavy and large barnacle goose males had a higher
probability of dispersing successfully from their natal colony because they
could overcome these costs. Thus, dispersal tactics can differ among
individuals with different phenotypes. This study also suggested, as well as a
small number of other studies, that the direction of the relationship between
phenotype and natal dispersal rate might be determined by the quality of the
natal habitat relative to the quality of alternative habitats. There is now
growing evidence that natal dispersal in birds does not always result from
poor-quality individuals being expelled from their natal area and that low
return rates of heavy individuals are probably not a result from stabilizing
selection, but result from phenotypic effects on dispersal.
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ACKNOWLEDGEMENTS |
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I wish to thank all the volunteers that sent their winter observations to me which enabled me to identify such a large number of dispersers. Karen Blaakmeer and Gunnar Kärrholt participated in collecting the data on the behavior of 1-year-old birds. Kjell Larsson is greatly acknowledged for his continuous participation in the fieldwork, guidance, and sharing ideas, and for allowing me to use data he and Pär Forslund collected before I joined the project. Anders Forsman and Sami Merilaita advised on the statistical analyses. Kjell Larsson, Thomas Pärt, Ineke van der Veen, Marlene Zuk, and two anonymous referees commented on an earlier version of the manuscript. Financial support was provided by the Stiftelsen för Zoologisk Forskning and the Royal Swedish Academy of Sciences.
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