Geographical variation in reproductive ageing patterns and life‐history strategy of a short‐lived passerine bird

We investigated differences in ageing patterns in three measures of breeding performance in populations of barn swallows Hirundo rustica L. from Spain and Denmark differing in breeding latitude and hence migration distance and duration of the breeding season. We found differences in ageing patterns between populations. Generally, young (i.e. yearling) and old females (i.e. ≥ 5 years of age) laid their first eggs later and produced smaller clutches than middle‐aged females (i.e. 2–4 years of age) in both populations. The southernmost population (i.e. Spanish) showing the shorter migratory distance experienced a greater within‐individual increase in timing of breeding and clutch size in early life and a greater within‐individual decrease in laying date but not in clutch size during senescence compared with the northernmost population (i.e. Danish). We also found that the number of fledglings produced annually was related to the age of the two members of the breeding pairs with pairs composed of young and old females performing less well than breeding pairs composed of middle‐aged females. We did not find reproductive senescence for the age of the male while controlling for the age of the female on the number of fledglings produced annually by the breeding pair. Differential survival between individuals did not explain age effects on laying date or annual clutch size in neither population. However, the increase in the number of fledglings produced annually with age was partly explained by the disappearance of poor‐quality members of the pairs, mainly poor‐quality males. Age‐related breeding success (i.e. number of fledglings) was similar for barn swallows from Spain and Denmark. Therefore, the study of ageing patterns and life‐history strategies in free‐ranging animals from more than a single population can throw new light on life‐history theory, population dynamics and evolutionary studies of senescence.     

Density-dependent selection on horn phenotype in Soay sheep.

In the naturally regulated population of Soay sheep (Ovis aries L.) on Hirta (St Kilda), 12% of males and 24% of females have scurred horns. This trait reduces the competitive ability of males in the mating season but is associated with higher overwinter survival rates in both sexes (Moorcroft et al. 1996). In this paper, we show that scurred females also show higher conception rates and weaning rates than non-scurred ones, and that these differences are associated with heavier maternal body weight as well as higher birth weights of offspring. Selection pressures favouring scurredness in females vary with population density and are generally more pronounced among younger animals than adults. We discuss these results with reference to recent suggestions that temporal fluctuation in selection pressures may help to maintain genetic diversity within populations. We suggest that selection against scurredness operating through male mating success is opposed by selection favouring this trait, operating through the breeding success of females and the survival of both sexes.     

Differential reproductive success and facultative adjustment of sex ratios among captive female bonnet macaques (Macaca radiata)

In a group of captive bonnet macaques (Macaca radiata) housed at the California Primate Research Center, variance in reproductive success among females is primarily due to differences in infant survival. The infants of low-ranking females have a smaller probability of surviving to 6 months of age than do the infants of other females. In addition, the juvenile daughters of low-ranking females are more vulnerable to behaviourally induced mortality than are other immature animals. Observational evidence indicates that this mortality is the direct result of aggression by unrelated, higherranking adult females. Although infants' sex is not consistently related to survival, yearly fluctuations in the survival of male and female infants are reflected in the extent and direction of the skew in the sex ratio of offspring produced the following year. Years in which the highest proportion of male infants survive are followed by years in which the largest proportions of the birth cohorts are composed of males, and years in which the largest proportions of females survive are followed by years in which the largest proportions of birth cohorts are composed of females. For infant females the probability of surviving is reduced when a substantial proportion of the birth cohort is composed of females. The same pattern is evident among the sons of low-ranking females. The adaptive significance of behaviourally induced variation in reproductive success among females is considered in relation to these data.     

Social bonds enhance reproductive success in male macaques

For animals living in mixed-sex social groups, females who form strong social bonds with other females live longer and have higher offspring survival [1-3]. These bonds are highly nepotistic, but sometimes strong bonds may also occur between unrelated females if kin are rare [2, 3] and even among postdispersal unrelated females in chimpanzees and horses [4, 5]. Because of fundamental differences between the resources that limit reproductive success in females (food and safety) and males (fertilizations), it has been predicted that bonding among males should be rare and found only for kin and among philopatric males [6] like chimpanzees [7-9]. We studied social bonds among dispersing male Assamese macaques (Macaca assamensis) to see whether males in multimale groups form differentiated social bonds and whether and how males derive fitness benefits from close bonds. We found that strong bonds were linked to coalition formation, which in turn predicted future social dominance, which influenced paternity success. The strength of males' social bonds was directly linked to the number of offspring they sired. Our results show that differentiated social relationships exert an important influence on the breeding success of both sexes that transcends contrasts in relatedness.     

Effects of spatio-temporal variation in food supply on red squirrel Sciurus vulgaris body size and body mass and its consequences for some fitness components

Food availability is likely to influence body condition and, in turn, fitness. The intensity of this response may vary between populations of the same species on a small spatial and temporal scale. We used 5 yr of data from 6 Eurasian red squirrel Sciurus vulgaris populations from the southern Alps to explore differences in body size and body mass among neighbouring populations, in relation to habitat type and variation in food supply. We also investigated sexual dimorphism in these traits and whether phenotypic variation affects local survival and female reproductive success. Mean hind foot length, a measure of body size, did not differ between sexes but differed between areas. Seasonal variation in body mass was small with no evidence for fattening in autumn. Females were slightly heavier than males, but this difference was largely explained by mass gain of females during reproduction. The size of conifer seed crops, the major food supply, varied strongly over years and between habitats, but this variation corresponded only weakly with autumn body mass. Differences in size and mass between populations were partially explained by habitat‐related differences in body size and variability of seed‐crops, suggesting differential selection for smaller squirrels in spruce‐larch forests against selection for larger and heavier animals in mixed broadleaves and conifer forests and in Scots pine forests with more stable seed production. The probability of reproduction by females increased with body mass, but varied strongly between habitats and years, with more females reproducing in years with rich seed‐crops. In both sexes, body mass positively affected probability of settlement and length of residency. Our results suggest that in temporally variable environments that differ in overall amount of food resources, individual variation in body mass is related to habitat type, and that having a relatively high body mass, within each population, positively affects male and female settlement success and local survival, and female reproductive success.     

No evidence of inbreeding avoidance despite demonstrated survival costs in a polygynous rodent

Individuals are generally predicted to avoid inbreeding because of detrimental fitness effects. However, several recent studies have shown that limited inbreeding is tolerated by some vertebrate species. Here, we examine the costs and benefits of inbreeding in a largely polygynous rodent, the yellow-bellied marmot (Marmota flaviventris). We use a pedigree constructed from 8 years of genetic data to determine the relatedness of all marmots in our study population and examine offspring survival, annual male reproductive success, relatedness between breeding pairs and the effects of group composition on likelihood of male reproduction to assess inbreeding in this species. We found decreased survival in inbred offspring, but equal net reproductive success among males that inbred and those that avoided it. Relatedness between breeding pairs was greater than that expected by chance, indicating that marmots do not appear to avoid breeding with relatives. Further, male marmots do not avoid inbreeding: males mate with equal frequency in groups composed of both related and unrelated females and in groups composed of only female relatives. Our results demonstrate that inbreeding can be tolerated in a polygynous species if the reproductive costs of inbreeding are low and individuals that mate indiscriminately do not suffer decreased reproductive success.     

Understanding contributions of cohort effects to growth rates of fluctuating populations

1. Understanding contributions of cohort effects to variation in population growth of fluctuating populations is of great interest in evolutionary biology and may be critical in contributing towards wildlife and conservation management. Cohort-specific contributions to population growth can be evaluated using age-specific matrix models and associated elasticity analyses. 2. We developed age-specific matrix models for naturally fluctuating populations of stoats Mustela erminea in New Zealand beech forests. Dynamics and productivity of stoat populations in this environment are related to the 3-5 year masting cycle of beech trees and consequent effects on the abundance of rodents. 3. The finite rate of increase (lambda) of stoat populations in New Zealand beech forests varied substantially, from 1.98 during seedfall years to 0.58 during post-seedfall years. Predicted mean growth rates for stoat populations in continuous 3-, 4- or 5-year cycles are 0.85, 1.00 and 1.13. The variation in population growth was a consequence of high reproductive success of females during seedfall years combined with low survival and fertility of females of the post-seedfall cohort. 4. Variation in population growth was consistently more sensitive to changes in survival rates both when each matrix was evaluated in isolation and when matrices were linked into cycles. Relative contributions to variation in population growth from survival and fertility, especially in 0-1-year-old stoats, also depend on the year of the cycle and the number of transitional years before a new cycle is initiated. 5. Consequently, management strategies aimed at reducing stoat populations that may be best during one phase of the beech seedfall cycle may not be the most efficient during other phases of the cycle. We suggest that management strategies based on elasticities of vital rates need to consider how population growth rates vary so as to meet appropriate economic and conservation targets.     

Infanticide in captive collared peccaries (Tayassu tajacu)

Housing animals in groups that are typical of the normal social system can be very important to the success of captive husbandry. Collared peccaries normally live in mixed-sex territorial groups of 13–40 individuals with little evidence for movement between groups. We recorded 21–30 cases of infanticide in a captive group composed of animals from several wild groups. In five observed cases, the attacker was a female unrelated to the mother, but related females helped defend neonates. Victims did not differ in sex ratio or size from individuals that survived. We recommend that peccaries be housed as related female lineages.     

Establishment success and resulting fitness consequences for vole dispersers

Dispersal is one of the most important, yet least understood phenomena of evolutionary ecology. Triggers and consequences of dispersal are difficult to study in natural populations since dispersers can typically only be identified a posteriori. Therefore, a lot of work on dispersal is either of a theoretical nature or based on anecdotal observation. This is especially true for cryptic species such as small mammals. We conducted an experiment on the common vole, Microtus arvalis, in semi‐natural enclosures and investigated the spatial and genetic establishment success of residents and dispersers in their natal and new populations. Our study uses genetic data on the reproductive success of 1255 individuals to measure the fitness trajectories of the residents and dispersing individuals. In agreement with past studies, we found that dispersal was highly male‐biased, and was most probably induced by the agonistic encounters with conspecifics, suggesting it could act as an inbreeding avoidance mechanism. There was low breeding success of dispersers into new populations. Although nearly 26% of identified dispersers reproduced in their natal populations, only seven percent reproduced in the new populations. Settlement appeared to be a pre‐requisite for reproduction in both sexes, and animals that did not spatially settle into a new population dispersed again, usually on the same day of immigration. In the event that dispersers reproduced in the new population, they did so at relatively low population densities. We also found age‐related differences between the sexes in breeding success, and male dispersers that subsequently established in the new population were young individuals that had not reproduced in their natal population, whereas successful females had already reproduced in their natal population. In conclusion, with our detailed field data on establishment and substantial parentage assignments to understand breeding success, we were able to gain an insight into the fitness of dispersers, and how the two sexes optimise their fitness. Taken together, our results help to further understand the relative advantages and costs of dispersal in the common vole.     

Female choice and the relatedness of mates in the guppy (<Emphasis Type="Italic">Poecilia reticulata</Emphasis>)

Several studies suggest that females may offset the costs of genetic incompatibility by exercising pre-copulatory or post-copulatory mate choice to bias paternity toward more compatible males. One source of genetic incompatibility is the degree of relatedness among mates; unrelated males are expected to be genetically more compatible with a female than her relatives. To address this idea, we investigated the potential for inbreeding depression and paternity biasing mechanisms (pre- and post-copulatory) of inbreeding avoidance in the guppy, Poecilia reticulata. Inbreeding resulted in a reduction in offspring number and quality. Females mated to siblings gave birth to significantly fewer offspring compared to females mated to non-siblings and inbred male offspring took longer to reach sexual maturity. There was no evidence of inbreeding avoidance in pre-copulatory behaviors of females or males. Sexual responsiveness of females to courting males and the number of sexual behaviors males directed at females did not decrease as a function of the relatedness of the two individuals. We also tested whether female guppies can use post-copulatory mechanisms to bias sperm usage toward unrelated males by comparing the number of offspring produced by females mated to two of their siblings (SS), two males unrelated to the female (NN), or to one unrelated male and a sibling male (NS). We found that NS females produced a number of offspring not significantly different than what would be expected if fertilization success were halfway between completely outbreeding (NN) and completely inbreeding (SS) females. This suggests that there is no significant improvement in the number of offspring produced by females mating to both related and unrelated males, relative to that which would be expected if sperm from both males were used equally. Our results suggest that female guppies do not discriminate against closely related males or their sperm.     

Double-brooding and annual breeding success of great tits in urban and forest habitats

Urban areas differ from natural habitats in several environmental features that influence the characteristics of animals living there. For example, birds often start breeding seasonally earlier and fledge fewer offspring per brood in cities than in natural habitats. However, longer breeding seasons in cities may increase the frequency of double-brooding in urban compared with nonurban populations, thus potentially increasing urban birds’ annual reproductive output and resulting in lower habitat difference in reproductive success than estimated by studies focusing on first clutches only. In this study, we investigated 2 urban and 2 forests great tit Parus major populations from 2013 to 2019. We compared the probability of double-brooding and the total number of annually fledged chicks per female between urban and forest habitats, while controlling for the effects of potentially confounding variables. There was a trend for a higher probability of double-brooding in urban (44% of females) than in forest populations (36%), although this was not consistent between the 2 urban sites. Females produced significantly fewer fledglings annually in the cities than in the forest sites, and this difference was present both within single- and double-brooded females. Furthermore, double-brooded urban females produced a similar number of fledglings per season as single-brooded forest females. These results indicate that double-brooding increases the reproductive success of female great tits in both habitats, but urban females cannot effectively compensate in this way for their lower reproductive output per brood. However, other mechanisms like increased post-fledging survival can mitigate habitat differences in reproductive success.     

MEASURING SELECTION ON A POPULATION OF DAMSELFLIES WITH A MANIPULATED PHENOTYPE

The estimation of the relationship between phenotype and fitness in natural populations is constrained by the distribution of phenotypes available for selection to act on. Because selection is blind to the underlying genotype, a more variable phenotypic distribution created by using environmental effects can be used to enhance the power of a selection study. I measured selection on a population of adult damselflies (Enallagma boreale) whose phenotype had been modified by raising the larvae under various levels of food availability and density. Selection on body size (combination of skeletal and mass at emergence) and date of emergence was estimated in two consecutive episodes. The first episode was survival from emergence to sexual maturity and the second was reproductive success after attaining sexual maturity. Female survival to sexual maturity was lower, and therefore opportunity for selection greater, than males in both years. Opportunity for selection due to reproductive success was greater for males. The total opportunity for selection was greater for males one year and for females the other. Survival to sexual maturity was related to mass gain between emergence and sexual maturity. Females gained more mass and survived less well than males in both years but there was no linear relationship between size at emergence and survival for females in either year. However, females in the tails of the phenotype distribution were less likely to survive than those near the mean. In contrast, small males consistently gained more mass than large males and survived less well in one year. There was significant selection on timing of emergence in both years, but the direction of selection changed due to differences in weather; early emerging females were more successful one year and late emerging males and females the other. The number of clutches laid by females was independent of body size. Because the resources used to produce eggs are acquired after emergence and this was independent of size at emergence, female fitness did not increase with size. Small males may have had lower survival to sexual maturity but they had higher mating success than large males. Resources acquired prior to sexual maturity are essential for reproductive success and may in some species alter their success in inter- and intrasexual competition. Therefore, ignoring the mortality associated with resource acquisition will give an incomplete and potentially misleading picture of selection on the phenotype.     

Reproductive success and failure: the role of winter body mass in reproductive allocation in Norwegian moose

A life history strategy that favours somatic growth over reproduction is well known for long-lived iteroparous species, especially in unpredictable environments. Risk-sensitive female reproductive allocation can be achieved by a reduced reproductive effort at conception, or the subsequent adjustment of investment during gestation or lactation in response to unexpected environmental conditions or resource availability. We investigated the relative importance of reduced investment at conception compared with later in the reproductive cycle (i.e. prenatal, perinatal or neonatal mortality) in explaining reproductive failure in two high-density moose (Alces alces) populations in southern Norway. We followed 65 multiparous, global positioning system (GPS)-collared females throughout the reproductive cycle and focused on the role of maternal nutrition during gestation in determining reproductive success using a quasi-experimental approach to manipulate winter forage availability. Pregnancy rates in early winter were normal (≥0.8) in all years while spring calving rates ranged from 0.4 to 0.83, with prenatal mortality accounting for most of the difference. Further losses over summer reduced autumn recruitment rates to 0.23–0.69, despite negligible predation. Over-winter mass loss explained variation in both spring calving and autumn recruitment success better than absolute body mass in early or late winter. Although pregnancy was related to body mass in early winter, overall reproductive success was unrelated to pre-winter body condition. We therefore concluded that reproductive success was limited by winter nutritional conditions. However, we could not determine whether the observed reproductive allocation adjustment was a bet-hedging strategy to maximise reproduction without compromising survival or whether females were simply unable to invest more resources in their offspring.     

Selection for synchronous breeding in the European starling

Colonial birds often demonstrate considerable breeding synchrony. In southern Sweden the semi-colonial European starling initiated the vast majority of clutches within one week. Laying dates were positively skewed so that many birds initiated clutches at similar dates early in the season. Breeding was further synchronised by a particularly strong clutch-size reduction equivalent to one third of an egg per day during the first part of the breeding season. The decline in clutch size with season also held true for separate age-classes of females, for individual females laying at different times at different years and for individual females laying at different times the same year. Trends in breeding success during nestling rearing were unlikely to explain the high degree of breeding synchrony or the seasonal decline in clutch size; nestling survival and growth were weakly related or unrelated to reproductive timing. In contrast recruitment success of fledged offspring declined sharply with season. Even within the synchronous laying period, defined as clutches initiated during the first week each year, local recruitment success declined. It is suggested that the early seasonal decline is caused by selection for synchronous fledging permitting the immediate formation of flocks after fledging, whereas the late seasonal trends may be caused by either population differences in female quality or deteriorating conditions for raising young.     

Genetic variation among females affects paternity in a dioecious plant

Flowering plants rely on vectors for pollen transfer, and cannot choose their mates. Although recipient plants are unable to choose which pollen they receive, post‐pollination selection (acting pre‐ or post‐zygotically) may modify the outcome of pollination. Here we show that genetic variation among pollen recipients can predict the outcome of pollen competition (seed paternity) in the dioecious white campion. To investigate whether genetic variation among pollen recipients affects paternity, we applied the same pollen mixture from two males to three females, two of which full sisters and the third one chosen at random (unrelated). To control for maternal environmental effects, the plants used for these crosses were greenhouse‐reared F₁. We replicated this in two populations, for a total of 51 crosses, and genotyped a total of 772 offspring to assign paternity. If genetic variation affects paternity, we expected greater similarity of paternity success of the focal male with the sisters, compared to the unrelated female. Paternity of the focal male was significantly more repeatable over sisters, compared to repeatability over the mean of sisters and the unrelated females. When populations were analyzed separately, this was significant in one of the two populations. Paternity was not significantly correlated with stigma size. This provides evidence that in at least one population, genetic variation among individual plants influences the donors’ paternity success, as assessed through genetic analysis of the seedling. Since due to gravity‐dispersed seeds natural patches may often consist of related plants, the observed effect may contribute to variation in male reproductive success.     

Survival rates in a small hibernator,the edible dormouse: a comparison across Europe

Understanding how local environmental factors lead to temporal variability of vital rates and to plasticity of life history tactics is one of the central questions in population ecology. We used long‐term capture‐recapture data from five populations of a small hibernating rodent, the edible dormouse Glis glis, collected over a large geographical range across Europe, to determine and analyze both seasonal patterns of local survival and their relation to reproductive activity. In all populations studied, survival was lowest in early summer, higher in late summer and highest during hibernation in winter. In reproductive years survival was always lower than in non‐reproductive years, and females had higher survival rates than males. Very high survival rates during winter indicate that edible dormice rarely die from starvation due to insufficient energy reserves during the hibernation period. Increased mortality in early summer was most likely caused by high predation risk and unmet energy demands. Those effects have probably an even stronger impact in reproductive years, in which dormice were more active. Although these patterns could be found in all areas, there were also considerable differences in average survival rates, with resulting differences in mean lifetime reproductive success between populations. Our results suggest that edible dormice have adapted their life history strategies to maximize lifetime reproductive success depending on the area specific frequency of seeding events of trees producing energy‐rich seeds.     

The effects and limits of territoriality on population regulation in grey red-backed voles,Clethrionomys rufocanus bedfordiae

The effects of breeding territoriality on the stability of grey red-backed vole (Clethrionomys rufocanus bedfordiae) populations were investigated on a control grid and a grid on which the voles were fed, in an outdoor enclosure in Hokkaido, Japan. Vole populations were monitored by live trapping from 1984 to 1986: (1) Population density was 2–7 times greater on the experimental grid to which food was added than on the control grid. Reproductive output was more closely associated with the difference in density between grids than survival or dispersal (immigration and emigration) rates. (2) The number of adult females and pregnancy rate of the experimental population were significantly greater than those of the control one. The difference in the number of adult females between the populations was greater than that in pregnancy rate. (3) The proportion of successful litters and the number of weanlings per litter were not significantly different between the control and experimental population. (4) Adult females held territories on both the control and experimental grid; they were spaced out more than would be expected from random occupation. The territories overlapped more on the experimental grid than on the control grid. (5) Mean territory size of adult females on the experimental grid was about half of that on the control grid. The territory size was correlated negatively with population density. (6) The proportion of trap sites that were used by adult females was significantly greater on the experimental grid than on the control grid. This suggests that adult females on the experimental grid used the area more extensively. This factor, in association with territory size and overlapping of territory, was also important in causing the difference in the number of adult females between the grids. (7) These results call into question the hypothesis that territoriality stabilizes the density in populations of Clethrionomys.     

Maintenance of gynodioecy in Wurmbea biglandulosa (Colchicaceae): gender differences in seed production and progeny success

 In gynodioecious species, females contribute genes to future generations only through ovules, and to persist in populations they must have a compensatory advantage compared with hermaphrodites that reproduce via ovules and pollen. This compensation can result from greater fecundity and/or superior success of progeny from females. We examined differences in seed production and progeny success between females and hermaphrodites in the geophyte Wurmbea biglandulosa to explain the maintenance of females. Females produced more ovuliferous flowers and had more ovules per flower than did hermaphrodites but this did not necessarily result in greater fecundity, in part because seed production of females was pollen-limited. Over four years in one population, open-pollinated females produced 1.32 more seeds than open-pollinated hermaphrodites (range 1.09–1.63). In two other populations examined for one year only females produced 1.07 and 0.79 as many seeds as hermaphrodites. Seed production of open-pollinated females and hermaphrodites was only 55% and 73% that of cross-pollinated plants, respectively, indicating that both genders were pollen-limited but females more so than hermaphrodites. Open-pollinated seeds from females were 1.18–1.27 times more likely to germinate than seeds from hermaphrodites. No gender differences existed in seedling growth or survival. Hermaphrodites were self-compatible, but selfed seed set was only 80% that of crossed seed set. Crossed seed set of females and hermaphrodites did not differ. Assuming nuclear control of male sterility, relative female fitness is insufficient to maintain females at their current frequencies of 17%, and substantial female fitness advantages at later life-cycle stages are required. Received May 4, 2001 Accepted February 25, 2002     

Comparative survival and recovery of Ross's and lesser snow geese from Canada's central arctic

Large increases in several populations of North American arctic geese have resulted in ecosystem-level effects from associated herbivory. Consequently, some breeding populations have shown density dependence in recruitment through declines in food availability. Differences in population trajectories of lesser snow geese (Chen caerulescens caerulescens; hereafter snow geese) and Ross's geese (C. rossii) breeding in mixed-species colonies south of Queen Maud Gulf (QMG), in Canada's central arctic, suggest that density dependence may be limiting snow goose populations. Specifically, long-term declines in age ratios (immature:adult) of harvested snow geese may have resulted from declines in juvenile survival. Thus, we focused on juvenile (first-year) survival of snow and Ross's geese in relation to timing of reproduction (annual mean nest initiation date) and late summer weather. We banded Ross's and snow geese from 1991 to 2008 in the QMG Migratory Bird Sanctuary. We used age-structured mark-recapture models to estimate annual survival rates for adults and juveniles from recoveries of dead birds. Consistent with life history differences, juvenile snow geese survived at rates higher than juvenile Ross's geese. Juvenile survival of both species also was lower in late seasons, but was unrelated to arctic weather measured during a 17-day period after banding. We found no evidence of density dependence (i.e., a decline in juvenile survival over time) in either species. We also found no interspecific differences in age-specific hunting vulnerability, though juveniles were more vulnerable than adults in both species, as expected. Thus, interspecific differences in survival were unrelated to harvest. Lower survival of juvenile Ross's geese may result from natural migration mortality related to smaller body size (e.g., greater susceptibility to inclement weather or predation) compared to juvenile snow geese. Despite lower first-year survival, recruitment by Ross's geese may still be greater than that by snow geese because of earlier sexual maturity, greater breeding propensity, and higher nest success by Ross's geese. © 2012 The Wildlife Society.     

Demographic balancing of migrant and resident elk in a partially migratory population through forage–predation tradeoffs

Partial migration is widespread in ungulates, yet few studies have assessed demographic mechanisms for how these alternative strategies are maintained in populations. Over the past two decades the number of resident individuals of the Ya Ha Tinda elk herd near Banff National Park has been increasing proportionally despite an overall population decline. We compared demographic rates of migrant and resident elk to test for demographic mechanisms partial migration. We determined adult female survival for 132 elk, pregnancy rates for 150 female elk, and elk calf survival for 79 calves. Population vital rates were combined in Leslie‐matrix models to estimate demographic fitness, which we defined as the migration strategy‐specific population growth rate. We also tested for differences in factors influencing risk of mortality between migratory strategies for adult females using Cox‐proportional hazards regression and time‐varying covariates of exposure to forage biomass, wolf predation risk, and group size. Despite higher pregnancy rates and winter calf weights associated with higher forage quality, survival of migrant adult females and calves were lower than resident elk. Resident elk traded high quality food to reduce predation risk by selecting areas close to human activity, and by living in group sizes 20% larger than migrants. Thus, residents experienced higher adult female survival and calf survival, but lower pregnancy and calf weights. Cause‐specific mortality of migrants was dominated by wolf and grizzly bear mortality, whereas resident mortality was dominated by human hunting. Demographic differences translated into slightly higher (2–3%), but non‐significant, resident population growth rate compared to migrant elk, suggesting demographic balancing between resident strategies during our study. Despite statistical equivalence, our results are also consistent with slow long‐term declines in migrants because of high predation because of higher wolf‐caused mortality in migrants. These results emphasize that migrants and residents will make different tradeoffs between forage and risk may affect the demographic balance of partially migratory populations, which may explain recent declines in migratory behavior in many ungulate populations around the world.