Behavioral Ecology Vol. 11 No. 4: 416-420
© 2000 International Society for Behavioral Ecology
Parental nepotism enhances survival of retained offspring in the Siberian jay
kan Tegelströmca Evolutionary Biology Centre, Department of Population Biology, Uppsala University, Norbyvägen 18D, SE-752 36 Uppsala, Sweden b Department of Zoology, Stockholm University, SE-106 91 Stockholm, Sweden c Evolutionary Biology Centre, Department of Genetics, Uppsala University, Norbyvägen 18D, SE-752 36 Uppsala, Sweden
Address correspondence to J. Ekman. E-mail : jan.ekman{at}ebc.uu.se .
Received 24 July 1999; revised 7 December 1999; accepted 10 December 1999.
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ABSTRACT |
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The evolutionary payoff accruing to parents from breeding offspring could be an incentive for prolonged investments in the offspring. Enhanced survival for offspring as a result of such a prolonged parental investment would increase the value of remaining in the natal territory for the offspring. Here we show that first-year survival in Siberian jays is higher in the company of their parents. Two observations point to that the enhanced survival of retained offspring is due to nepotistic parents rather than to the quality of a shared habitat. First, winter survival is higher only for those retained offspring whose parents have survived too ; this precludes the possibility that the link between timing of dispersal and survival should reflect a higher phenotypic quality of retained offspring in general. Second, there is no support for the more parsimonious explanation that this link between the survival of parents and retained offspring reflects habitat quality of a shared territory. We could, with high statistical power, reject the possibility of a correlation between the survival of parental birds and unrelated immigrants to the territory. Such a correlation would have been expected if survival reflected habitat quality and not kinship. Our data instead suggest a direct fitness gain to retained offspring in enhanced survival through parental nepotism (parental facilitation). The behavior of parents in allowing retained offspring access to food that is denied to immigrants is one proximate mechanism mediating a benefit of delayed dispersal.
Key words: delayed dispersal, facilitation, kin, nepotism, parental behavior, Perisoreus infaustus, retained offspring, Siberian jays, survival, winter.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Offspring that reach reproductive status are an evolutionary asset to their parents. The potential for such fitness gains may be an incentive for parents to help their offspring after independence. Such help has been described as parental facilitation and is defined as "...facilitation by parents... of achieving breeding status by their young" (Brown and Brown, 1984
: 206).
Acquisition of breeding status requires survival, and parents may be able to
promote the survival prospects of retained offspring by conceding resources
(Ekman and Rosander, 1992 ;
McNamara et al., 1994 ;
Taylor, 1988).
There are several examples of parental nepotism where parents allow
retained offspring an access to food that they deny non-kin
(Barkan et al., 1986 ;
Ekman et al., 1994 ;
Scott, 1980). Any improvement
in survival as a result of such parental tolerance would enhance the prospects
for the offspring to become independent breeders. Parental nepotism,
furthermore, would be an incentive for offspring to delay dispersal, as the
offspring should be more likely to forgo dispersal when survival prospects are
better in the natal territory than elsewhere
(Brown, 1978).
The Siberian jay Perisoreus infaustus is a species in which
parents show nepotistic behavior (Ekman et
al., 1994). In this species, retained offspring enjoy enhanced
access to food compared to nonrelatives
(Sklepkovych, 1997). The
concession of resources by parents has to enhance offspring survival and,
eventually, reproductive success, to be of any evolutionary consequence. To
test if such concession of resources by the parents has consequences for the
fitness value for offspring remaining in their natal territory, we analyzed
data on winter survival of Siberian jays. In particular, we wanted to test the
hypothesis of survival among retained offspring being conditional on the
presence of the parents and their tolerance of offspring present in winter.
Our analysis is based on a data set of the variation in the timing of natal
dispersal across years, allowing us to compare the survival of retained
offspring in their first year with that of offspring that disperse in their
first year.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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We studied winter survival in a population of individually color-banded Siberian jays, northwest of Arvidsjaur, northern Sweden (65°40' N, 19°0' E), 1990-1999. The number of breeding territories has varied between 13 and 15 in the study site, and the spring population has fluctuated between 30 and 60 birds. The Siberian jay is a territorial, singular breeder, but in two cases retained sons have been allowed to breed in the territory of their parents. The Siberian jay lives in small (range two to seven individuals) all-year flocks formed around the locally breeding pair in company with unrelated immigrants and retained offspring. Siberian jay offspring show a large variation in the timing of natal dispersal. Approximately two out of every three offspring disappear in their first summer of life. Five such birds were found as immigrants to other groups in the study area. At the same time locally hatched offspring disappear, there is an influx of unbanded immigrants (n = 44) hatched outside the study area. Such immigrants may be either first-year birds or older birds, some of which may have delayed dispersal (Ekman et al., 1999
). However, approximately one-third of the offspring forgo
dispersal in their first year of life, and they often postpone dispersal for
several years.
Retained offspring (n = 49) could be identified from banding of
nestlings or capture of fledged offspring in company with their parents. The
reproductive success of pairs was usually recorded by following breeding
attempts. We located nests by radiotracking females. Alternatively, if no nest
was found, we checked the behavior of the flock members, and in particular of
the dominant pairs. In late summer we checked for retained offspring among the
first-year birds in flocks using behavior and DNA fingerprinting. Parents are
distinctly less aggressive to their retained offspring
(Ekman et al., 1994). This
identification of retained offspring was checked and confirmed by multilocus
DNA fingerprinting (see Ekman et al.,
1994, for methods). With this procedure we located 10 offspring
after they had fledged. The evidence from behavior and DNA fingerprinting was
consistent in all 10 retained offspring from those nine broods (two sibs
retained from one brood). Birds classified from behavior as retained offspring
had band sharing with putative parents in the range of 0.565-0.683, which
identified them as offspring. The degree of band sharing was 0.186 (SE, 0.091
; n = 17) within breeding pairs and 0.173 (SE, 0.032 ; n =
7) between breeders and immigrants, and band sharing between parents (both
sexes) and nestlings was 0.625 (SE, 0.088 ; n = 91, 17 broods).
Siberian jays are sexually monomorphic with a substantial overlap in size
between sexes. We therefore determined sex using molecular techniques
(Griffiths et al., 1998 ;
Ogawa et al., 1997). The sex
determinations using these two techniques were first checked for individuals
for which we had independent information on sex from roles during copulation
and from carrying eggs. The observation that the technique consistently
yielded one male and one female determination for breeding pairs is another
check of the reliability of our method of sexing. Finally, we checked the two
techniques for internal consistency. All tests confirmed that molecular sexing
was reliable. Sex determination agreed with our behavioral observations
(n = 21), and breeding pairs consistently produced one male and one
female determination (n = 18 pairs). Furthermore, there were no
internal inconsistencies, and the outcomes of the two molecular techniques
were in agreement for all birs (n = 123).
The survival estimates were based on resightings of colorbanded birds. Losses may be due to mortality and emigration. To locate emigrants we searched territories neighboring the study site and around one-third of territories next to neighboring sites on an annual basis. This search effort was coordinated with a ringing program of the Siberian jay by a local bird watcher. Likewise, local residents were alerted to the possibility that banded Siberian jays might turn up, and they were asked to report. There is no evidence indicating that winter losses (October-March) should be due to emigration. Resightings in March of both retained offspring and immigrants were consistently made within the territory where the bird was last seen in September-October. Furthermore, none of the 39 first-year birds that disappeared between October and March was ever retrieved elsewhere. This lack of evidence for emigration in winter is in contrast to conditions during the breeding season. No less than 12 (80%) out of 15 first-year birds disappearing in April and May were later resighted within or outside our study site, demonstrating a substantial mobility after winter. These resightings furthermore testify to our potential to detect actual emigration, should any of the birds disappearing in winter have left our study area.
All birds were marked and reobserved repeatedly on sampling occasions, at the beginning (September-October) and the end (March) of the winter. Sampling covered several weeks each, and a multiple mark-recapture analysis of the resightings also confirmed that the probability of individuals escaping detection within the study area was negligible. The analysis of survival consequences of delayed dispersal is based on a comparison between the survival of retained offspring and immigrants. Immigrants in their first year cannot have delayed dispersal, whereas older immigrants may have been retained offspring elsewhere. We therefore analyzed survival for first-year birds separately from older birds. Age determination was based on the shape of rectrices. Comparison of rectrices from retained offspring collected in their first year and subsequent years (n = 29) showed that first-year birds are easily distinguished from older birds by the shape of rectrices, which changes from being rounded to a more angular shape at first molt.
Survival is a binary response variable, and we therefore tested correlates with logistic regression in a model where survival was entered as survive = 1 and disappear = 0. The analyses controlled for several discrete nominal variables that function as identifiers without any numerical significance (territory, year, and offspring sex). They were therefore entered as categorical variables, and we analyzed the logistic regression with SAS Proc CATMOD (SAS Institute, Cary, North Carolina), which is designed to handle such variables.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Survival of retained offspring and immigrants
Local winter survival was higher among retained offspring in their first year than among immigrants. First-year birds were significantly more likely to be resighted when they spent the winter with their parents (Table 1). The overall probability of a retained first-year offspring present in October being resighted in March was 0.73 (pooled data for all years). In comparison, the corresponding probability of first-year immigrants being resighted was only 0.36. However, we found no enhanced survival from associating with the parents beyond the age of 1 year. Out of 38 retained offspring older than 1 year (actually offspring years ; multiple years for individuals), 12 birds (proportion 0.32) disappeared in October-March, whereas the corresponding losses among immigrants were 5 out of 20 birds (proportion 0.25 ; X2 = 0.048, df = 2, ns).
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Winter survival of parents, offspring, and immigrants
Retained offspring associate with their parents, and enhanced winter
survival could be due to fitness gained through active help and/or concession
of resources by the parents (parental facilitation). Alternatively, the lower
survival of immigrant group members in winter relative to retained offspring
may merely reflect phenotypic quality. To decouple offspring phenotypic
quality from the role of parental facilitation, we made a separate analysis in
which we compared winter survival among retained offspring in their first year
relative to survival of their parents (none, one, or two surviving parents
after winter). This analysis was confined to territories where the pair had
bred successfully. Retained offspring in their first year are possible only in
such groups. We further controlled for group size, territory, year, and
offspring sex. The latter three variables were entered as categorical
variables (see Methods above). The logistic regression analysis revealed a
significant positive effect only for the number of surviving parents for the
first-year survival of retained offspring between October and March
(Table 2). The probability of
the retained first-year birds to be reobserved in March was 0.92 if both
parents had survived (n = 33), 0.5 if only one parent had survived
(n = 12), and 0 if both parents had disappeared (n = 4). The
variables season and territory were included in the test to control for
confounding effects. They were entered in the analysis as fixed variables, as
they were not randomly drawn. With this design we can control for variation in
survival between our territories and between years during the specific period
of our study and exclude that such effects confound the influence of parental
facilitation on the survival of retained offspring. However, we cannot exclude
year or territory effects in general from their lack of significant influence
on survival.
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The higher survival of retained offspring associating with parents that have survived and therefore were present in winter is consistent with the hypothesis that parental behavior promotes offspring survival. In contrast, we found no support for the more parsimonious explanation that habitat quality of a shared environment accounts for the improved survival of retained offspring in territories where the survival of parents was high. If survival rates of parents and retained offspring were linked through the quality of a shared environment, then immigrants also should have survived better in territories with high parent survival. However, no such relationship was found in a logistic regression testing how winter survival among unrelated immigrants (n = 40 individuals) was related to the number of surviving parents breeding in the territory the previous spring (Table 2). In the logistic regression we controlled for group size, year, territory, and sex in the same manner as in the test of survival among retained offspring.
Our rejection of the alternative hypothesis of a habitat effect is based on
failure to falsify the null hypothesis that winter survival among immigrants
was unrelated to survival among parents. A correlation would have been
expected if differences in survival between territories reflected habitat
quality. However, there is a risk that a test fails to falsify the null
hypothesis, even when it is untrue, because of low power (type II error). To
assess the risk of committing a type II error in rejecting the alternative
hypothesis of a habitat effect for survival, we performed a power analysis
(Cohen, 1988). Given that
survival rates reflect habitat quality and not kinship, our alternative
hypothesis states that immigrants should gain from staying on good territories
just as retained offspring do. The power analysis shows that the risk of
committing a type II error was small when we failed to falsify the null
hypothesis, given there is an effect size
(Cohen, 1988) of the magnitude
observed for retained offspring. The power analysis showed that our test then
had a probability (power) above 0.99 of rejecting the null hypothesis at the
5% significance level (two-tailed) with our sample size. Hence, our rejection
of the alternative hypothesis that the habitat quality of territories explains
the correlation between survival rates of retained offspring and their parents
does not suffer from low power of the test. Further, we found no evidence
suggesting that the dispersal-related differences in survival should reflect
sex because we found no deviation from a balanced sex ratio among either
retained offspring or immigrants.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Lack of vacant habitat has been the focus of numerous attempts to explain delayed dispersal (e.g., Emlen, 1982
; Kokko and Johnstone,
1999 ; Selander,
1964 ; Stacey,
1979). Selander
(1964) recognized a link
between delayed dispersal and ecological constraints on access to vacant space
for species such as the Siberian jay living in territorial systems. Field data
have corroborated Selander's suggestion to the extent that retained offspring
can be induced to disperse by creating openings through removals of
territorial birds (Komdeur,
1992 ; Pruett-Jones and Lewis,
1990). Siberian jay offspring improve their survival by being
retained, but access to the natal territory itself is not sufficient to
account for this benefit of delayed dispersal. It is the presence of parents
that appears to entail a direct fitness gain to the offspring in the form of
enhanced survival.
Selander's (1964) claim
that ecological constraints on space force offspring to delay dispersal was
based on the given range. Considering the potential for species to use areas
beyond their current range, Brown
(1969) later recognized that it
was not the lack of space itself, but its suitability, which was the issue.
This emphasis on the suitability of a vacant habitat is more than just
semantics. With Brown's (1969)
approach, dispersal will change from being the best option, although sometimes
constrained for lack of space, into a trade-off decision that takes into
account potential benefits to be gained in the natal territory.
Implicit in Brown's concept of "suitability" is that it
incorporates benefits available in the natal territory, later coined
"benefits of philopatry"
(Stacey and Ligon, 1991), as
part of a cost-benefit trade-off between remaining on the natal territory and
dispersing elsewhere. It is obvious from the lack of cooperative breeding in
the Siberian jay that this benefit of philopatry does not necessarily accrue
in the form of indirect fitness gains of raising nondescendent offspring.
Attempts to identify other benefits of philopatry have focused on the habitat.
The natal territory may represent a better environment when the decline in
quality toward vacant habitat is steep
(Koenig and Pitelka, 1981), as
may be the case when there are large local variations in habitat quality
(Stacey and Ligon, 1991).
Remaining on the natal territory, despite having to share it, could then offer
better prospects compared to dispersing to a vacant habitat of poor quality.
However, our data demonstrate a direct fitness gain for retained offspring
from associating with tolerant parents. This is a true benefit of philopatry,
as offspring, by definition, can gain it only by associating with their
parents, which in territorial species implies remaining in the natal
territory.
Our evidence for a direct fitness benefit from associating with the parents
is not confined to survival consequences. The behavior of Siberian jay parents
provides a proximate mechanism that can account for this fitness gain.
Nepotistic parents allow their retained offspring access to food in winter
that they deny to immigrants (Ekman et
al., 1994). There appears to be a genuine survival gain for
retained offspring from such nepotistic behavior by Siberian jay parents,
which confirms the suggestion of Brown and
Brown (1984) that retained offspring can gain in direct fitness
from prolonged care by the parents (parental facilitation). Neither phenotypic
quality nor habitat quality appears to be sufficient to account for survival
differences among first-year birds. First, survival is higher for retained
offspring whose parents also survive the winter. This parent-related
difference in survival, within the group of retained offspring, excludes that
the higher survival of such offspring solely reflects higher phenotypic
quality. Second, this link between the survival of parents and retained
offspring is more than just the product of a shared environment. There is no
corresponding link between the survival of parent birds and unrelated
immigrants. Such a correlation would be expected if the link between the
survival of parents and retained offspring were simply a by-product of a
shared environment.
Parental facilitation may be more than just an additional factor promoting
delayed dispersal. Such benefits of remaining in the natal territory offer an
alternative to the ecological constraints approach, which fails as a general
explanation for delayed dispersal. Many species do not postpone dispersal,
even though access to space is just as constrained for them as for species
with delayed dispersal (Brown,
1969 ; Heinsohn et al.,
1990). There is, for instance, a thoroughly documented lack of
delayed dispersal among several group-living tits (Parus sp.) despite
experimentally verified ecological constraints on space
(Ekman, 1989 ;
Matthysen, 1990). This lack of
consistency can at best be reconciled by the fact that ecological constraints
on access to habitat can be a necessary, but not a sufficient, factor for
delayed dispersal. Some other factors appear to be required. The offspring
could be constrained to stay in the natal territory by costs of dispersal
(Emlen, 1982), but benefits
gained through parental facilitation on the natal territory could also provide
a sufficient benefit so that the offspring do better when they forgo dispersal
(Brown and Brown, 1984).
Parental tolerance is one conceivable mechanism providing parental
facilitation, and in the Siberian jay such tolerance is associated with a
direct fitness gain for retained offspring. This incentive to delay dispersal
would be absent without parental tolerance. However, if delayed dispersal
requires parental tolerance, then plasticity in parental facilitation may
account for the lack of consistency in how dispersal responds to ecological
constraints on space. Theory predicts that parents should be generous and
concede resources to retained offspring only when their own prospects are good
(Ekman and Rosander, 1992 ;
McNamara and Houston, 1994 ;
Taylor, 1988). The prediction
that parents should not always be tolerant seems robust, as it has been
obtained with different modeling techniques. The models predict parental
nepotism at high parental survival. Parental nepotism could thus be a
proximate mechanism that acts as the link between parental longevity and
delayed dispersal indicated by comparative analyses and stated as the
"life-history hypothesis"
(Arnold and Owens, 1998 ;
Brown, 1987).
Although parental facilitation, providing direct fitness gains to retained
offspring, has the potential to account for delayed dispersal, its generality
remains to be explored. However, the Siberian jay is not the only species
where parents show nepotistic behavior. Belding's ground squirrel
(Spermophilus beldingi) engages in nepotistic alarm calling (Sherman,
1977,
1981), and Scott (1982)
discovered that parents protect their offspring in feeding winter flocks of
the Bewick Swan (Cygnus bewickii). Parental concession of resources
to retained offspring has also been found in the Mexican jay (Aphelocoma
ultramarina) (Barkan et al.,
1986) and in other studies of the Siberian jay
(Ekman et al., 1994). For the
Siberian jay, we here confirm direct fitness benefits to retained offspring as
a result of enhanced survival from associating with the parents during the
winter. Tolerance, forming the behavioral background for parental
facilitation, could be a prolonged brood care
(Ekman and Rosander, 1992 ;
Ekman et al., 1994) promoted
by an immediate and direct fitness gain when offspring survival is enhanced.
From this aspect, parental facilitation differs from the indirect fitness
gains of cooperative breeding, which are delayed until after the offspring
have postponed dispersal. Hence, the fitness gain from cooperative breeding
requires the prior existence of coherent families, and for this reason
cooperative breeding has been considered to be a consequence of family units
being formed through delayed dispersal rather than its cause
(Brown, 1987). Relieved from
the requirement of first accounting for how coherent families are formed,
parental facilitation could be a cause of delayed dispersal rather than its
consequence.
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ACKNOWLEDGEMENTS |
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This study would not have been possible without all the support and advice from Folke and Maj Lindgren. We are most grateful for the hospitability of our host couple Nils-Gunnar and Ingrid Persson, who provided a perfect base at "Lappugglan." Staffan Ulfstrand read the manuscript and made many useful suggestions. We thank Bohdan Sklepkovych for field companionship. The study was supported by the Swedish Natural Science Research Council and Stiftelsen Olle Engkvist Byggmästare (to J.E.)
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