Social transmission of migratory knowledge: quantifying the risk of losing migratory behavior

When migration is a learned behavior, small populations have a significant problem of maintaining migratory knowledge across generations. These populations risk losing migratory behavior entirely, which may exacerbate existing stressors on population size. Here we investigated the success of various behavioral, demographic, and social factors towards maintaining migration within small populations. Using a discrete-time probabilistic model to simulate repeated migrations, we found that migratory group size plays an important role in maintaining migratory knowledge within the population. Rare, large groups allow for migratory knowledge to be spread to many individuals at once. When a population learns migration information incrementally, the presence of individuals that can learn quickly, therefore transitioning rapidly into leaders, has a profound impact on migrational persistence. Furthermore, small populations are better able to maintain migratory behavior when groups rely on informed leaders as compared to using collective group knowledge, even when that collective knowledge is heavily weighted towards knowledgeable individuals. Finally, we found that both species with short lifespans and species that migrate with fixed group compositions are at especially high risk of losing their migration behavior at small population sizes.     

Partial migration: growth varies between resident and migratory fish

Partial migration occurs in many taxa and ecosystems and may confer survival benefits. Here, we use otolith chemistry data to determine whether fish from a large estuarine system were resident or migratory, and then examine whether contingents display differences in modelled growth based on changes in width of otolith growth increments. Sixty-three per cent of fish were resident based on Ba : Ca of otoliths, with the remainder categorized as migratory, with both contingents distributed across most age/size classes and both sexes, suggesting population-level bet hedging. Migrant fish were in slightly better condition than resident fish based on Fulton''s K condition index. Migration type (resident versus migratory) was 56 times more likely to explain variation in growth than a model just incorporating year- and age-related growth trends. While average growth only varied slightly between resident and migratory fish, year-to-year variation was significant. Such dynamism in growth rates likely drives persistence of both life-history types. The complex relationships in growth between contingents suggest that management of species exhibiting partial migration is challenging, especially in a world subject to a changing climate.     

Search and navigation in dynamic environments – from individual behaviors to population distributions

Animal movement receives widespread attention within ecology and behavior. However, much research is restricted within isolated sub-disciplines focusing on single phenomena such as navigation (e.g. homing behavior), search strategies (e.g. Levy flights) or theoretical considerations of optimal population dispersion (e.g. ideal free distribution). To help synthesize existing research, we outline a unifying conceptual framework that integrates individual-level behaviors and population-level spatial distributions with respect to spatio-temporal resource dynamics. We distinguish among (1) non-oriented movements based on diffusion and kinesis in response to proximate stimuli, (2) oriented movements utilizing perceptual cues of distant targets, and (3) memory mechanisms that assume prior knowledge of a target's location. Species' use of these mechanisms depends on life-history traits and resource dynamics, which together shape population-level patterns. Resources with little spatial variability should facilitate sedentary ranges, whereas resources with predictable seasonal variation in spatial distributions should generate migratory patterns. A third pattern, 'nomadism', should emerge when resource distributions are unpredictable in both space and time. We summarize recent advances in analyses of animal trajectories and outline three major components on which future studies should focus: (1) integration across alternative movement mechanisms involving links between state variables and specific mechanisms, (2) consideration of dynamics in resource landscapes or environments that include resource gradients in predictability, variability, scale, and abundance, and finally (3) quantitative methods to distinguish among population distributions. We suggest that combining techniques such as evolutionary programming and pattern oriented modeling will help to build strong links between underlying movement mechanisms and broad-scale population distributions.     

Cultural transmission and flexibility of partial migration patterns in a long‐lived bird,the great bustard Otis tarda

Factors responsible for individual variation in partial migration patterns are poorly known, and identifying possible causes of these changes is essential for understanding the flexibility in migratory behavior. Analyzing 190 life histories of great bustards Otis tarda radio‐tagged in central Spain, we investigated the changes in migratory tendency across lifetime in this long‐lived bird, and how migratory flexibility is related to individual condition. In females migratory behavior was not fixed individually. For every age class there was a fraction of ca 15–30% of females that changed their migratory pattern between consecutive years. Migrant females tended to remain sedentary in years when they had dependent young to attend. These findings show that the female migratory tendency is a behaviorally flexible, condition‐dependent trait. Immature females usually acquired their migratory behavior by learning from the mother in their first winter or by social transmission from other migratory females in their second winter. As for immature males, their summer migratory behavior was not related to mother–offspring transmission, but learned from adult males. We found that their age‐related increase in migratory tendency was associated to a greater integration in flocks of migrant adult males. These results show that within the partial migration system, cultural transmission mechanisms, either mediated by kin or not, and individual condition, may contribute to shape the migratory tendency. Our study reinforces the view that the migratory behavior is an evolutionary complex trait conditioned by the interaction of individual, social and environmental factors. Particularly in long‐lived species with extended parental care, the inherited migration program may be shaped by mother–offspring and social transmission of migratory patterns.     

Evaluating the reproductive cost of migration for females in a partially migrating pond-breeding amphibian

Populations of animals with resident and migratory individuals provide an ideal system for addressing questions concerning the evolution of migration. Partially migratory populations may persist because residents and migrants have equal fitness or because migration is based on conditional asymmetries. Studies measuring the costs and benefits of migration provide empirical data to test hypotheses concerning the maintenance of partial migration within a population. In this study, we measured the reproductive differences between resident and migrant females in a pond-breeding amphibian, the red-spotted newt Notophthalmus viridescens . We used large field enclosures to repeatedly sample egg laying over the prolonged breeding season of this species. Resident females did not lay a greater number of eggs or begin laying eggs earlier, despite beginning the breeding season earlier and having a higher mass than migrant newts. The only difference in reproduction we detected was that the eggs of resident females hatched into larger larvae compared with the larvae of migrant females. We discuss this result in the context of other potential trade-offs between residency and migration. This study illustrates the phenology of egg laying of N. viridescens and our results contribute to understanding the population dynamics of partially migratory species.     

Mysticete migration revisited: are Mediterranean fin whales an anomaly?

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Correlates of alternative migratory strategies in western bluebirds

Partial migration occurs when only some animals in a population migrate. While evidence suggests that migratory strategies are partially controlled by genes, individual and environmental conditions which alter the cost‐benefit trade‐off of migration among individuals are also likely to play a role. Three hypotheses have been advanced to explain condition‐dependent partial migration: the arrival time, dominance and body size hypotheses. In this study, we asked whether these hypotheses explained differences in migratory strategy among individuals in a partially migratory population of western bluebirds Sialia mexicana breeding in southern British Columbia, Canada. We used stable hydrogen isotope signatures in claw tissue to determine migratory strategy of individual bluebirds, and examined patterns of migration at both individual and population levels. The proportion of resident bluebirds varied significantly over the three years of the study, and across study sites. Several migrants switched to the resident strategy between years; however, we found no evidence of strategy switching in the opposite direction. Young birds were significantly more likely to be resident than older birds, a pattern which could arise if early arrival is particularly important for birds obtaining a territory for the first time. Furthermore, young females were the most likely of all sex–age classes to be resident, which may reflect a survival advantage of residency for young females. Finally, birds mated assortatively by migratory strategy and isotopic evidence suggests that members of a pair often wintered in the same place. Our results provided no support for the dominance or body size hypotheses, and only limited support for the arrival time hypothesis in bluebirds. However, taken together, we suggest that our findings indicate that social factors may influence migratory strategies in this system.     

Population and evolutionary dynamics in spatially structured seasonally varying environments

Increasingly imperative objectives in ecology are to understand and forecast population dynamic and evolutionary responses to seasonal environmental variation and change. Such population and evolutionary dynamics result from immediate and lagged responses of all key life‐history traits, and resulting demographic rates that affect population growth rate, to seasonal environmental conditions and population density. However, existing population dynamic and eco‐evolutionary theory and models have not yet fully encompassed within‐individual and among‐individual variation, covariation, structure and heterogeneity, and ongoing evolution, in a critical life‐history trait that allows individuals to respond to seasonal environmental conditions: seasonal migration. Meanwhile, empirical studies aided by new animal‐tracking technologies are increasingly demonstrating substantial within‐population variation in the occurrence and form of migration versus year‐round residence, generating diverse forms of ‘partial migration’ spanning diverse species, habitats and spatial scales. Such partially migratory systems form a continuum between the extreme scenarios of full migration and full year‐round residence, and are commonplace in nature. Here, we first review basic scenarios of partial migration and associated models designed to identify conditions that facilitate the maintenance of migratory polymorphism. We highlight that such models have been fundamental to the development of partial migration theory, but are spatially and demographically simplistic compared to the rich bodies of population dynamic theory and models that consider spatially structured populations with dispersal but no migration, or consider populations experiencing strong seasonality and full obligate migration. Second, to provide an overarching conceptual framework for spatio‐temporal population dynamics, we define a ‘partially migratory meta‐population’ system as a spatially structured set of locations that can be occupied by different sets of resident and migrant individuals in different seasons, and where locations that can support reproduction can also be linked by dispersal. We outline key forms of within‐individual and among‐individual variation and structure in migration that could arise within such systems and interact with variation in individual survival, reproduction and dispersal to create complex population dynamics and evolutionary responses across locations, seasons, years and generations. Third, we review approaches by which population dynamic and eco‐evolutionary models could be developed to test hypotheses regarding the dynamics and persistence of partially migratory meta‐populations given diverse forms of seasonal environmental variation and change, and to forecast system‐specific dynamics. To demonstrate one such approach, we use an evolutionary individual‐based model to illustrate that multiple forms of partial migration can readily co‐exist in a simple spatially structured landscape. Finally, we summarise recent empirical studies that demonstrate key components of demographic structure in partial migration, and demonstrate diverse associations with reproduction and survival. We thereby identify key theoretical and empirical knowledge gaps that remain, and consider multiple complementary approaches by which these gaps can be filled in order to elucidate population dynamic and eco‐evolutionary responses to spatio‐temporal seasonal environmental variation and change.     

Evolutionary aspects of migratory behavior in European warblers

During the past 20 years, European Sylvia warblers have been used for a model study of the control mechanisms of bird migration and of evolutionary aspects of migratory behavior. Endogenous annual rhythms (‘circannual’ rhythms) and photo-period have proved to be the essential internal and external controlling factors. It is unknown whether this basic system, that also controls migration in other bird species, is currently evolutionarily stable or is instead adapting birds to the present slightly changing environmental conditions. Using the Blackcap, the control of partial migration in a bird species was analyzed. Two-way selective breeding experiments demonstrated a large selection response and high heritability values. These experiments have also indicated that a partially migratory population can become either almost completely migratory, or sedentary, in two to five generations. Hence, genetic influences are very important and presumably dominant over environmental factors in the expression of migratory or sedentary behavior. The large selection response implies a strikingly high evolutionary potential with respect to strong selection pressures. Further, in the Blackcap, migratory orientation behavior (in addition to migratory activity) was immediately transmitted into a F₁-generation when a cross-breeding experiment was performed using birds from a migratory and a resident population. The hybrids displayed their migratory activity along an axis that is used by their migratory parents. Finally, a rapidly developing novel migratory habit (new migratory direction to new wintering areas) in the Blackcap is discussed with respect to a positive feedback-mechanism, possibly including a series of advantages leading to above-average fitness.     

Comparing competitive ability and associated metabolic traits between a resident and migratory population of bull trout against a non-native species

Juvenile bull trout Salvelinus confluentus from two geographically and ecologically distinct populations were compared with regard to their ability to compete with non-native brook trout Salvelinus fontinalis in an artificial stream, and with respect to their rates of oxygen consumption. Bull trout collected from a migratory population foraged more successfully against brook trout competitors than those from a resident population, capturing more of a limited amount of food items presented. The migratory population was also more aggressive (measured by the number of nips, chases and lateral threat displays) against brook trout competitors than the resident population. Bull trout from the migratory population had a higher oxygen consumption rate (203 mg O₂ kg · hr^-1) in the field than similar sized fish from the resident population (183 mg O₂ kg · hr^-1). These results suggest native bull trout have population-level variation in competitive ability against a non-native species and such competitive ability is positively associated with metabolism and migratory life history.     

Diversity of migration strategies among great frigatebirds populations

Migratory behavior varies extensively between bird taxa, from long distance migration to purely sedentary behavior. Variability in migratory behavior also occurs within taxa, where individuals within some species, or even populations, show mixed strategies. The same variability occurs in seabird species. We examined the migratory behavior of distinct populations of great frigatebirds Fregata minor in three distant oceanographic basins. Great frigatebird populations showed extensive variation in post‐breeding migratory behavior. Birds from Europa Island (Mozambique Channel) made long‐distance migration to numerous distinct roosting sites in the Indian Ocean, New Caledonia birds made shorter distance migrations to roosting sites in the southwestern Pacific Ocean, and Galapagos birds were resident within the archipelago year round. Juvenile birds from Europa Is. and New Caledonia dispersed widely whereas Galapagos juveniles were resident year round. The migratory behavior of Europa Is. and New Caledonia resulted in complete separation of foraging grounds between breeding adults, non‐breeding adults, and juveniles, whereas in the Galapagos the overlap was complete. We suggest that population variability in migratory behavior may have arisen because of different environmental conditions at sea, and also depends on the availability of suitable roosting sites on oceanic islands. The results also highlight the capacity of frigatebirds to remain airborne most of the time even outside the breeding season when they have to molt.     

International migrations toward the city of Parma (Northern Italy): Evolution during a decade (1989–2000)

The population of Parma can be considered a paradigm in order to describe the rapid demographic transformations characterizing Northern Italy communities, in which immigration flows play undoubtedly an important role. The present study uses data from the ‘Osservatorio Biodemografico’ (Biodemographic Observatory), a data bank set up in 1989 with the aim to analyze longitudinally the dynamics of that urban population. This data bank traces individual and family life-histories since 12 years, updating them year by year. The presence of migrants has significantly risen in this last decade: at the beginning of 1989, foreign people living in Parma were 1.341 (751 men and 590 women; 0.78% of the resident population); in the year 2000 foreign residents were 6.491 (3.583 men and 2.908 women; 3.9% of the resident population). Immigrants can be analyzed since their arrival in the city (or, better, since their inclusion in the Population Register) according to their characteristics and modalities of arrival, duration of permanence in Parma, co-residential patterns, dynamics of family rejoining. Numerous profiles, characterized by different and very complex individual and familial trajectories, have been described. Actually, despite of a common region of origin, migratory trajectories differ greatly one each other, in consequence of complex dynamics involving, sex age and kinship. We wish to thank the Fyssen Foundation, thanks to which the post doctoral studies of Dott.ssa Pellegrino are carried out.     

The need for integrative approaches to understand and conserve migratory ungulates

Over the last two centuries overhunting, anthropogenic barriers and habitat loss have disrupted many ungulate migrations. We review the literature on ungulate migration disruptions and find that for many species the disruption of migratory routes causes a rapid population collapse. Previous research has focused on the proximal ecological factors that might favour migration, particularly spatiotemporal variation in resources and predation. However, this does not provide an adequate basis for understanding and mitigating anthropogenic effects on migratory populations. Migration is a complex behaviour and we advocate an integrative approach that incorporates population dynamics, evolution, genetics, behaviour and physiology, and that borrows insights and approaches from research on other taxa. We draw upon research on avian migration to illustrate research approaches that might also be fruitful in ungulates. In particular, we suggest that the migratory cycle should be evaluated in the context of seasonal population limitation, an approach we highlight with a preliminary demographic perturbation analysis of the Serengeti wildebeest ( Connochaetes taurinus ) population. We provide suggestions for avenues of future research and highlight areas where we believe rapid progress can be made by applying recent advances in theory, technology and analytical approaches.     

From intracellular signaling to population oscillations: bridging size- and time-scales in collective behavior

Collective behavior in cellular populations is coordinated by biochemical signaling networks within individual cells. Connecting the dynamics of these intracellular networks to the population phenomena they control poses a considerable challenge because of network complexity and our limited knowledge of kinetic parameters. However, from physical systems, we know that behavioral changes in the individual constituents of a collectively behaving system occur in a limited number of well-defined classes, and these can be described using simple models. Here, we apply such an approach to the emergence of collective oscillations in cellular populations of the social amoeba Dictyostelium discoideum. Through direct tests of our model with quantitative in vivo measurements of single-cell and population signaling dynamics, we show how a simple model can effectively describe a complex molecular signaling network at multiple size and temporal scales. The model predicts novel noise-driven single-cell and population-level signaling phenomena that we then experimentally observe. Our results suggest that like physical systems, collective behavior in biology may be universal and described using simple mathematical models.     

Summer elk calf survival in a partially migratory population

Decomposing variation in juvenile recruitment is a key component of understanding population dynamics for partially migratory ungulates. We investigated reproductive parameters of adult female elk (Cervus canadensis) with calves at heel, and survivorship, cause-specific mortality, and intrinsic and extrinsic factors affecting risk of mortality for calves in a partially migratory elk population from 2013–2016 in Alberta, Canada. Elk calves born to resident mothers had 45% lower survivorship on average compared to migrant calves (0.24 vs. 0.69) and nearly twice the mortality rate (0.37 vs. 0.19) from bears (Ursus spp.), the dominant source of mortality. Contrary to our predictions, we found that increasing levels of maternal ingesta-free body fat were associated with increasing risk of calf mortality, indicating predation may have overwhelmed nutritional effects. We found no evidence that timing of calf birth or birth weight differed between migratory tactics or influenced mortality risk. We found that as percentage of cut forest increased, risk of calf mortality marginally decreased, which benefited migrant elk that were exposed to more clear-cuts compared to residents. Exposure to bear predation risk was unimportant during the hiding phase (≤10 days after birth) for either migratory tactic, presumably because neonatal hiding behavior reduced vulnerability. In contrast, bear predation risk was important for mortality risk after 10 days in age, especially for resident elk calves, which were exposed to higher bear predation risk compared to migrants. We conclude that relative differences in bear predation between migratory tactics are contributing to the dynamics of partial migration in this population through additive effects on calf mortality. Thus, wildlife managers should anticipate that recovering grizzly bear (U. arctos) populations may substantially lower elk recruitment through effects on summer calf survival, especially in areas with diverse carnivore assemblages.     

Drivers and consequences of partial migration in an alpine bird species

1. Partial migration, where a portion of the population migrates between winter and summer (breeding) areas and the rest remain year‐round resident, is a common phenomenon across several taxonomic groups. Several hypotheses have been put forward to explain why some individuals migrate while others stay resident, as well as the fitness consequences of the different strategies. Yet, the drivers and consequences of the decision to migrate or not are poorly understood.
2. We used data from radio‐tagged female (n = 73) willow ptarmigan Lagopus lagopus in an alpine study area in Central Norway to test if (i) the decision to migrate was dependent on individual state variables (age and body weight), (ii) individuals repeated migratory decisions between seasons, and (iii) the choice of migratory strategy was related to reproductive success.
3. Partially supporting our prediction that migratory strategy depends on individual state, we found that juvenile birds with small body sizes were more likely to migrate, whereas large juveniles remained resident. For adult females, we found no relationship between the decision to migrate or stay resident and body weight. We found evidence for high individual repeatability of migratory decision between seasons. Migratory strategy did not explain variation in clutch size or nest fate among individuals, suggesting no direct influence of the chosen strategy on reproductive success.
4. Our results indicate that partial migration in willow ptarmigan is related to juvenile body weight, and that migratory behavior becomes a part of the individual life history as a fixed strategy. Nesting success was not affected by migratory strategy in our study population, but future studies should assess other traits to further test potential fitness consequences.

     

Stress hormone dynamics: an adaptation to migration?

The hormone corticosterone (CORT) is an important component of a bird’s response to environmental stress, but it can also have negative effects. Therefore, birds on migration are hypothesized to have repressed stress responses (migration-modulation hypothesis). In contrast to earlier studies on long-distance migrants, we evaluate this hypothesis in a population containing both migratory and resident individuals. We use a population of partially migratory blue tits (Cyanistes caeruleus) in southern Sweden as a model species. Migrants had higher CORT levels at the time of capture than residents, indicating migratory preparations, adaptation to stressors, higher allostatic load or possibly low social status. Migrants and residents had the same stress response, thus contradicting the migration-modulation hypothesis. We suggest that migrants travelling short distances are more benefited than harmed by retaining the ability to respond to stress.     

Characterizing population and individual migration patterns among native and restored bighorn sheep (Ovis canadensis)

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Modelling invasibility in endogenously oscillating tree populations: timing of invasion matters

The timing of introduction of a new species into an ecosystem can be critical in determining the invasibility (i.e. the sensitivity to invasion) of a resident population. Here, we use an individual-based model to test how (1) the type of competition (symmetric versus asymmetric) and (2) seed masting influence the success of invasion by producing oscillatory dynamics in resident tree populations. We focus on a case where two species (one resident, one invader introduced at low density) do not differ in terms of competitive abilities. By varying the time of introduction of the invader, we show that oscillations in the resident population favour invasion, by creating “invasibility windows” during which resource is available for the invader due to transiently depressed resident population density. We discuss this result in the context of current knowledge on forest dynamics and invasions, emphasizing the importance of variability in population dynamics.     

Brown planthopper Nilaparvata lugens was concentrated at the rear of the typhoon Soudelor in Eastern China in August 2015

Sometimes, extreme weather is vital for the population survival of migratory insects by causing sudden population collapse or outbreak. Several studies have shown that rice planthopper migration was significantly influenced by typhoons in eastern Asia. Most typhoons occur in the summer, especially in August. In August, brown planthopper Nilaparvata lugens (Stål) migrates northward or southward depending on wind direction, and thus typhoons can potentially influence its migration process and population distribution. However, this has not yet been studied. This paper reported a case study on the effects of Typhoon Soudelor on the summer migration of N. lugens in eastern China in 2015. The migration pathways of N. lugens were reconstructed for the period under the influence of a typhoon by calculating the trajectories and migration events in eight counties of the Yangtze River Valley region with ancillary information. Trajectory modelling showed that most migrants took short distance migrations (less than 200 km) under the influence of the Typhoon Soudelor. Numerous N. lugens migrants were concentrated and deposited at the rear of the typhoon during the last 5 days of Typhoon Soudelor on August 9–13 due to horizontal convergence, and this led to an outbreak population. These results indicated that the N. lugens population was redistributed by the typhoon in the summer and that the population dynamics at the rear of a typhoon should be kept under close surveillance. This study provided insight into migratory organisms adapting to atmospheric features.