Isotopic variation across the Audubon's–myrtle warbler hybrid zone

Differences in seasonal migratory behaviours are thought to be an important component of reproductive isolation in many organisms. Stable isotopes have been used with success in estimating the location and qualities of disjunct breeding and wintering areas. However, few studies have used isotopic data to estimate the movements of hybrid offspring in species that form hybrid zones. Here, we use stable hydrogen to estimate the wintering locations and migratory patterns of two common and widespread migratory birds, Audubon's (Setophaga auduboni) and myrtle (S. coronata) warblers, as well as their hybrids. These two species form a narrow hybrid zone with extensive interbreeding in the Rocky Mountains of British Columbia and Alberta, Canada, which has been studied for over four decades. Isotopes in feathers grown on the wintering grounds or early on migration reveal three important patterns: (1) Audubon's and myrtle warblers from allopatric breeding populations winter in isotopically different environments, consistent with band recovery data and suggesting that there is a narrow migratory transition between the two species, (2) most hybrids appear to overwinter in the south‐eastern USA, similar to where myrtle warblers are known to winter, and (3) some hybrid individuals, particularly those along the western edge of the hybrid zone, show Audubon's‐like isotopic patterns. These data suggest there is a migratory divide between these two species, but that it is not directly coincident with the centre of the hybrid zone in the breeding range. We interpret these findings and discuss them within the context of previous research on hybrid zones, speciation and migratory divides.     

Migrating Birds as Dispersal Vehicles for West Nile Virus

Whereas migrating birds have been implicated in the spread of West Nile virus (WNV), there is no direct evidence of birds actively migrating while infectious. The role of birds in WNV dispersal is difficult to assess in the field. However, this role can be evaluated experimentally because birds in migratory disposition display increased locomotor activity or restlessness under captive conditions. We tested the following hypotheses: (1) migrating passerine birds continue to exhibit migratory activity while infectious with WNV and (2) the migratory state of the individual affects the magnitude of viremia. We examined the migratory activity of two neoarctic-neotropical passerine migrants, Swainson’s thrush (Catharus ustulatus) and gray catbird (Dumetella carolinensis), during acute WNV infection. All gray catbirds and six of nine Swainson’s thrushes exhibited migratory activity while infectious. Moreover, migratory status did not appear to influence viremia titers, as might be expected if individuals were immunosuppressed during migration. Therefore, we demonstrate that migrating passerine birds are potential dispersal vehicles for WNV.     

Migratory restlessness in captive individuals predicts actual departure in the wild

In captivity, migratory birds show increased activity during the time that they would normally migrate. The phenology and intensity of such ‘migratory restlessness’ has been shown to mirror species- and population-specific migration patterns observed in the wild and has consequently been used as a proxy for the motivation to migrate. Many studies doing so, however, were aiming to explain among-individual variation in migratory behaviour or traits, and not species- or population-specific traits. These studies thus assumed that, also at the level of the individual, migratory restlessness is an accurate proxy for the motivation to migrate. We tested this assumption for the first time and found that it holds; individuals showing very little migratory restlessness remained at stopover for longer than one night, whereas most individuals showing more restlessness departed sooner. This finding validates the use of migratory restlessness as a proxy for the motivation to migrate, thereby justifying the conclusions made in a large body of research on avian migration.     

An inventory of the possible effects of climatic change on western palearctic migratory birds

This paper presents an assessment of the possible effects of future climatic change on migratory birds. The assessment is based on two approaches: firstly an inventory is made of the environmental factors that may change which directly affect migratory birds. These factors include physical (temperature, hydrology, ocean and air streaming patterns) as well as biological (floral and faunal composition of ecosystems) and landuse aspects of the environment.Secondly, these possible changes were related to the annual cycles of migratory birds in order to estimate the problems that different groups of migratory birds have to cope with at various stages in their annual cycle. It is concluded that many migratory bird species will be influenced by climatic change, leading to adaptations in the birds annual cycle. The biggest problems may arise for those birds which depend on wetlands, because many of these wetlands may dessicate.     

Effects of habitat heterogeneity and local adaptation on the body condition of a forest passerine at the edge of its distributional range

We studied fluctuating asymmetry and feather growth rates as indicators of fitness of blackcaps ( Sylvia atricapilla ) breeding at the border of their distribution range in the Iberian Peninsula. Iberian blackcaps increase their abundance with increasing rainfall and ground cover of brambles. In border habitats, they are sedentary and morphologically different from migrants, suggesting that they could be adapted to peripheral conditions. We tested whether juvenile body condition depends on (1) distance from the centre of the range, (2) mean precipitation or (3) bramble cover. Controlling for environmental variation, we tested for differences between migratory and sedentary populations. Body condition varied across the Iberian gradient in parallel with changes in precipitation. Controlling for this effect, sedentary populations were in better body condition than migratory populations. Our results support the idea that environmental heterogeneity causes fitness to fluctuate across species' ranges, and also that local adaptation may mean that peripheral populations are more than a 'tail end' of the species.  © 2003 The Linnean Society of London . Biological Journal of the Linnean Society , 2003, 78 , 479–488.     

Phenotypic divergence during speciation is inversely associated with differences in seasonal migration

Differences in seasonal migration might promote reproductive isolation and differentiation by causing populations in migratory divides to arrive on the breeding grounds at different times and/or produce hybrids that take inferior migratory routes. We examined this question by quantifying divergence in song, colour, and morphology between sister pairs of North American migratory birds. We predicted that apparent rates of phenotypic differentiation would differ between pairs that do and do not form migratory divides. Consistent with this prediction, results from mixed effects models and Ornstein–Uhlenbeck models of evolution showed different rates of divergence between these groups; surprisingly, differentiation was greater among non-divide pairs. We interpret this finding as a result of variable rates of population blending and fusion between partially diverged forms. Ancient pairs of populations that subsequently fused are now observed as a single form, whereas those that did not fuse are observable as pairs and included in our study. We propose that fusion of two populations is more likely to occur when they have similar migratory routes and little other phenotypic differentiation that would cause reproductive isolation. By contrast, pairs with migratory divides are more likely to remain reproductively isolated, even when differing little in other phenotypic traits. These findings suggest that migratory differences may be one among several isolating barriers that prevent divergent populations from fusing and thereby increase the likelihood that they will continue differentiating as distinct species.     

Sex‐biases in distribution and resource use at different spatial scales in a migratory shorebird

In migratory species, sexual size dimorphism can mean differing energetic requirements for males and females. Differences in the costs of migration and in the environmental conditions occurring throughout the range may therefore result in sex‐biases in distribution and resource use at different spatial scales. In order to identify the scale at which sexual segregation operates, and thus the scale at which environmental changes may have sex‐biased impacts, we use range‐wide tracking of individually color‐ringed Icelandic black‐tailed godwits (Limosa limosa islandica) to quantify sexual segregation at scales ranging from the occupation of sites throughout the non‐breeding range to within‐site differences in distribution and resource use. Throughout the range of this migratory shorebird, there is no evidence of large‐scale sex differences in distribution during the non‐breeding season. However, the sexes differ in their selection of prey types and sizes, which results in small‐scale sexual segregation within estuaries. The scale of sexual segregation therefore depends on the scale of variation in resource distribution, which, in this system, is primarily within estuaries. Sexual segregation in within‐site distribution and resource use means that local‐scale anthropogenic impacts on estuarine benthic prey communities may disproportionately affect the sexes in these migratory shorebirds.     

Individual differences in migratory behavior shape population genetic structure and microhabitat choice in sympatric blackcaps (Sylvia atricapilla)

In migratory birds, traits such as orientation and distance are known to have a strong genetic background, and they often exhibit considerable within‐population variation. How this variation relates to evolutionary responses to ongoing selection is unknown because the underlying mechanisms that translate environmental changes into population genetic changes are unclear. We show that within‐population genetic structure in southern German blackcaps (Sylvia atricapilla) is related to individual differences in migratory behavior. Our 3‐year study revealed a positive correlation between individual migratory origins, denoted via isotope (δ²H) values, and genetic distances. Genetic diversity and admixture differed not only across a recently established migratory polymorphism with NW‐ and SW‐migrating birds but also across δ²H clusters within the same migratory route. Our results suggest assortment based on individual migratory origins which would facilitate evolutionary responses. We scrutinized arrival times and microhabitat choice as potential mechanisms mediating between individual variation in migratory behavior and assortment. We found significant support that microhabitat choice, rather than timing of arrival, is associated with individual variation in migratory origins. Moreover, examining genetic diversity across the migratory divide, we found migrants following the NW route to be genetically more distinct from each other compared with migrants following the traditional SW route. Our study suggests that migratory behavior shapes population genetic structure in blackcaps not only across the migratory divide but also on an individual level independent of the divide. Thus, within‐population variation in migratory behavior might play an important role in translating environmental change into genetic change.     

Bat head contains soft magnetic particles: Evidence from magnetism

Recent behavioral observations have indicated that bats can sense the Earth's magnetic field. To unravel the magnetoreception mechanism, the present study has utilized magnetic measurements on three migratory species (Miniopterus fuliginosus, Chaerephon plicata, and Nyctalus plancyi) and three non‐migratory species (Hipposideros armiger, Myotis ricketti, and Rhinolophus ferrumequinum). Room temperature isothermal remanent magnetization acquisition and alternating‐field demagnetization showed that the bats' heads contain soft magnetic particles. Statistical analyses indicated that the saturation isothermal remanent magnetization of brains (SIRM_{1T_brain}) of migratory species is higher than those of non‐migratory species. Furthermore, the SIRM_{1T_brain} of migratory bats is greater than their SIRM_{1T_skull}. Low‐temperature magnetic measurements suggested that the magnetic particles are likely magnetite (Fe₃O₄). This new evidence supports the assumption that some bats use magnetite particles for sensing and orientation in the Earth's magnetic field. Bioelectromagnetics 31:499–503, 2010. © 2010 Wiley‐Liss, Inc.     

Wind patterns as a potential driver in the evolution and maintenance of a North American migratory suture zone

Suture zones are areas where range contact zones and hybrid zones of multiple taxa are clustered. Migratory divides, contact zones between divergent populations that breed adjacent to one another but use different migratory routes, are a particular case of suture zones. Although multiple hypotheses for both the formation and maintenance of migratory divides have been suggested, quantitative tests are scarce. Here, we tested whether a novel factor, prevailing winds, was sufficient to explain both the evolution and maintenance of the Cordilleran migratory divide using individual‐based models. Empirical observations of eastern birds suggest a circuitous migratory route across Canada before heading south. Western breeders, however, travel south along the Pacific coast to their wintering grounds. We modeled the effect of wind on bird migratory flights by allowing them to float at elevation using spatially explicit modeled wind data. Modeled eastern birds had easterly mean trajectories, whereas western breeders showed significantly more southern trajectories. We also determined that a mean airspeed of 18.5 m s^–1 would be necessary to eliminate this difference in trajectory, a speed that is achieved by waterfowl and shorebirds, but is faster than songbird flight speeds. These results lend support for the potential importance of wind in shaping the phylogeographic history of North American songbirds.     

Migratory Disturbance Thresholds with Mule Deer and Energy Development

Fine-scale movement data has transformed our knowledge of ungulate migration ecology and now provides accurate, spatially explicit maps of migratory routes that can inform planning and management at local, state, and federal levels. Among the most challenging land use planning issues has been developing energy resources on public lands that overlap with important ungulate habitat, including the migratory routes of mule deer (Odocoileus hemionus). We generally know that less development is better for minimizing negative effects and maintaining habitat function, but we lack information on the amount of disturbance that animals can tolerate before reducing use of or abandoning migratory habitat. We used global positioning system data from 56 deer across 15 years to evaluate how surface disturbance from natural gas well pads and access roads in western Wyoming, USA, affected habitat selection of mule deer during migration and whether any disturbance threshold(s) existed beyond which use of migratory habitat declined. We used resource and step selection functions to examine disturbance thresholds at 3 different spatial scales. Overall, migratory use by mule deer declined as surface disturbance increased. Based on the weight of evidence from our 3 independent but complementary metrics, declines in migratory use related to surface disturbance were non-linear, where migratory use sharply declined when surface disturbance from energy development exceeded 3%. Disturbance thresholds may vary across regions, species, or migratory habitats (e.g., stopover sites). Such information can help with management and land use decisions related to mineral leasing and energy development that overlap with the migratory routes of ungulates. © 2020 The Wildlife Society.     

寄主型和迁飞型棉蚜的交配行为

棉蚜Aphis gossypii Glover种群存在寄主利用和迁飞能力上的明显分化,有棉花型和瓜型、迁飞型和滞留型之分。但是,棉花型与瓜型之间,以及迁飞型与滞留型之间是否能发生交配行为,尚无研究报道。本文在低温和短光照条件下分别诱导棉花型、瓜型、迁飞型和滞留型棉蚜的性蚜,并进行性蚜间的交配行为观察。结果表明,迁飞型和滞留型性蚜间可以发生交配行为,杂交时发生交配行为的个体比率与自交时无显著差异,但是杂交时雄蚜寻找配偶所花的时间要显著长于自交时,并且迁飞型雌蚜(M♀)与滞留型雄蚜(S♂)杂交时的交配持续时间也显著长于自交时。迁飞型和滞留型棉蚜同型交配容易完成,棉蚜的迁飞型和滞留型已在交配行为上产生了一定的分化。棉花型和瓜型棉蚜的性蚜间可以发生交配行为,并且两品系在正交和反交时雄蚜的寻偶时间与交配持续时间均无显著差异。     

Assessing the effects of habitat characteristics and co-occurrence with closely related species on occupancy of resident ‘wood-warblers’ in a Neotropical working landscape

The simultaneous effects of habitat traits and interspecific interactions determine the occurrence and habitat use of wildlife populations. However, little research has been devoted to examining spatial co-occurrence among closely related species while considering the effect of habitat variation and imperfect detectability of species in the field. In this study, we focused on migratory and resident 'wood-warblers' that coexist during the winter in a Neotropical working landscape in southern Mexico to understand if habitat occupancy of resident wood-warblers is influenced by habitat characteristics and by the presence of other species of resident and migratory wood-warblers. For this purpose, we implemented two-species occupancy models, which account for the imperfect detectability of these birds in the field. Our results revealed that habitat occupancy of resident wood-warblers was positively influenced by the presence of other closely related species (both migratory and resident). These positive relationships may be explained by the fact that different species of wood-warblers frequently participate in mixed-species flocks. However, these patterns of species co-occurrence were more evident among resident species than between migratory and resident species, which may be explained by micro-habitat segregation and differences in behaviours between resident and migratory wood-warblers. We also found that some habitat characteristics may mediate the observed patterns of species co-occurrence. Specifically, sites with larger trees were associated with the co-occurrence of some species of resident wood-warblers. In addition, we discuss the possibility that species co-occurrence might be the result of shared preferences for environmental factors that we did not consider. Our study highlights the importance of the interplay between species co-occurrence and habitat traits in determining the presence and habitat use of resident birds in Neotropical working landscapes.     

Hybrid songbirds employ intermediate routes in a migratory divide

Migratory divides are contact zones between populations that use different routes to navigate around unsuitable areas on seasonal migration. Hybrids in divides have been predicted to employ intermediate and potentially inferior routes. We provide the first direct test of this hypothesis, using light‐level geolocators to track birds breeding in a hybrid zone between Swainson's thrushes in western Canada. Compared to parental forms, hybrids exhibited increased variability in their migratory routes, with some using intermediate routes that crossed arid and mountainous regions, and some using the same routes as one parental group on fall migration and the other on spring migration. Hybrids also tended to use geographically intermediate wintering sites. Analysis of genetic variation across the hybrid zone suggests moderately strong selection against hybrids. These results indicate that seasonal migratory behaviour might be a source of selection against hybrids, supporting a possible role for migration in speciation.     

Weak migratory connectivity,loop migration and multiple non-breeding site use in British breeding Whinchats Saxicola rubetra

Determining the links between breeding populations and the pressures, threats and conditions they experience presents a challenge for the conservation of migratory birds which can use multiple sites separated by hundreds to thousands of kilometres. Furthermore, migratory connectivity – the connections made by migrating individuals between networks of breeding and non-breeding sites – has important implications for population dynamics. The Whinchat Saxicola rubetra is declining across its range, and tracking data from a single African non-breeding site implies high migratory spread. We used geolocators to describe the migration routes and non-breeding areas of 20 Whinchats from three British breeding populations. As expected, migratory spread was high, with birds from the three populations overlapping across a wide area of West Africa. On average, in non-breeding areas, British breeding Whinchats were located 652 km apart from one another, with some likely to share non-breeding areas with individuals from breeding populations as far east as Russia. Four males made a direct non-breeding season movement to a second, more westerly, non-breeding location in January. Autumn migration was through Iberia and around the western edge of the Sahara Desert, whereas spring migration was more direct, indicating an anticlockwise loop migration. Weak migratory connectivity implies that Whinchat populations are somewhat buffered against local changes in non-breeding conditions. If non-breeding season processes have played a role in the species’ decline, then large-scale drivers are likely to be the cause, although processes operating on migration, or interactions between breeding and non-breeding processes, cannot be ruled out.     

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.     

Limitations and mechanisms influencing the migratory performance of soaring birds

Migration is costly in terms of time, energy and safety. Optimal migration theory suggests that individual migratory birds will choose between these three costs depending on their motivation and available resources. To test hypotheses about use of migratory strategies by large soaring birds, we used GPS telemetry to track 18 adult, 13 sub‐adult and 15 juvenile Golden Eagles Aquila chrysaetos in eastern North America. Each age‐class had potentially different motivations during migration. During spring, the migratory performance (defined here as the directness of migratory flight) of adults was higher than that of any other age‐classes. Adults also departed earlier and spent less time migrating. Together, these patterns suggest that adults were primarily time‐limited and the other two age‐classes were energy‐limited. However, adults that migrated the longest distances during spring also appeared to take advantage of energy‐conservation strategies such as decreasing their compensation for wind drift. During autumn, birds of all age‐classes were primarily energy‐minimizers; they increased the length of stopovers, flew less direct routes and migrated at a slower pace than during spring. Nonetheless, birds that departed later in autumn flew more directly, indicating that time limitations may have affected their decision‐making. During both seasons, juveniles had the lowest performance, sub‐adults intermediate performance and adults the highest performance. Our results show age‐ and seasonal variation in time and energy‐minimization strategies that are not necessarily exclusive of one another. Beyond time and energy, a complex suite of factors, including weather, experience and navigation ability, influences migratory performance and decision‐making.     

Animal migrations and parasitism: reciprocal effects within a unified framework

Migrations, i.e. the recurring, roundtrip movement of animals between distant and distinct habitats, occur among diverse metazoan taxa. Although traditionally linked to avoidance of food shortages, predators or harsh abiotic conditions, there is increasing evidence that parasites may have played a role in the evolution of migration. On the one hand, selective pressures from parasites can favour migratory strategies that allow either avoidance of infections or recovery from them. On the other hand, infected animals incur physiological costs that may limit their migratory abilities, affecting their speed, the timing of their departure or arrival, and/or their condition upon reaching their destination. During migration, reduced immunocompetence as well as exposure to different external conditions and parasite infective stages can influence infection dynamics. Here, we first explore whether parasites represent extra costs for their hosts during migration. We then review how infection dynamics and infection risk are affected by host migration, thereby considering parasites as both causes and consequences of migration. We also evaluate the comparative evidence testing the hypothesis that migratory species harbour a richer parasite fauna than their closest free-living relatives, finding general support for the hypothesis. Then we consider the implications of host migratory behaviour for parasite ecology and evolution, which have received much less attention. Parasites of migratory hosts may achieve much greater spatial dispersal than those of non-migratory hosts, expanding their geographical range, and providing more opportunities for host-switching. Exploiting migratory hosts also exerts pressures on the parasite to adapt its phenology and life-cycle duration, including the timing of major developmental, reproduction and transmission events. Natural selection may even favour parasites that manipulate their host's migratory strategy in ways that can enhance parasite transmission. Finally, we propose a simple integrated framework based on eco-evolutionary feedbacks to consider the reciprocal selection pressures acting on migratory hosts and their parasites. Host migratory strategies and parasite traits evolve in tandem, each acting on the other along two-way causal paths and feedback loops. Their likely adjustments to predicted climate change will be understood best from this coevolutionary perspective.     

Integrating stable isotopes,parasite, and ring‐reencounter data to quantify migratory connectivity—A case study with Barn Swallows breeding in Switzerland,Germany, Sweden,and Finland

Ecosystems around the world are connected by seasonal migration. The migrant animals themselves are influenced by migratory connectivity through effects on the individual and the population level. Measuring migratory connectivity is notoriously difficult due to the simple requirement of data conveying information about the nonbreeding distribution of many individuals from several breeding populations. Explicit integration of data derived from different methods increases the precision and the reliability of parameter estimates. We combine ring‐reencounter, stable isotope, and blood parasite data of Barn Swallows Hirundo rustica in a single integrated model to estimate migratory connectivity for three large scale breeding populations across a latitudinal gradient from Central Europe to Scandinavia. To this end, we integrated a non‐Markovian multistate mark‐recovery model for the ring‐reencounter data with normal and binomial mixture models for the stable isotope and parasite data. The integration of different data sources within a mark‐recapture modeling framework enables the most precise quantification of migratory connectivity on the given broad spatial scale. The results show that northern‐breeding populations and Southern Africa as well as southern‐breeding populations and Western–Central Africa are more strongly connected through Barn Swallow migration than central European breeding populations with any of the African wintering areas. The nonbreeding distribution of Barn Swallows from central European breeding populations seems to be a mixture of those populations breeding further north and south, indicating a migratory divide.