Delayed dispersal and the costs and benefits of different routes to independent breeding in a cooperatively breeding bird

Why sexually mature individuals stay in groups as nonreproductive subordinates is central to the evolution of sociality and cooperative breeding. To understand such delayed dispersal, its costs and benefits need to be compared with those of permanently leaving to float through the population. However, comprehensive comparisons, especially regarding differences in future breeding opportunities, are rare. Moreover, extraterritorial prospecting by philopatric individuals has generally been ignored, even though the factors underlying this route to independent breeding may differ from those of strict philopatry or floating. We use a comprehensive predictive framework to explore how various costs, benefits and intrinsic, environmental and social factors explain philopatry, prospecting, and floating in Seychelles warblers (Acrocephalus sechellensis). Not only floaters more likely obtained an independent breeding position before the next season than strictly philopatric individuals, but also suffered higher mortality. Prospecting yielded similar benefits to floating but lower mortality costs, suggesting that it is overall more beneficial than floating and strict philopatry. While prospecting is probably individual‐driven, although limited by resource availability, floating likely results from eviction by unrelated breeders. Such differences in proximate and ultimate factors underlying each route to independent breeding highlight the need for simultaneous consideration when studying the evolution of delayed dispersal.     

Experimentally increased food resources in the natal territory promote offspring philopatry and helping in cooperatively breeding carrion crows

Kin-based societies, where families represent the basic social unit, occur in a relatively small number of vertebrate species. In the majority of avian kin societies, families form when offspring prolong their association with the parents on the natal territory. Therefore, the key to understanding the evolution of families in birds is to understand natal philopatry (i.e. the tendency to remain on the natal territory). It has been shown that, within populations, the strength of the association between parents and offspring (i.e. family stability) increases when offspring dispersal is constrained by external environmental factors, but it is unclear whether and how family wealth influences juvenile dispersal decisions. Here, we show that young carrion crows (Corvus corone corone) from territories that were food-supplemented year-round were more philopatric and more likely to help at their family's nest than the unfed ones. The results suggest that offspring philopatry and helping behaviour are influenced by the quality of 'home' and that the availability of food resources positively affects the cohesion of the family.     

Dispersal costs set the scene for helping in an atypical avian cooperative breeder

The ecological constraints hypothesis is suggested to explain the evolution of cooperative breeding in birds. This hypothesis predicts that the scene for cooperative breeding is set when ecological factors constrain offspring from dispersal. This prediction was tested in the atypical cooperative breeding system of the long-tailed tit, Aegithalos caudatus, by comparing the degree of philopatry and cooperation in an isolated and a contiguous site whilst experimentally controlling for confounding aspects of reproduction. No difference was found between the two sites in the survival of offspring but a greater proportion were found to remain philopatric in the isolated site. This difference was caused by greater philopatry of normally dispersive females suggesting, as predicted, that dispersal costs were greater from this site. Furthermore, a greater proportion of males and females cooperated following breeding failure in the isolated site than in the contiguous site. Thus, as has been suggested for typical avian cooperative breeders, dispersal costs, relative to philopatric benefits, appear to set the scene for cooperative breeding in long-tailed tits.     

Social and ecological influences on dispersal and philopatry in the plateau pika (Ochotona curzoniae)

Benefits and costs of dispersal and philopatry of the socialplateau pika (Ochotona curzoniae) were studied on the Tibetanplateau for 3 years. Although short-lived, plateau pikas livein cohesive family groups that occupy burrow systems in sedgemeadow habitat Most (57.8%) plateau pikas were philopatric,and dispersal movements were extremely restricted. No juvenilefemales or adult pikas moved more than two family ranges betweenyears; the greatest observed dispersal distances were by twojuvenile males that moved five family ranges from the familyof their birth. Traversing unfamiliar habitat was not a costof pika dispersal because most dispersers settled in familiesthat they could easily visit before dispersal. Dispersal movementsappeared to result in equalization of density among pika families,an expected result if competition for environmental resourcesinfluenced dispersal. Males did not disperse to gain advantagesin competition for mates, as evidenced by their moving to familieswith significantly fewer females. Females, however, moved tofamilies with significantly more males. Males provide abundantpaternal care, and significantly more offspring per female survivedto become adults from families with more adult males per adultfemale. Evidence concerning the influence of inbreeding avoidanceon natal dispersal was indirect. Some males exhibited natalphilopatry; thus some families had opportunity for dose inbreeding.Males and females that dispersed had no opposite-sex relativesin their new families. Philopatric pikas may have benefitedby remaining in families that exhibited low local densities,and philopatric females might have benefited from social cooperationwith relatives.     

The evolution of social philopatry in female primates

The transition from solitary life to sociality is considered one of the major transitions in evolution. In primates, this transition is currently not well understood. Traditional verbal models appear insufficient to unravel the complex interplay of environmental and demographic factors involved in the evolution of primate sociality, and recent phylogenetic reconstructions have produced conflicting results. We therefore analyze a theoretical model for the evolution of female social philopatry that sheds new light on the question why most primates live in groups. In individual-based simulations, we study the evolution of dispersal strategies of both resident females and their offspring. The model reveals that social philopatry can evolve through kin selection, even if retention of offspring is costly in terms of within-group resource competition and provides no direct benefits. Our model supports the role of predator avoidance as a selective pressure for group-living in primates, but it also suggests that a second benefit of group-living, communal resource defense, might be required to trigger the evolution of sizable groups. Lastly, our model reveals that seemingly small differences in demographic parameters can have profound effects on primate social evolution.     

Delayed dispersal as a route to breeding: territorial inheritance, safe havens, and ecological constraints

The relative roles of ecological constraints, the benefits of philopatry, and the role of life history continue to be debated in the evolution of natal philopatry and cooperative breeding. We compare three routes to breeding: departing to search for territories as a floater, staying and queuing to inherit the natal territory, or queuing and eventually shifting to a neighboring vacancy. Our model assumed a dominance-structured population. It quantifies the benefits of philopatry for varying-rank subordinates and contrasts it against the benefit of dispersal. We apply the model to data on Siberian jay Perisoreus infaustus, a species in which retained offspring do not help at the nest. The results indicate that territorial inheritance plays a small role in this species (presumably due to inbreeding avoidance), and territory acquisition is less constrained for dispersing than philopatric offspring. Nevertheless, small family groups-one or, at the most, two same-sex queuers-are predicted to form because philopatric offspring gain nepotistic benefits that improve their survival. This fits with data on group sizes and supports the idea of the natal territory as a safe haven for waiting for breeding opportunities. We also discuss our predictions in the light of ecological constraints and clarify recent confusingly different predictions on the role of habitat saturation as an explanation for delayed dispersal and cooperative breeding. We argue that "ecological constraint" is too wide a term to yield useful predictive power and that it is more appropriate to examine the consequences of specific life-history traits on the success of dispersers.     

Microhabitat heterogeneity influences offspring sex allocation and spatial kin structure in possums

1. Sex allocation theory predicts that where dispersal is sex biased, the fitness consequences of producing male or female offspring are mediated by resource availability and maternal competitive ability. Females in poorer condition are expected to favour dispersing offspring to minimize resource competition with kin. Environmental heterogeneity may drive spatial variation in sex allocation through resource competition-related benefits to females and territory quality benefits to dispersing or philopatric offspring. 2. Here, we demonstrate that microhabitat heterogeneity can drive extremely fine-scale spatial heterogeneity in offspring sex allocation. Female bobucks (Trichosurus cunninghami) in temperate rainforest were more likely to produce male offspring than those in surrounding Eucalyptus forest. 3. A maternal physiological effect was identified, in that females of lower body mass were more likely to produce male offspring. This finding is consistent with resource competition predictions, in that smaller females are expected to have poorer competitive ability. 4. Genetic spatial autocorrelation analysis identified males as the more dispersing sex. Furthermore, overproduction of males by mothers in the rainforest habitat was geographically concordant with reduced philopatry, as inferred from spatial genetic analysis. This provides empirical validation of dispersal-related explanations of offspring sex allocation: that production of offspring of the dispersing sex minimizes the potential for resource competition with kin. 5. Spatial variation in dispersal via sex allocation responses to environmental heterogeneity can potentially contribute to spatial patterns in population dynamics.     

Delayed dispersal and prolonged brood care in a family‐living beetle

Delayed juvenile dispersal is an important prerequisite for the evolution of family‐based social systems, such as cooperative breeding and eusociality. In general, young adults forego dispersal if there are substantial benefits to remaining in the natal nest and/or the likelihood of dispersing and breeding successfully is low. We investigate some general factors thought to drive delayed juvenile dispersal in the horned passalus beetle, a family‐living beetle in which young adults remain with their families in their natal nest for several months before dispersing. Fine‐scale population genetic structure indicated high gene flow between nest sites, suggesting that constraints on mobility are unlikely to explain philopatry. Young adults do not breed in their natal log and likely disperse before reaching breeding age, suggesting that they do not gain direct reproductive benefits from delayed dispersal. We also examined several ways in which parents might incentivize delayed dispersal by providing prolonged care to adult offspring. Although adult beetles inhibit fungal growth in the colony by manipulating both the nest site and deceased conspecifics, this is unlikely to be a major explanation for family living as both parents and adult offspring seem capable of controlling fungal growth. Adult offspring that stayed with their family groups also neither gained more mass nor experienced faster exoskeleton development than those experimentally removed from their families. The results of these experiments suggest that our current understanding of the factors underlying prolonged family living may be insufficient to explain delayed dispersal in at least some taxa, particularly insects.     

Philopatry and natal dispersal of Montagu's harriers (Circus pygargus) breeding in Spain: a review of existing data

Natal dispersal is an important component of bird ecology, plays a key role in many ecological and evolutionary processes, and has important conservation implications. Nevertheless, detailed knowledge on natal dispersal is still lacking in many bird species, especially raptors. We review and compile existing information from five tagging programmes of juvenile Montagu's harriers (Circus pygargus) in different Spanish regions, with PVC rings or wing tags, to provide an assessment of philopatry and natal dispersal of the species in Spain. Only 7% of all tagged harriers were observed as breeders in subsequent years. The percentage of philopatric (i.e. breeding within 10?km of the natal site) males and females was lower that 5%. Overall, there were no sexual differences in percentage of philopatric birds or dispersal distances, but we found study area differences. The low philopatry observed suggests a high capacity for natal dispersal in this species, for both sexes, and therefore high genetic mixing between populations. Differences in philopatry between study areas may be influenced by the different observation effort or detectability, or else reflect different philopatric strategies among populations. Finally, we found no significant differences in philopatry rate or dispersal distances related to tagging method, suggesting that tagging technique has a smaller effect than monitoring effort or observation ease on observation probability. Developing tagging programmes at a small scale and without procuring very large-scale and intensive subsequent monitoring is not worthwhile for evaluating philopatry and natal dispersal in this species.     

Sex-biased survival and philopatry in birds: Do they interact?

A review of studies of sex-biased dispersal and philopatry and sex-biased survival in birds is presented. The comparison between sex-related mortality and natal and breeding dispersal at the species-level shows that dispersing birds (mainly females) suffer higher mortality, while philopatric birds (mainly males) have higher survival. The interaction between sex-biased survival and spatial behavior is a crucial component of avian vital strategy, which determine population dynamics and genetic structure.     

Impact of dispersal status on estimates of local population growth rates in a Temminck's stint Calidris temminckii population

Due to different costs and benefits associated with dispersal and philopatry, life history traits of immigrants and philopatric individuals may differ. Despite of the apparent effects, dispersal status is only rarely considered in analyses of population dynamics. We analysed whether dispersal status explains variation in life history traits of an endangered Temminck's stint Calidris temminckii population breeding at the Baltic Sea. We also estimated the impact of immigration and dispersal status on the population growth rate (λ) with a population matrix model, in which immigrants and philopatric individuals are separated to their own stages. We found that philopatric individuals had a higher apparent survival than immigrants in both sexes. In reproductive parameters, variation due to dispersal status was not clear. Nests incubated by philopatric individuals survived better than those of immigrants, but this did not translate in hatchling production per breeding attempt. Models described a sink population in which the inclusion of both immigration rate to the population and the dispersal status of individuals into the model increased estimates of λ. When the better success of philopatric individuals was considered, the population growth appeared more stable (λ=0.972). If this was not taken into account, λ implied a strong decline (λ=0.911). The results support the hypothesis that immigrants exhibit lower components of lifetime reproductive success and therefore contribute less to population growth and the gene pool than local recruits. While we cannot distinguish whether this difference reflects higher mortality or permanent emigration, the latter explanation seems more plausible. Our results highlight the importance of considering immigration and dispersal status in population modelling. In the case of the endangered study population, the results implied that management options directed to improve local recruitment would be a profitable option.     

Female‐biased dispersal in a bat with a female‐defence mating strategy

The ultimate causes for predominant male‐biased dispersal (MBD) in mammals and female‐biased dispersal (FBD) in birds are still subject to much debate. Studying exceptions to general patterns of dispersal, for example, FBD in mammals, provides a valuable opportunity to test the validity of proposed evolutionary pressures. We used long‐term behavioural and genetic data on individually banded Proboscis bats (Rhynchonycteris naso) to show that this species is one of the rare mammalian exceptions with FBD. Our results suggest that all females disperse from their natal colonies prior to first reproduction and that a substantial proportion of males are philopatric and reproduce in their natal colonies, although male immigration has also been detected. The age of females at first conception falls below the tenure of males, suggesting that females disperse to avoid father–daughter inbreeding. Male philopatry in this species is intriguing because Proboscis bats do not share the usual mammalian correlates (i.e. resource‐defence polygyny and/or kin cooperation) of male philopatry. They have a mating strategy based on female defence, where local mate competition between male kin is supposedly severe and should prevent the evolution of male philopatry. However, in contrast to immigrant males, philopatric males may profit from acquaintance with the natal foraging grounds and may be able to attain dominance easier and/or earlier in life. Our results on Proboscis bats lent additional support to the importance of inbreeding avoidance in shaping sex‐biased dispersal patterns and suggest that resource defence by males or kin cooperation cannot fully explain the evolution of male philopatry in mammals.     

Benefits, constrainsts and the evolution of the family

Families form when offspring delay dispersal and remain associated with their parents into adulthood. What are the ultimate causes of such philopatry and prolonged association? Two schools of thought have emerged, providing seemingly opposing answers to this question - the 'benefits of philopatry' and the 'ecological constraints' schools. A recent threshold model for delayed dispersal helps clarify the confused semantics of these arguments and demonstrates the complimentarity of the two approaches. Many of the current advantages of family living may have evolved secondarily, however, suggesting that the initial origin of families occurred under conditions of severe competition for breeding vacancies.     

The role of survival for the evolution of female philopatry in a solitary forager, the grey mouse lemur (Microcebus murinus)

It is widely accepted that natal philopatry is a prerequisite for the evolution of sociality. The life-history hypothesis maintains that longevity of adults results in extended territory tenure and thus limits breeding vacancies for offspring, which makes natal philopatry more likely. Here, we tested the importance of longevity for natal philopatry in females of a basal primate, the grey mouse lemur (Microcebus murinus). This species is regarded as being solitary due to its foraging habits but while males disperse, female offspring in this species forgo dispersal and form long-term sleeping groups with their mothers. We tested whether high adult survival could be a cause for natal philopatry of female offspring. In addition, we assessed costs and benefits associated with space sharing between mothers and daughters and whether mothers actively increase survival of daughters by beqeauthal of territories, information transfer about resources or thermoregulation. Contrary to our predictions, adult females had low-survival rates. Space sharing appeared to improve survival of both, mothers and daughters. This could be a result of information transfer about sleeping sites and thermoregulatory benefits. Our results cast doubt on the idea that longevity predisposes species for social traits and provide support for benefits of philopatry.     

Dispersal as a source of variation in age-specific reproductive strategies in a wild population of lizards

Dispersal syndromes describe the patterns of covariation of morphological, behavioural, and life-history traits associated with dispersal. Studying dispersal syndromes is critical to understanding the demographic and genetic consequences of movements. Among studies describing the association of life-history traits with dispersal, there is anecdotal evidence suggesting that dispersal syndromes can vary with age. Recent theory also suggests that dispersive and philopatric individuals might have different age-specific reproductive efforts. In a wild population of the common lizard (Zootoca vivipara), we investigated whether dispersive and philopatric individuals have different age-specific reproductive effort, survival, offspring body condition, and offspring sex ratio. Consistent with theoretical predictions, we found that young dispersive females have a higher reproductive effort than young philopatric females. Our results also suggest that the early high investment in reproduction of dispersive females trades-off with an earlier onset of senescence than in philopatric females. We further found that young dispersive females produce smaller offspring in lower body condition than do young philopatric females. Overall, our results provide empirical evidence that dispersive and philopatric individuals have different age-specific life-history traits.     

Inbreeding avoidance and male-biased natal dispersal in Antechinus spp. (Marsupialia: Dasyuridae)

Members of the dasyurid marsupial genus Antechinus and their close relatives are unique among terrestrial vertebrates because male generations are discrete, being punctuated by an abrupt post-mating mortality. Amost all juvenile males disperse shortly after they are weaned, while females are strongly philopatric. Circumstantial and experimental evidence suggests that mothers cause the dispersal of their sons and recruit unrelated males to live with themselves and their daughters during this dispersal phase. Dispersal generally involves all the males in a litter and occurs in litters with a single male, those consisting entirely of males, and even where the mother will not breed again. These data suggest the benefits of inbreeding avoidance are a sufficient cause for the evolution of sex-biased dispersal. Alternative causes that have been suggested in the literature are inadequate because of the absence of a class of experienced males that would promote dispersal among subordinates, and the failure of the dispersal to reduce group-size in nests.     

Nepotistic alarm calling in the Siberian jay, Perisoreus infaustus

From a life history perspective, parents have an incentive to protect their reproductive investment, and so may provide care even after their offspring are independent. Such prolonged parental care could lead to postponed dispersal of the offspring and thereby facilitate the formation of kin groups. We tested whether alpha birds in Siberian jays protected their independent, retained offspring by giving alarm calls during simulated predator attacks. We compared the responses to predator attacks simulated by flying a hawk model over a dyad of birds on a feeder for dyads composed of an alpha bird and either a relative or a nonrelative. Alpha females were nepotistic in their alarm-calling behaviour, in that they called more frequently when accompanied by their retained offspring than by unrelated immigrants, but alpha males called indiscriminately. This difference in alarm calling could reflect dominance relationships in Siberian jay groups, because the presence of immigrants may be less costly to alpha males, but alpha females are more vulnerable to competition from immigrants. Alarm calls were usually given during escape, when both individuals in the dyad had left the feeding site. However, results of a playback experiment suggest that alarm calls conveyed information about danger and incited an immediate escape reaction. Our results indicate that alarm calling can be nepotistic, and that factors other than kinship influence alarm-calling behaviour. Nepotistic antipredator behaviours are benefits that offspring can gain only in their natal territory. Hence, in the absence of preferential treatment by their parents, offspring may be more likely to disperse and kin groups are prevented from forming.     

Maternal influence on philopatry and space use by juvenile brushtail possums (Trichosurus vulpecula)

1. The causes of juvenile sex-biased philopatry and space use in mammals remain poorly understood, and results of previous research have been conflicting. Experimental interventions and manipulations on wild populations are rare, but can play an important role in establishing the factors governing offspring space use. 2. We experimentally removed mothers of independent juvenile brushtail possums from the maternal home range and examined changes in offspring space use with global positioning system collars. We examined the influence of mother absence on philopatric behaviour, and determined whether or not maternal presence affected offspring space use. 3. We fitted a longitudinal linear mixed effects model to demonstrate a change over time in the home range size of juveniles following experimental treatment by the removal of their mothers. When mothers were removed from the natal range, juveniles occupied significantly larger home range areas, with average increases of 175% in 95% kernel density estimates and 289% in minimum convex polygon estimates. This increase occurred within the first month following mother absence and was independent of juvenile sex. Home ranges of control juveniles did not change during the same time period. 4. Changes in the spatial structure of mammalian populations in response to removal of individuals have important implications for pest management. The impacts of management strategies which target particular individuals in a population may counteract conservation benefits through their effect on the space use of survivors. Studies involving experimental removals provide important information on consequences of control and also yield insights into the causes of mammalian space use, philopatric behaviours and ultimately dispersal.     

Philopatry and within-colony movements in Columbian ground squirrels

Philopatry and dispersal result in selection of habitat locations that may differ in resources and social environment and thus should influence fitness components like survival and reproduction. We examined short-distance movements of young and adult females from natal or previous nesting sites within a colony of Columbian ground squirrels (Urocitellus columbianus) in the Rocky Mountains of Alberta, Canada, over a 17-year period. Females of all ages were strongly philopatric, yet a few (10-15%) exhibited movements that took them to new home ranges. We tested three hypotheses to explain the pattern of female natal and breeding movements: (1) that movements of philopatric females promote proximity to close kin; (2) that range shifts favour close kin via bequeathal of territory and (3) that dispersers move to lower density areas where competition for resources is lower. Tests of these three hypotheses revealed that: (1) philopatry and movements of young and older philopatric females led to proximity to mothers and local presence of close kin; (2) breeding dispersal did not result in bequeathal of home range to daughters, but movements of philopatric females suggested that they shared space with close kin and (3) adult females moved to new ranges with lower local densities, though dispersing females also left ranges where local density was significantly lower than for philopatric females. Natal and breeding movements among years produced two opportunities for territorial females: close spatial proximity to close kin via short philopatric movements, and habitats with fewer competitors via longer dispersal movements.     

Lifetime philopatry in the blue-footed booby: a longitudinal study

Philopatry over the lifetime and its relationship with reproductivesuccess were examined using longitudinal records of nest locationand reproduction of individual blue-footed boobies. Males showedshorter natal dispersal than females, and natal dispersal distanceof both sexes were unrelated to either first reproductive successor lifetime reproductive success. Throughout the early lifetime,males and females nested closer to their first breeding sitesthan to their natal sites, and comparison with a simulationof successive breeding dispersals in random directions showedthat male and female blue-footed boobies are philopatric tothe first breeding site. Therefore, throughout the early lifetime,the first breeding site seems to function as a point of referencefor breeding site use together with the previous season's site.Males and females with shorter natal dispersal distances showedstronger lifetime philopatry to their first breeding sites,suggesting stable individual variation in competitive abilityor dispersal phenotype. However, early lifetime philopatry tofirst breeding sites was unrelated to annual breeding success.Compared with simple fidelity to previous breeding sites, lifetimephilopatry to first breeding sites should result in increasedkin interactions and greater selection for kin recognition,altruism and inbreeding avoidance, as well as long-term familiaritywith neighbors.