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Behavioral Ecology Vol. 12 No. 3: 287-294
© 2001 International Society for Behavioral Ecology
Ecologically determined natal philopatry within a colony of great cormorants
Department of Zoology, Institute of Biological Sciences, Building 135, University of Aarhus, DK-8000 Aarhus C, Denmark
Address correspondence to S. Schjørring, who is now at the Department of Evolutionary Ecology, Max Planck Institute for Limnology, August-Thienemann-Str. 2, D-24306 Plön, Germany. E-mail: schjoerring{at}mpil-ploen.mpg.de .
Received 24 March 2000; revised 25 July 2000; accepted 25 July 2000.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Dispersal patterns of individuals within populations have implications for the social and genetic structure of local populations. Knowing what factors determine individual dispersal behavior is essential for predicting how the population structure will be influenced by environmental and demographic changes. In this study, I investigated whether the settling pattern of individuals breeding for the first time within a colony of great cormorants was determined by ecological or genetic factors. Furthermore, I examined the possible effects of age and gender. First-time breeders that came back to breed within their natal colony showed strong philopatry toward their natal breeding sites. Because of the simultaneous strong fidelity of breeders toward their former breeding sites, this caused kin to cluster to some extent around the natal site. However, genetic factors (attraction to close kin) are less likely to explain natal philopatry than ecological ones (attraction to the natal site itself). Younger first-time breeders were more philopatric than older ones, in accordance with a decrease in the predictability of the quality of breeding sites with increasing time lags. Furthermore, males dispersed farther from the natal breeding site than females. This result is contrary to what is generally expected for a breeding system where the male is dependent on a breeding territory for mate acquisition. I suggest that this sex difference could arise because first-time breeding males are constrained from settling in the natal site by interference competition with older males or because males are better informed about alternative breeding sites of high quality within the colony.
Key words: great comorants, habitat selection, kin selection, natal dispersal distance, Phalacrocorax carbo sinensis, philopatry.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Dispersal patterns of organisms are a fundamental aspect of their ecology as they modify the genetic and social structure of local populations. Factors in the ecological and social environment that influence the settling pattern of individuals may provide evolutionary explanations for the maintenance of dispersal in natural populations.
The ecological environment may influence the dispersal behavior of
individuals by determining the variation in intrinsic quality among potential
breeding sites. Availability of mates and other essential resources, risk of
predation and parasitism, and exposure to harsh weather are examples of
factors that may determine the suitability of breeding sites. When breeding
for the first time, spotted sandpipers (Actitis macularia), pine
voles (Microtus pinetorum), and female plateau pikas (Ochotona
curzoniae), for example, prefer breeding areas where the sex ratio is
higher for the opposite sex (Dobson et
al., 1998
; Reed and Oring,
1992; Solomon et al.,
1998), presumably because they thereby increase their chances of
finding a mate. In the colonial cliff swallow (Hirundo pyrrhonota),
infestations of the hematophagous swallow bug (Oeciacus vicarius)
severely depress nestling body condition and survivorship
(Brown and Brown, 1986).
Individuals that experience high levels of infestation as nestlings are more
likely to disperse to other colonies to breed than those that experience lower
levels of infestation (Brown and Brown,
1992). The hatching success in the Manx shearwaters (Puffinus
puffinus) is strongly determined by the incidence of rainfall during the
incubation period (Thompson and Furness,
1991). Eggs in flooded nest burrows are much less likely to hatch
than those in dry burrows. The nest burrows vary in their susceptibility to
flooding, and breeding birds tend to move to new burrows after flooding
(Thompson and Furness,
1991).
Interactions with conspecifics may constrain an individual's settling
options or affect its expected benefits by invoking competition or
cooperation. In the lesser kestrel (Falco naumanni) and the marsh tit
(Parus palustris), for example, competition for breeding territories
increases with density, and individuals breeding for the first time tend to
disperse away from areas of high density
(Negro et al., 1997;
Nilsson, 1989). Settling in a
familiar breeding area may be beneficial because of increased competitive
ability (Krebs, 1982;
Zack and Stutchbury, 1992),
fewer or more benign aggressive encounters with familiar neighbors
(Beletsky and Orians, 1989;
Eason and Hannon, 1994;
Stamps, 1991;
Widemo, 1997), or more
efficient antipredator tactics in collaboration with familiar neighbors
(Beletsky and Orians,
1991).
If the interacting individuals are close kin, additional costs of
competition and benefits of cooperation may be invoked by indirect fitness
effects. Aggression, for example, is commonly less intense among kin than
among unrelated individuals (Waldman,
1988). Male western gulls (Larus occidentalis) nesting
adjacent to their fathers or brothers fledge on average 26% more young per
breeding attempt than males nesting adjacent to the territory of their
deceased fathers (Spear et al.,
1998). Breeding close to related individuals may also imply an
increased probability of incestuous breeding if kin recognition is poorly
developed or if the number of alternative unrelated mates is limited.
Inbreeding may be beneficial if conditions favor the maintenance of locally
adapted or intrinsically coadapted gene complexes
(Shields, 1987). However,
breeding with close kin can result in inbreeding depression (reviewed by
Crnokrak and Roff, 1999) due
to the unmasking of recessive, deleterious alleles
(Lande, 1994), increased
homozygosity, or reduced allozyme variability
(Vrijenhoek, 1994).
Wheelwright and Mauck
(1998) found that first-time
breeding savannah sparrows (Passerculus sandwichensis) settled
farther from their natal site if it was still occupied by the parent of
opposite sex. Additionally, they settled farther from their siblings than
expected by chance. This suggests that individuals actively attempt to avoid
inbreeding. Some of the strongest evidence that inbreeding avoidance can
affect individual settling patterns comes from studies of small mammals
(Pusey, 1987). The motivation
for dispersal of female gray-sided voles (Clethrionomys rufocanus)
increases with increasing male bias in their natal litter
(Ims, 1989). Studies of other
microtines suggest that this effect is linked to a difference in exposure to
male hormones during fetal development
(Holekamp et al., 1984;
Vom Saal and Bronson, 1978).
Female voles with many brothers may thus experience higher concentrations of
male hormones than those with fewer brothers
(Ims, 1989). Such a relatively
inflexible trigger mechanism of dispersal in one of the sexes is more likely
to be based on inbreeding avoidance than, for example, on competition for
resources, where a more plastic, conditional response is likely to be
advantageous (Ims, 1989;
Pusey, 1987).
The influential ecological and genetic factors may interact to some extent.
Kin-biased behaviors, for example, may be conditional on resource availability
if the indirect fitness gain is compensated by a direct fitness loss due to
shared resources. The more scarce the resource, the larger the direct cost
relative to the indirect fitness benefit
(Brown and Brown, 1993;
Stacey and Ligeon, 1987).
Individual differences in dispersal behavior within a population may result
from an age- and sex-related variation in the costs and benefits involved.
Settling close to kin, for example, may facilitate breeding site acquisition
(Watson et al., 1994). If only
one sex is territorial and younger breeders are competitively inferior to
older ones, young first-time breeders of the territorial sex would benefit
more from settling close to kin of the same sex than both older first-time
breeders and first-time breeders of the nonterritorial sex. Furthermore, the
older an individual is when it starts breeding, the higher the probability is
that its close kin will have died (Waser
and Jones, 1983). The expected benefits or costs of interacting
with close kin should therefore decrease with increasing age at first
breeding. Similarly, philopatry of first-time breeders can be advantageous if
the natal site provides the individual with an experience in the year of birth
that will increase its probability of breeding successfully upon breeding
start (Waser and Jones, 1983).
However, the older an individual is when it starts breeding, the less
"up to date" it will be with the conditions at the natal breeding
site, and the smaller its expected benefit of being philopatric will be.
In this study, I investigated the settling pattern of first-time breeders
within a colony of great cormorants, Phalacrocorax carbo sinensis, to
determine whether settling is caused by ecological or genetic factors.
Furthermore, I examined whether the pattern is affected by age at first
breeding. An earlier study of the colony showed that the reproductive success
was spatially autocorrelated and temporally predictable from one year to the
next, suggesting that breeding areas within the colony vary in quality in a
predictable manner (Schjørring et
al., 2000). I extended this analysis to include longer time lags
in order to examine how the decrease in the temporal autocorrelation of
reproductive success agrees with a possible age dependency of the settling
pattern. Finally, I considered the effect of gender. In the great cormorant,
males are, for example, more involved in the establishment and defense of the
breeding territory than females (Cramp,
1977). Thus, if certain ecological or genetic factors act to
facilitate or impede the establishment of a breeding territory, then settling
of males should be more strongly affected by these factors than settling of
females.
Throughout this study, I disregard the possible interaction of extrapair
paternity (EPP; Birkhead and Møller,
1992) with the effect of close kin on the settling pattern of
first-time breeders. A high rate of EPP could disrupt kin selection favoring
attraction to or avoidance of the male parent. Unfortunately, no information
is available on the frequency of EPPs in populations of great cormorants.
However, in the shag, Phalacrocorax aristotelis, a closely related
species with a life history similar to the great cormorant's, the frequency of
EPPs within a colony can vary from almost zero in some years to 18% in others
(Graves et al., 1993). These
results suggest that similar rates of EPP could be found in the great
cormorant and thus could weaken the influence of male parents on the settling
pattern of first-time breeders. However, because most families are founded by
"faithful" mothers, the effect of EPPs would at most be weak.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Great cormorants of the subspecies sinensis breeding in north-west Europe are migratory, piscivorous, aquatic birds. Males arrive at the breeding colony before females in early spring, and they establish and defend a breeding territory, from which they attract females by display and other courtship behavior. Females generally lay four or five eggs; between zero and four young fledge. Both sexes provide care for the young. Age at first breeding is between 2 and 4 years, and birds are assumed to breed for an average of 6 years (Cramp, 1977
). The data presented here are from a long-term study of a Danish colony of great cormorants, conducted by the National Environmental Research Institute. The colony, which is entirely tree nesting, is situated on the island of Vorsø, Jutland. Between 1977 and 1996, 10,861 chicks were marked with engraved plastic leg bands that are legible from a distance. Since 1981, daily searches for ringed individuals have been performed from a blind that overlooks part of the colony, with an average daily observation period of 2 h. Presence, breeding status, and position of breeding site were registered and mapped for ringed individuals within the study area.
An individual was assumed to have attempted to breed if it had acquired a mate and had started building a nest. Only individuals that were attempting to breed for the first time and that were younger than 5 years were included in the analyses.
Settling of first-time breeders relative to the natal breeding site
and close kin
Initially, I examined whether first-time breeders settle randomly with
respect to their natal breeding site and their close kin (parents and siblings
of the same clutch). In 841 cases the position of the natal site and the sex
and breeding position of a first-time breeder were known. For 53 of these,
neither their ringed parents nor their ringed siblings from the same clutch
were observed to breed in the colony in their first year of breeding. This
subset was used to test whether first-time breeders settled randomly with
respect to the natal site. The sex and breeding position were known for a
ringed parent in 44 cases and for a ringed sibling of the same clutch in 33
cases, when sex and breeding position (but not necessarily the position of the
natal site) of first-time breeders were known. These two subsets of data were
used to test for random settling of first-time breeders with respect to
parents and sibling. In the few cases where the breeding position of both a
parent and a sibling was known, first-time breeders were randomly assigned to
one of the two latter subsets of data. Thus, no first-time breeder appeared in
more than one of the three subsamples of data.
For each of the three subsamples of first-time breeders the distance between the first-time breeder and its parent, sibling or natal site was measured and summed over all similar pairs and all years, keeping the different combinations of sex separate. If first-time breeders are attracted to kin or their natal site when they settle, this sum of distances will be smaller than if they have settled randomly with respect to kin and the natal breeding site. If, on the other hand, they try to avoid settling close to kin or their natal site, the sum of distances will be larger than if they have settled randomly.
I tested for a significant difference from random settling by permutating the position of the focal first-time breeders among the breeding positions of all first-time breeders of the same sex and age in the same year (i.e., among what I assumed to be the available breeding positions that year). For each of the three situations, the permutation was repeated 1000 times, and the significance of the sum of the observed distances was estimated from the resulting distribution of sums of distances. Because there is no a priori expectation about the local dispersal pattern, the tests are all two-tailed.
Effect of close kin or the natal breeding site on choice of breeding
site
Site fidelity is strongly developed among breeding birds in the study
colony (Schjørring et al.,
2000). To determine whether a nonrandom settling pattern by
first-time breeders is caused by an effect related to the natal breeding site
(ecological factors) or to kin (genetic factors), I controlled for the spatial
independence of the two by examining how the distance of first-time breeders
to their close kin relative to their natal dispersal distance covaries with
the distance between kin and the natal breeding site. A significant
correlation is expected whether the nonrandom settling of first-time breeders
is caused by an effect of kin or natal site. However, the sign will be
opposite for the two cases: a positive relationship is expected if philopatry
is determined by an attraction to the natal breeding site, whereas a negative
relationship would indicate an attraction to close kin. For 74 first-time
breeders information was available on the position of both kin and natal site
within the colony. The response variable in the correlation analysis was the
difference in log-transformed distances of a first-time breeder to its close
kin and its natal breeding site, and the independent variable was the
log-transformed distance of kin to the natal breeding site. The regression
line was forced through the origin due to the definition of the variables in
the model.
The value of the response variable is constrained to values smaller than or equal to the numerical value of the independent variable; the significance of the regression could therefore not be determined by evaluating the ratio of explained to unexplained variance in an F distribution. Instead, I determined the significance level with a permutation test. To each observed value of the independent variable, I assigned a random value of the response variable within its possible range. I then fitted a least squares regression line through the data points and calculated the ratio of explained variance to unexplained variance. This procedure was repeated 1000 times, and the test probability of the regression line fitted through the observed data was determined from the resulting distribution.
Because I was interested in how the coefficient of determination changes
with age at first breeding, the analysis was performed separately for
individuals with an age at first breeding of 2 years and those with an age of
3 or 4 (data for first-time breeders that were 3 and 4 years old were merged
due to small sample sizes). This was only possible when examining settling of
first-time breeders with respect to their parent or their sibling of opposite
sex because of the small sample of first time breeding siblings of the same
sex. For the same reason, the power of the regression was not expected to be
sufficiently high to reveal a significant interaction with sex, and I
therefore performed the regression on the three sex combinations of siblings
separately. For the settling of first-time breeders with respect to their
parents, on the other hand, sex of both the first-time breeder and the parent
was initially included in an unconstrained analysis of covariance. However,
the effect of sex was nonsignificant, and since this test is less conservative
than the final constrained linear regression, sex was not included in the
latter. The year of hatching was initially included in all the regression
models as a confounding covariate. The colony expanded rapidly during the time
period considered in the analyses
(Bregnballe, 1996), and several
studies have documented a density dependence of individual dispersal
propensity (Greenwood et al.,
1979); Negro et al.,
1997; Nilsson,
1989). However, due to consistent nonsignificance, year of
hatching was excluded from the final model.
Change in predictability of the quality of breeding sites with
increasing time lag
I used reproductive success as an estimate of the quality of a breeding
site. I determined whether the predictability of the site quality decreases as
expected when the time lag is increased to 2, 3, or 4 years, the ages at which
most individuals breed for the first time, with the method described by
Schjørring et al.
(2000). Briefly, a coefficient
quantifying the temporal autocorrelation between pairs of years is calculated,
and the test probabilities are determined with a Monte Carlo permutation
method (Sokal and Rohlf,
1995). An overall level of significance is then obtained with a
meta-analysis (see Schjørring et
al., 2000, for a more detailed description of the method).
Sex and age differences in natal dispersal distance
Natal dispersal distances commonly follow a geometric distribution
(Buechner, 1987). I therefore
used a log negative binomial regression model
(Hilbe, 1994) to test for sex
and age differences in natal dispersal distance. In addition, the year of
hatching was included as a confounding variable. All possible interactions
were allowed in the analysis, but only significant ones were included in the
final model. The preceding analyses showed that close kin did not
significantly affect the settling decisions of first-time breeders, and all
841 first-time breeders of known sex and breeding position were therefore
included in the analysis.
The same data were used to test the significance of predictability of the
spatially autocorrelated reproductive success at different time lags.
Furthermore, the data used in the linear regression analysis form a subset of
the data used in the permutation tests. Accordingly, the significance levels
were adjusted with Bonferroni methods
(Sokal and Rohlf, 1995).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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The sums of natal dispersal distances were significantly smaller than those obtained when randomly assigning individuals to breeding sites (Table 1). Thus, within the colony of great cormorants, first-time breeders settled significantly closer to both their close kin and their natal breeding site than expected if they had settled at random.
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The difference in distance of first-time breeders to their close kin and their natal breeding site increased with the distance of close kin to the natal breeding site in all but one of the linear regressions (Table 2, Figure 1), with the exception that 3- and 4-year-old first-time breeders settled independently of both their natal breeding site and their parent (Table 2, Figure 1a). Thus, the farther away an individual's close kin settled from the natal breeding site, the closer the individual settled to its natal breeding site relative to its kin. In the cases where close kin were a parent or a sibling of the opposite sex, sample sizes allowed me to distinguish younger first-time breeders (2 years old) from older ones (3 or 4 years old). In both cases, the coefficient of determination was substantially smaller for older first-time breeders than for younger ones (Figure 1a,b).
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The predictability of the quality of breeding sites decreased with increasing time lag, as indicated by an increase in the test probability of the temporal autocorrelation with increasing time lag (Table 3). The temporal autocorrelation coefficients and test probabilities of each of the permutation tests are given in the Appendix.
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The log negative binomial regression model satisfactorily fitted the data
(Pearson 
2 = 837.16, df = 837, p =.492) and revealed
that male first-time breeders dispersed farther from the natal breeding site
than females (Table 4,
Figure 2a). Age also had a
significant effect on natal dispersal distance, with older first-time breeders
(3 or 4 years old) settling farther away from their natal breeding site than
younger ones (2 years old; Table
4, Figure 2b).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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The factors eliciting the strongest effect on the settling patterns of individuals should be those contributing most to the variation in quality among breeding sites. However, the extent to which animals can be expected to match variation in quality with their dispersal behavior depends on the detectability and predictability of the important factors. The specific choice of a breeding site of high quality requires that some cue reflects the variation in quality. If no cue is available, or if it is too costly to detect, individuals cannot make a specific choice and may settle randomly or according to some settling rule. Natal philopatry, the tendency to return to the natal breeding site, is one possible settling rule that could, under some circumstances, provide an individual with a higher probability of obtaining a breeding site of high quality than if it settled at random. In species where kin recognition is poorly developed, but breeding near kin is nonetheless beneficial (due to indirect fitness benefits from cooperation, for example), the return of offspring to the natal breeding site would be a way of ensuring kin as neighbors (Komdeur and Hatchwell, 1999
). Furthermore, the effect of factors determining the
intrinsic quality of breeding sites may in some cases be predictable from one
breeding season to the next (Burger,
1982; Danchin et al.,
1998). Under these circumstances, returning to a natal site that
is known to have been of high quality in the year of birth (since the
individual survived being born there;
Ashmole, 1962), would give an
individual a higher chance of ending up in a high-quality site than if it
settled at random. First-time breeding great cormorants that returned to breed within the study colony settled closer to both their natal breeding site and their close kin than expected if they had settled at random (Table 1). This pattern was caused by an attraction to the natal breeding site and not to close kin, because first-time breeders settled relatively closer to the natal breeding site than to kin when kin had moved some distance from the natal site (Figure 1, Table 2).
The benefits that accrue from a settling rule like natal philopatry would decrease with increasing age at maturity because the likelihood that close kin will have died, or that the intrinsic quality of the natal site will have changed, increase when the time between birth and breeding start increases. Thus, within a population, older individuals are expected to show less natal philopatry than younger ones. In accordance with this expectation, older first-time breeders settled less dependently of their natal breeding site than younger ones (Figure 1a,b, Table 2). Confirming this result, younger first-time breeders settled shorter distances from their natal breeding site than older ones (Figure 2b, Table 4).
The settling pattern of first-time breeders within the study colony thus seemed to be determined by a natal philopatry that diminishes with increasing age at first breeding. For the cormorant, it does not seem likely that this settling behavior is caused by kin selection because this explanation would assume poor kin recognition. However, for at least some periods of the life cycle, kin recognition is strongly developed. During a prolonged feeding period after fledging, for example, parents are able to locate their offspring, and vice versa, among thousands of individuals and far from the natal breeding site (S. Schjørring, personal observation). Furthermore, of the 664 breeding pairs within the colony where both individuals were ringed, only 1 pair consisted of close kin (a female and her 3-year-old son), and they did not even reach the egg-laying stage. Thus, pairing of close kin occurs extremely rarely considering the proximity at which kin appear to breed (Table 1). This suggests an ability to recognize kin and avoid them as mates.
The age-dependent attraction to the natal site is best explained by a settling pattern determined by ecological factors. An important assumption of this explanation is that the quality of breeding sites is predictable among years and that this predictability decreases with increasing time periods. The study colony complied with this requirement. The quality of breeding sites was temporally autocorrelated from one year to the next, and the significance of this autocorrelation diminished as the time span in years increased (Table 3).
Within the study colony, males tended to disperse farther from the natal
breeding site than females (Figure
2a, Table 4). This
result is contrary to what is generally expected for monogamous species where
one sex requires a territory to acquire its mate
(Greenwood, 1980). The
majority of studies confirm that with such a breeding system the natal
dispersal distance of the nonterritorial sex is usually farther than that of
the territorial sex (Osorio-Beristain and
Drummond, 1993;
Pärt,
1990; Spear et al.,
1998; Verhulst et al.,
1997), although several studies have failed to find any sex bias
(Arcese, 1989;
Negro et al., 1997;
Payne, 1991;
Wheelwright and Mauck, 1998).
The longer natal dispersal distances of males could be due to a sex difference
in competition for breeding sites. Competition among males for breeding
territories implies direct interference among individuals, whereas competition
for mates among females rarely involves direct interference (S.
Schjørring, personal observation) and may be best characterized as a
scramble for mates (Andersson and Iwasa,
1996). In interference competition, young individuals are often
inferior to older ones because they lack experience, physical strength, or
status (Arcese, 1989;
Potts et al., 1980;
Sutherland and Parker, 1985).
Male first-time breeders may therefore be more constrained than females when
acquiring breeding sites. Thus, even if males are not less motivated to obtain
a breeding site close to their natal site, they could be less likely to obtain
one.
Alternatively, males and females may differ in their motivation to
disperse. Many cormorants are present in the breeding colony in the years
before they start breeding. This prospecting behavior appears to provide them
with information about the position of potential future breeding sites of high
quality (Schjørring et al.,
1999). Males prospect more than females presumably because
prospecting provides males with the additional benefit of knowledge about
future neighbors (Schjørring et
al., 1999). First-time breeding males may thus on average have
more information than females about the positions of good breeding sites.
Thus, a lack of information about the quality of breeding sites that are not
their natal site may constrain natal dispersal of females.
Fidelity of breeding birds toward former breeding sites
(Schjørring et al.,
2000) and the simultaneous attraction of siblings to the natal
breeding site caused kin to cluster to some extent around the natal breeding
site. However, this does not necessarily mean that the population per se is
dominated by family clusters. The frequency of simultaneous breeding of
parents and offspring in the colony may be too low, or migration among
colonies too high, to allow local dynamics to generate an overall genetic
patchiness in the population. Furthermore, it is important to keep in mind
that genes may move even though individuals do not disperse, either via
extrapair paternity (Birkhead and
Møller, 1992) or egg dumping
(Yom-Tov, 1980). Thus,
conclusions at this level are not justifiable unless molecular methods are
used to determine the genetic structure within the colony.
Overall, the data I have presented allow an understanding of some of the
factors that determine the settling decisions of great cormorants. Such an
understanding is crucial for formulating realistic predictions about how
changes in demographic and environmental factors might affect the dispersal
behavior of individuals and thereby the structure and dynamics of
populations.
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ACKNOWLEDGEMENTS |
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I thank the Danish National Environmental Research Institute for allowing me access to their long-term data set on great cormorants. Furthermore, I express my thanks to all the people that collected and organized the data, especially J. Gregersen, T. Bregnballe, L. Abrahamsen, K. Halberg, M. Nitschke, and E. Fritze. I also thank A. P. Møller, J. C. Koella, and S. Toft for their useful comments on earlier versions of the manuscript.
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