Research progress in avian dispersal behavior

Dispersal, defined as a linear spreading movement of individuals away from others of the population is a fundamental characteristic of organisms in nature. Dispersal is a central concept in ecological, behavioral and evolutionary studies, driven by different forces such as avoidance of inbreeding depression, density-dependent competition and the need to change breeding locations. By effective dispersal, organisms can enlarge their geographic range and adjust the dynamic, sex ratio and genetic compositions of a population. Birds are one of the groups that are studied intensively by human beings. Due to their diurnal habits, diverse life history strategies and complex movement, birds are also ideal models for the study of dispersal behaviors. Certain topics of avian dispersal including sex-biased, asymmetric dispersal caused by differences in body conditions, dispersal processes, habitat selection and long distance dispersal are discussed here. Bird-ringing or marking, radio-telemetry and genetic markers are useful tools widely applied in dispersal studies. There are three major challenges regarding theoretical study and methodology research of dispersal: (1) improvement in research methodology is needed, (2) more in-depth theoretical research is necessary, and (3) application of theoretical research into the conservation efforts for threatened birds and the management of their habitats should be carried out immediately. __________ Translated from Acta Ecologica Sinica, 2008, 28(4): 1354–1365 [译自: 生态学报]     

Research progress in avian dispersal behavior

Dispersal, defined as a linear spreading move-ment of individuals away from others of the population is a fundamental characteristic of organisms in nature. Dispersal is a central concept in ecological, behavioral and evolutionary studies, driven by different forces such as avoidance of inbreeding depression, density-dependent competition and the need to change breeding locations. By effective dispersal, organisms can enlarge their geo-graphic range and adjust the dynamic, sex ratio and gen-etic compositions of a population. Birds are one of the groups that are studied intensively by human beings. Due to their diurnal habits, diverse life history strategies and complex movement, birds are also ideal models for the study of dispersal behaviors. Certain topics of avian dispersal including sex-biased, asymmetric dispersal caused by differences in body conditions, dispersal pro-cesses, habitat selection and long distance dispersal are discussed here. Bird-ringing or marking, radio-telemetry and genetic markers are useful tools widely applied in dispersal studies. There are three major challenges regard-ing theoretical study and methodology research of dis-persal: (1) improvement in research methodology is needed, (2) more in-depth theoretical research is neces-sary, and (3) application of theoretical research into the conservation efforts for threatened birds and the manage-ment of their habitats should be carried out immediately.     

Reproductive consequences of natal dispersal in a highly philopatric seabird

Natal and breeding dispersal have a major impact on gene flowand population structure. We examined the consequences of nataldispersal on the reproductive success (proportion of pairs rearingchicks) of colonial-breeding Thick-billed murres (Uria lomvia).Reproductive success increased with distance dispersed for thefirst and second breeding attempt. The increase in breedingsuccess leveled off at natal dispersal distances above 7 m.Our results were consistent with the idea that the relationshipbetween dispersal and reproductive success is caused by siteavailability and mate choice as birds willing to disperse fartherhad a greater choice of potential sites and mates. This hypothesiswas supported by the fact that birds dispersing farther weremore likely to pair with an experienced breeder, which increasesthe likelihood of breeding success for young breeders. Explanationsfor increasing breeding success with increased dispersal basedon inbreeding effects were unlikely because most breeding failureswere caused by egg loss rather than infertility or nestlingdeath. However, we could not explain why >50% of birds returnwithin 3 m of the natal site, despite having an up to 50% lowerreproductive success than birds dispersing 7 m or more.     

A comparison of techniques for assessing dispersal behaviour in gundis: revealing dispersal patterns in the absence of observed dispersal behaviour

Knowledge of the dispersal status of group members is important to understanding how sociality may have evolved within a species. I assessed the effectiveness of four techniques for elucidating dispersal behaviour in a rock-dwelling rodent ( Ctenodactylus gundi ) with small group sizes (2–10 animals): genetic parentage assignment, haplotype data and kinship analyses, assignment testing, and F -statistics. The first two methods provided the greatest insight into gundi dispersal behaviour. Assignment testing and F -statistics proved of limited use for elucidating fine-scale dispersal, but could detect large-scale patterns despite low sex-biased dispersal intensity (1.9 : 1) because of moderate genetic differentiation among groups ( F _ST = 0.10). Findings are discussed in light of current dispersal theory. In general, gundi dispersal is plastic, and seems to be dependent on body weight (for males), group composition, and scale of analysis (total dispersal events recorded within the population were almost twice the immigration rate into the population). Most groups were comprised of a single matriline and one immigrant male. Immigrant rather than philopatric males bred with group females. Dispersal among groups was male-biased, but dispersal or philopatry could occur by either sex. During a drought, both sexes delayed dispersal and cooperative social units formed. Whether such behaviour resulted directly from the drought or not remains unclear, however, since comparative information was not available from nondrought years. Combining fine-scale analyses with information on large-scale patterns provided substantial insight into gundi dispersal behaviour despite the limited movement of animals during a drought, and may prove useful for elucidating dispersal behaviour in other social animals.     

Variation in fine‐scale genetic structure and local dispersal patterns between peripheral populations of a South American passerine bird

The distribution of suitable habitat influences natal and breeding dispersal at small spatial scales, resulting in strong microgeographic genetic structure. Although environmental variation can promote interpopulation differences in dispersal behavior and local spatial patterns, the effects of distinct ecological conditions on within‐species variation in dispersal strategies and in fine‐scale genetic structure remain poorly understood. We studied local dispersal and fine‐scale genetic structure in the thorn‐tailed rayadito (Aphrastura spinicauda), a South American bird that breeds along a wide latitudinal gradient. We combine capture‐mark‐recapture data from eight breeding seasons and molecular genetics to compare two peripheral populations with contrasting environments in Chile: Navarino Island, a continuous and low density habitat, and Fray Jorge National Park, a fragmented, densely populated and more stressful environment. Natal dispersal showed no sex bias in Navarino but was female‐biased in the more dense population in Fray Jorge. In the latter, male movements were restricted, and some birds seemed to skip breeding in their first year, suggesting habitat saturation. Breeding dispersal was limited in both populations, with males being more philopatric than females. Spatial genetic autocorrelation analyzes using 13 polymorphic microsatellite loci confirmed the observed dispersal patterns: a fine‐scale genetic structure was only detectable for males in Fray Jorge for distances up to 450 m. Furthermore, two‐dimensional autocorrelation analyzes and estimates of genetic relatedness indicated that related males tended to be spatially clustered in this population. Our study shows evidence for context‐dependent variation in natal dispersal and corresponding local genetic structure in peripheral populations of this bird. It seems likely that the costs of dispersal are higher in the fragmented and higher density environment in Fray Jorge, particularly for males. The observed differences in microgeographic genetic structure for rayaditos might reflect the genetic consequences of population‐specific responses to contrasting environmental pressures near the range limits of its distribution.     

Mother-offspring interactions do not affect natal dispersal in a small rodent

According to kin selection and inbreeding avoidance hypotheses,natal dispersal should be facultatively adjusted to balancingthe costs and benefits of mother–offspring interactions.In polygynous mammals, it is hypothesized that female offspringshould seek to avoid local resource competition with their mother,whereas male dispersal should be determined by inbreeding avoidance.We tested these hypotheses with a field experiment investigatingthe relationship between territory acquisition and mother'spresence in the root vole Microtus oeconomus. This species hasa flexible social system in which sisters' and mother's homeranges overlap substantially, whereas sons disperse to a greaterextent. Immature sibling voles aged 20 days were released for20 days together with an unrelated adult male in a 2-patch systemeither in the presence of their mother or in the presence ofan unrelated adult female. Offspring movements were not influencedby mother's presence, but offspring, especially females, avoidedthe patch occupied by the adult female irrespective of kinship.Offspring remaining in contact with their mother were reproductivelysuppressed at the middle, but not by the end, of the experimentalperiod. These results indicate that juvenile root voles adoptedan opportunistic settlement strategy where they avoided theadult female irrespective of kinship and inbreeding risks.     

The link between behavioural type and natal dispersal propensity reveals a dispersal syndrome in a large herbivore

When individuals disperse, they modify the physical and social composition of their reproductive environment, potentially impacting their fitness. The choice an individual makes between dispersal and philopatry is thus critical, hence a better understanding of the mechanisms involved in the decision to leave the natal area is crucial. We explored how combinations of behavioural (exploration, mobility, activity and stress response) and morphological (body mass) traits measured prior to dispersal were linked to the subsequent dispersal decision in 77 roe deer Capreolus capreolus fawns. Using an unusually detailed multi-trait approach, we identified two independent behavioural continuums related to dispersal. First, a continuum of energetic expenditure contrasted individuals of low mobility, low variability in head activity and low body temperature with those that displayed opposite traits. Second, a continuum of neophobia contrasted individuals that explored more prior to dispersal and were more tolerant of capture with those that displayed opposite traits. While accounting for possible confounding effects of condition-dependence (body mass), we showed that future dispersers were less neophobic and had higher energetic budgets than future philopatric individuals, providing strong support for a dispersal syndrome in this species.     

Dispersal, interpatch movements, and survival in a shrubland breeding bird community

ABSTRACT Dispersal events can affect the distribution, abundance, population structure, and gene flow of animal populations, but little is known about long‐distance movements due to the difficulty of tracking individuals across space. We documented the natal and breeding dispersal of shrubland birds among 13 study sites in a 1000 km² area in southeastern Ohio. In addition, we radio‐marked and tracked 37 adult males of one shrubland specialist, the Yellow‐breasted Chat (Icteria virens). We banded 1925 juveniles and 2112 adults of nine shrubland species from 2002 to 2005. Of these, 33 (1.7%) juveniles were encountered in subsequent years (2003–2006) as adults (natal dispersal) and 442 (20.9%) birds initially banded as breeding adults were re‐encountered in subsequent years (breeding dispersal). Apparent survival of juvenile shrubland birds on their natal patches was 0.024 (95% CI 0.016–0.036). After accounting for the probability of detection, we found that 21% of birds banded as juveniles and recaptured as adults returned to their natal patches, whereas 78% of adult birds showed fidelity to the patch where they were originally captured. Moreover, natal dispersers tended to move farther than breeding dispersers (corrected natal median = 1.7 km ± 0.37; corrected breeding median = 0.23 km ± 0.10). We used our estimates of natal dispersal and annual apparent survival to estimate true survival at 0.11 (95% CI 0.07–0.18) for juveniles in their first year. However, this estimate was only applicable for birds dispersing within 7 km of their natal patches. Interpatch movements of radio‐marked Yellow‐breasted Chats were not uncommon, with 13 of 37 males located in more than one habitat patch. Overall, we observed low natal philopatry, but high adult site fidelity for shrubland birds in our study area. Considering the frequency of short‐distance movements observed (median = 531 m, range = 88–1045 m), clustering of patches within 1 km might facilitate use of shrubland habitat.     

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.     

Environmental conditions affect spatial genetic structures and dispersal patterns in a solitary rodent

The study of the spatial distribution of relatives in a population under contrasted environmental conditions provides critical insights into the flexibility of dispersal behaviour and the role of environmental conditions in shaping population relatedness and social structure. Yet few studies have evaluated the effects of fluctuating environmental conditions on relatedness structure of solitary species in the wild. The aim of this study was to determine the impact of interannual variations in environmental conditions on the spatial distribution of relatives [spatial genetic structure (SGS)] and dispersal patterns of a wild population of eastern chipmunks (Tamias striatus), a solitary rodent of North America. Eastern chipmunks depend on the seed of masting trees for reproduction and survival. Here, we combined the analysis of the SGS of adults with direct estimates of juvenile dispersal distance during six contrasted years with different dispersal seasons, population sizes and seed production. We found that environmental conditions influences the dispersal distances of juveniles and that male juveniles dispersed farther than females. The extent of the SGS of adult females varied between years and matched the variation in environmental conditions. In contrast, the SGS of males did not vary between years. We also found a difference in SGS between males and females that was consistent with male‐biased dispersal. This study suggests that both the dispersal behaviour and the relatedness structure in a population of a solitary species can be relatively labile and change according to environmental conditions.     

Social constraints limit dispersal and settlement decisions in a group-living bird species

Dispersal is a fundamental process affecting the genetic structureof populations, speciation, and extinction. Nevertheless, ourunderstanding of the evolution of dispersal is limited by ourpaucity of knowledge on dispersal decisions at the individuallevel. We investigated the effect of interactions between residentsand juvenile dispersers on individual dispersal and settlementdecisions in Siberian jays (Perisoreus infaustus). In this group-livingbird species, some offspring remain on the parental territoryfor up to 3 years (retained juveniles) whereas other offspringdisperse within 2 months of fledging (dispersers). We foundthat retained juveniles constrained settlement decisions ofdispersers by aggressively chasing dispersers off their territory,resulting in dispersers continuing to disperse and settlingin groups without retained juveniles. Experimental removal ofmale breeders during the dispersal period also demonstratedthat dispersers were unable to settle in high-quality breedingopenings, which were instead filled by older nonbreeding residents.Rather, dispersers immigrated into groups without retained offspringwhere they became subordinate group members, queuing for a breedingopening. Also, they preferably settled in groups with shortqueues where no same-sex juveniles were present. Dispersal didnot inflict a cost to dispersers through increased mortality.However, the presence of immigrants was costly for breedersbecause it increased the rate of conflicts during the breedingseason which negatively affected nestling condition. These resultsdemonstrate that resident individuals constrain both dispersaland settlement decisions of dispersers. Social interactionsbetween residents and dispersers can thus be a key factor tounderstand the evolution of dispersal.     

Sex-biased dispersal and natal philopatry in the diamondback terrapin, Malaclemys terrapin

Nesting ecology and population studies indicate that diamondback terrapins (Malaclemys terrapin) exhibit nest site fidelity and high habitat fidelity. However, genetic studies indicate high levels of gene flow. Because dispersal affects the genetics and population dynamics of a species, we used six highly polymorphic microsatellite markers to investigate sex-biased dispersal and natal philopatry of M. terrapin in Barnegat Bay, NJ. We compared results of spatial autocorrelation analysis, assignment methods and Wright's F(ST) estimators to a mark-recapture analysis. Mark-recapture analysis over a 4-year period indicated that most individuals have relatively small home ranges (<2 km), with mature females displaying greater home ranges than males. Goodness of fit analysis of our mark-recapture study indicated that some juvenile males were likely transient individuals moving through our study location. Mean assignment indices and first-generation migrant tests indicated that mature males were more prone to disperse than mature females, but first-generation migrant tests indicated that per capita there are more female than male dispersers. Thus, the relative importance of males and females on gene flow in terrapin populations may change in relation to population sex ratios. Spatial autocorrelation analysis indicated that mature females exhibited natal philopatry to nesting beaches, but first-generation migrant tests indicated that a small number of females failed to nest on natal beaches. Finally, we discuss the important conservation implications of male-biased dispersal and natal philopatry in the diamondback terrapin.     

Parasitism, host immune function, and sexual selection

Parasite-mediated sexual selection may arise as a consequence of 1) females avoiding mates with directly transmitted parasites, 2) females choosing less-parasitized males that provide parental care of superior quality, or 3) females choosing males with few parasites in order to obtain genes for parasite resistance in their offspring. Studies of specific host-parasite systems and comparative analyses have revealed both supportive and conflicting evidence for these hypotheses. A meta-analysis of the available evidence revealed a negative relationship between parasite load and the expression of male secondary sexual characters. Experimental studies yielded more strongly negative relationships than observations did, and the relationships were more strongly negative for ectoparasites than for endoparasites. There was no significant difference in the magnitude of the negative effect for species with and without male parental care, or between behavioral and morphological secondary sexual characters. There was a significant difference between studies based on host immune function and those based on parasite loads, with stronger effects for measures of immune function, suggesting that the many negative results from previous analyses of parasite-mediated sexual selection may be explained because relatively benign parasites were studied. The multivariate analyses demonstrating strong effect sizes of immune function in relation to the expression of secondary sexual characters, and for species with male parental care as compared to those without, suggest that parasite resistance may be a general determinant of parasite-mediated sexual selection.     

Age-dependent inbreeding risk and offspring fitness costs in female black grouse

Dispersal is an important mechanism used to avoid inbreeding. However, dispersal may only be effective for part of an individual's lifespan since, post-dispersal individuals that breed over multiple reproductive events may risk mating with kin of the philopatric sex as they age. We tested this hypothesis in black grouse Tetrao tetrix, and show that yearling females never mated with close relatives whereas older females did. However, matings were not with direct kin suggesting that short-distance dispersal to sites containing kin and subsequent overlap of reproductive lifespans between males and females were causing this pattern. Chick mass was lower when kinship was high, suggesting important fitness costs associated with inbred matings. This study shows that increased inbreeding risk might be a widespread yet rarely considered cost of ageing.     

The evolution of cooperative breeding in birds: kinship,dispersal and life history

The evolution of cooperation among animals has posed a major problem for evolutionary biologists, and despite decades of research into avian cooperative breeding systems, many questions about the evolution of their societies remain unresolved. A review of the kin structure of avian societies shows that a large majority live in kin-based groups. This is consistent with the proposed evolutionary routes to cooperative breeding via delayed dispersal leading to family formation, or limited dispersal leading to kin neighbourhoods. Hypotheses proposed to explain the evolution of cooperative breeding systems have focused on the role of population viscosity, induced by ecological/demographic constraints or benefits of philopatry, in generating this kin structure. However, comparative analyses have failed to generate robust predictions about the nature of those constraints, nor differentiated between the viscosity of social and non-social populations, except at a coarse level. I consider deficiencies in our understanding of how avian dispersal strategies differ between social and non-social species, and suggest that research has focused too narrowly on population viscosity and that a broader perspective that encompasses life history and demographic processes may provide fresh insights into the evolution of avian societies.     

Parasitism and dispersal potential of Sirex noctilio: implications for biological control

1 Sirex noctilio F. (Hymenoptera: Siricidae) is a wood‐boring wasp that attacks many pine species, including commercial trees planted throughout the world. Management of its populations is largely based on biological control using the nematode Beddingia siricidicola. Adult females are sterilized by the nematode, but are free to move and attack new trees, promoting nematode dispersal. Although generally successful, wasp management through nematode introductions has sometimes been inadequate. 2 We evaluated the effect of parasitism by B. siricidicola on flight performance of woodwasps under laboratory conditions. Using flight mills, we recorded a total of 46 flight trials over 23 h, obtained from infected and control (uninfected) females. 3 Although all wasps lost weight during flight, parasitized females were significantly smaller and suffered larger weight losses than uninfected females. In addition, total flight distance and velocity were lower in parasitized females. 4 Because nematode infection transmission relies on healthy wasps attacking trees previously visited by nematode‐bearing females, differential dispersal capacity could limit biological control success.     

Group structure, kinship, inbreeding risk and habitual female dispersal in plural-breeding mammals

In most plural-breeding mammals, female group members are matrilineal relatives but, in a small number of species, all adult females are immigrants who are seldom closely related to each other. Some explanations of contrasts in female philopatry suggest that these differences are a consequence of variation in resource distribution and feeding competition, whereas others argue that they reflect variation in the risk of close inbreeding to philopatric females. However, neither explanation has been tested against quantitative comparisons. Here, we use quantitative comparisons and phylogenetic reconstructions to show that contrasts in female philopatry in plural breeders are associated with the risk that a female's father is reproductively active in her group when she starts to breed, supporting the suggestion that habitual female dispersal has evolved to minimize the risk of inbreeding.