Age-related variation in wing shape of migratory and sedentary Blackcaps Sylvia atricapilla

In many passerines, juveniles have shorter and more rounded wings than adults. Given that (1) long and pointed wings improve endurance in migratory flights, (2) shorter and rounded wings improve manoeuvrability, and (3) juvenile birds are more vulnerable to predators than adults, it has been hypothesised that ontogenetic variation in wing shape results from a greater importance of predation avoidance relative to migration performance during the first year of life. If so, wing shape should not change with age in the absence of migration-related selection for longer and more pointed wings. We test this by studying the variation with respect to age in wing length and wing pointedness of migratory and sedentary Blackcaps wintering in southern Spain. Migratory Blackcaps had longer and more pointed wings than sedentary Blackcaps. Juveniles had shorter wings than adults in migratory populations, but not in sedentary populations. The variation with age in wing pointedness was less pronounced, and was found in migratory females only. These differences between the two traits could be related to a stronger selection for pointed wings than for longer wings with increasing distance of migration, and to an increased migratoriness of females in partially migratory Blackcap populations. We hypothesise that, in migratory Blackcaps, a shorter and more rounded wing in juveniles could be selected for if the decrease in predation rate compensated for the somewhat greater costs of the first migration attempt. On the other hand, there are no costs of migration in sedentary Blackcaps, which hence maintain a similar wing shape, giving high manoeuvrability, both as juveniles and as adults.     

Seasonal niche-tracking behaviour of two partially migratory passerines

Migratory birds spend periods of the year in different locations as a response to seasonal changes in environmental suitability. They are classified as either ‘niche-trackers’ or ‘niche-switchers’, depending on whether they track or switch environmental conditions throughout the year. However, the relationship between these strategies and their migratory behaviour is still unclear. Here we examine whether migratory European Robins Erithacus rubecula and Eurasian Blackcaps Sylvia atricapilla track environmental conditions between breeding and wintering areas and whether their behaviour differs from that of their sedentary counterparts. We used ringing data for both migratory and sedentary individuals wintering sympatrically in the Iberian Peninsula to assess the environmental conditions relating to their seasonal distributions. We explored seasonal niche-tracking using two multivariate analyses with alternative sets of predictor variables (landscape and climate) to generate different environmental scenarios. Our results show that migratory individuals track similar climatic conditions throughout their seasonal distributions, whereas sedentary birds cope with great variation in climate over the course of the year. In addition, migratory birds show less seasonal overlap in the landscape structure of their chosen habitats compared with sedentary individuals. These results suggest that there is a trade-off between, on the one hand, the cost for migrants of travelling long distances and, on the other, the increased flexibility required by sedentary birds if they are to tolerate a wider suite of environmental conditions within their permanent ranges. Given that sedentary populations in the southern Iberian Peninsula seem to be linked to populations of migratory individuals of these two species that started to move northwards after the last glacial cycle, the observed patterns suggest that migrant birds represent a fraction of the southern population that is specialized in the exploitation of a narrower range of environmental conditions.     

Migratory and resident Blackcaps Sylvia atricapilla wintering in southern Spain show no resource partitioning

When different populations of the same bird species share non‐breeding habitats, competition for food may promote resource partitioning. We studied food choice by resident and migratory Blackcaps Sylvia atricapilla in sympatric wintering grounds in southern Spain. Resident Blackcaps have a larger bill, which may allow them to feed on a broader range of fruit sizes, and they may know the distribution of food better than do migrants. Based on fruit and bird counts, we transformed both fruit crop and bird abundance to a common energy currency. During two winters with low and high fruit production, available energy from fruit in mid‐January was estimated to be 80 and 1300 times, respectively, the daily requirements of Blackcaps. Furthermore, Blackcap numbers did not track between‐winter changes in fruit abundance during 10 consecutive years of monitoring, further suggesting that fruit food is not limiting. Analysis of food items from 760 samples of 717 individuals showed that migrants and residents fed primarily on fruits of Wild Olive Olea europaea sylvestris, the most energetic fruit resource. There was no evidence that the larger bills of resident Blackcaps provided any foraging benefit. Migratory Blackcaps fed on Wild Olives and invertebrates, two resources with high energetic and structural value, more frequently than did residents. This food choice could be more important for migratory Blackcaps because they have lower body mass to reduce wing load. Our results suggest that the wintering grounds of Blackcaps in Iberia provide abundant food that is used by sympatric migrants and residents without resource partitioning. Slight differences in food choice suggest that migrants might benefit from feeding on more nutritive food than residents to counteract the energetic constraints associated with a smaller body size.     

Phylogeography of the Ryukyu robin (Erithacus komadori): population subdivision in land-bridge islands in relation to the shift in migratory habit

We analysed the mitochondrial control region sequences of 150 Ryukyu robins (Erithacus komadori) from five migratory and three sedentary populations. E. komadori is endemic to the subtropical Asian islands, is well differentiated among populations in morphology and migratory behaviour within its narrow habitat range, and is ideal for examining the effects of altered migratory habits on population differentiation. Maximum-likelihood analysis among the haplotypes resulted in the generation of a single tree in which two phylogenetic clades corresponding to the two subspecies were evident. Within the northern lineage, three distinctive subgroups of populations (one migratory and two sedentary groups) were observed by population genetic analysis. The migration-related wing morphology and molecular data were then compared among groups. The wing shape of the northern lineage was significantly more pointed than that of the southern sedentary lineage, and they were exclusively discriminated from each other. The difference was not so apparent between the sedentary and migratory groups within the northern lineage, suggesting that the present northern subspecies is primarily a migratory lineage. The different migratory behaviours of the robin played an apparent role in maintaining the genetic structure at two different levels--one between the southern and northern lineages and the other between the sedentary and migratory groups within the northern lineage. While gene flow had long been maintained among the islands occupied by migratory individuals, migrants have been unable to contribute to the gene pool of the sedentary populations despite passing through the breeding range of such populations twice each year.     

Divergent host phenotypes create opportunities and constraints on the distribution of two wing‐dwelling feather mites

The diversity of symbionts (commensals, mutualists or parasites) that share the same host species may depend on opportunities and constraints on host exploitation associated with host phenotype or environment. Various host traits may differently influence host accessibility and within‐host population growth of each symbiont species, or they may determine the outcome of within‐host interactions among coexisting species. In turn, phenotypic diversity of a host species may promote divergent exploitation strategies among its symbiotic organisms. We studied the distribution of two feather mite species, Proctophyllodes sylviae and Trouessartia bifurcata, among blackcaps Sylvia atricapilla wintering in southern Spain during six winters. The host population included migratory and sedentary individuals, which were unequally distributed between two habitat types (forests and shrublands). Visual mite counts showed that both mite species often coexisted on sedentary blackcaps, but were seldom found together on migratory blackcaps. Regardless of host habitat, Proctophyllodes were highly abundant and Trouessartia were scarce on migratory blackcaps, but the abundance of both mite species converged in intermediate levels on sedentary blackcaps. Coexistence may come at a cost for Proctophyllodes, whose load decreased when Trouessartia was present on the host (the opposite was not true). Proctophyllodes load was positively correlated with host wing length (wings were longer in migratory blackcaps), while Trouessartia load was positively correlated to uropygial gland size (sedentary blackcaps had bigger glands), which might render migratory and sedentary blackcaps better hosts for Proctophyllodes and Trouessartia, respectively. Our results draw a complex scenario for mite co‐existence in the same host species, where different mite species apparently take advantage of, or are constrained by, divergent host phenotypic traits. This expands our understanding of bird–mite interactions, which are usually viewed as less dynamic in relation to variation in host phenotype, and emphasizes the role of host phenotypic divergence in the diversification of symbiotic organisms.     

Acoustic but no genetic divergence in migratory and sedentary populations of blackcaps,Sylvia atricapilla

In songbirds, territorial songs are key regulators of sexual selection and are learned from conspecifics. The cultural transmission of songs leads to divergence in song characteristics within populations, which can ultimately lead to speciation. Many songbirds also migrate, and individual differences in migratory behaviours can influence population genetic structure and local song differentiation. Blackcaps, Sylvia atricapilla, exhibit versatile territorial songs and show diversity in migration behaviours. They therefore comprise a good model for investigating the relationships between migratory patterns, song variation, and genetic diversity. We studied a migratory population (two groups near Paris) and a sedentary population (three groups in Corsica). All of the birds were ringed and blood sampled to investigate genetic relatedness using 17 microsatellite loci. A detailed song analysis showed that this species has a complex repertoire (> 100 syllables), which required the development of a semi‐supervised method to classify different categories of syllables and compare sequences of syllables. Our analysis showed no genetic structuring among populations: individuals belonging to the same group were not genetically closer than those from different groups. However, we found a strong wingsize difference between sedentary and migratory populations. We also showed that geographical variations in songs rely at least on both syllable and sequence content. Unexpectedly, despite a higher turnover of individuals, migratory groups share as many syllables and sequences as sedentary groups, which raises interesting issues on song learning and the maintenance of dialects in migratory birds.     

Postglacial northward expansion and genetic differentiation between migratory and sedentary populations of the broad‐tailed hummingbird (Selasphorus platycercus)

Unlike other migratory hummingbirds in North America, the broad‐tailed hummingbird (Selasphorus platycercus) exhibits both long‐distance migratory behaviour in the USA and sedentary behaviour in Mexico and Guatemala. We examined the evolution of migration linked to its northward expansion using a multiperspective approach. We analysed variation in morphology, mitochondrial and nuclear DNA, estimated migration rates between migratory and sedentary populations, compared divergence times with the occurrence of Quaternary climate events and constructed species distribution models to predict where migratory and sedentary populations resided during the Last Glacial Maximum (LGM) and Last Interglacial (LIG) events. Our results are consistent with a recent northward population expansion driven by migration from southern sedentary populations. Phylogeographical analyses and population genetics methods revealed that migratory populations in the USA and sedentary populations in Mexico of the platycercus subspecies form one admixed population, and that sedentary populations from southern Mexico and Guatemala (guatemalae) undertook independent evolutionary trajectories. Species distribution modelling revealed that the species is a niche tracker and that the climate conditions associated with modern obligate migrants in the USA were not present during the LIG, which provides indirect evidence for recent migratory behaviour in broad‐tailed hummingbirds on the temporal scale of glacial cycles. The finding that platycercus hummingbirds form one genetic population and that suitable habitat for migratory populations was observed in eastern Mexico during the LIG also suggests that the conservation of overwintering sites is crucial for obligate migratory populations currently facing climate change effects.     

Variation in the mechanical properties of flight feathers of the blackcap Sylvia atricapilla in relation to migration

Migration causes temporal and energetic constraints during plumage development, which can compromise feather structure and function. In turn, given the importance of a good quality of flight feathers in migratory movements, selection may have favoured the synthesis of feathers with better mechanical properties than expected from a feather production constrained by migration necessities. However, no study has assessed whether migratory behaviour affects the relationship between the mechanical properties of feathers and their structural characteristics. We analysed bending stiffness (a feather mechanical property which is relevant to birds’ flight), rachis width and mass (two main determinants of variation in bending stiffness) of wing and tail feathers in migratory and sedentary blackcaps Sylvia atricapilla. Migratory blackcaps produced feathers with a narrower rachis in both wing and tail, but their feathers were not significantly lighter; in addition, bending stiffness was higher in migratory blackcaps than in sedentary blackcaps. Such unexpected result for bending stiffness remained when we statistically controlled for individual variation in rachis width and feather mass, which suggests the existence of specific mechanisms that help migratory blackcaps to improve the mechanical behaviour of their feathers under migration constraints.     

GENETIC BASE AND EVOLUTIONARY CHANGE OF MIGRATORY DIRECTIONS IN A EUROPEAN PASSERINE MIGRANT SYLVIA ATRICAPILLA

Helbig, A. J. 1994. Genetic basis and evolutionary change of migratory directions in a European Passerine migrant Sylvia atricapilla. Ostrich 65: 157–159.

Early displacement experiments and more recent experimental studies with hand-raised Blackcaps Sylvia atricapilla are reviewed in order to describe our current state of understanding of the genetic basis and recent evolutionary changes of migratory directions in birds. Hand-raised Blackcaps from east and west of the Central European migratory divide, when tested under identical conditions, exibited population-specific migratory directions in orientation cages. Cross-breeding of birds from these two populations demonstrated an intermediate mode of inheritance of this behavioural character.

New data on the mitochondrial genetic population structure of the Blackcap indicate that population differentiation at the behavioural level is not necessarily related to long periods of geographic isolation (e.g. during ice ages). Migratory adaptations may have evolved recently, in some cases rapidly, as is illustrated by the establishment of a new migration route of central European Blackcaps to winter quarters in the British Isles. This new route is shown (in a captive breeding experiment) to be based on a novel, genetically determined WNW migratory direction that must have spread from almost zero to 7–11% frequency in parts of central Europe within only three decades. The inheritance of this novel trait also follows a phenotypically intermediate mode and is not influenced by the origin of the female parent (i.e. non-genetic factors can be excluded). The evolutionary flexibility of migratory adaptations is discussed in relation to changes in the environment, both natural and accelerated by man. The need is stressed to study the population-genetic mechanisms of such adaptation with the help of molecular markers as well as large-scale ringing on the breeding grounds.     

Compensatory frugivory in migratory Sylvia warblers: geographical responses to season length

Seasonal shifts in the diets of Garden Warblers Sylvia borin , Blackcaps S. atricapilla and Lesser Whitethroats S. curruca at a North German site were studied from samples obtained by flushing the digestive tract of the birds. The significance of seven species of berries and pulpy fruits in the diet of these species while feeding in riparian vegetation during the premigratory and migratory seasons was assessed and compared with the pattern of frugivory shown by the same species in southern Germany. Because of the short time span between hatching and onset of migration, first-year long- and medium-distance migratory Sylvia warblers from northern Europe are forced to accelerate their juvenile development. Presumably to meet the higher nutritional demands of an accelerated development, juvenile Garden Warblers and Blackcaps in northern Germany ate relatively more fruit during premigration than Blackcaps in southern Germany. Fruit occurred in about 90% of their diet samples during both premigration and migration. However, this might not hold true for experienced adult birds. Garden Warblers accelerated the rate of fattening and reduced the time needed to prepare for migration when feeding on a mixed fruit diet. There were indications that Blackcaps directly influenced the fruit choice of Garden Warblers when fruits preferred by both species were scarce. Blackcaps and Garden Warblers showed subtle differences in their seasonal preferences for black elder Sambucus nigra and woody nightshade Solanum dulcamara .     

Wing size but not wing shape is related to migratory behavior in a soaring bird

Both wing size and wing shape affect the flight abilities of birds. Intra and inter‐specific studies have revealed a pattern where high aspect ratio and low wing loading favour migratory behaviour. This, however, have not been studied in soaring migrants. We assessed the relationship between the wing size and shape and the characteristics of the migratory habits of the turkey vulture Cathartes aura, an obligate soaring migrant. We compared wing size and shape with migration strategy among three fully migratory, one partially migratory and one non‐migratory (resident) population distributed across the American continent. We calculated the aspect ratio and wing loading using wing tracings to characterize the wing morphology. We used satellite‐tracking data from the migratory populations to calculate distance, duration, speed and altitude during migration. Wing loading, but not aspect ratio, differed among the populations, segregating the resident population from the completely migratory ones. Unlike what has been reported in species using flapping flight during migration, the migratory flight parameters of turkey vultures were not related to the aspect ratio. By contrast, wing loading was related to most flight parameters. Birds with lower wing loading flew farther, faster, and higher during their longer journeys. Our results suggest that wing morphology in this soaring species enables lower‐cost flight, through low wing‐loading, and that differences in the relative sizes of wings may increase extra savings during migration. The possibility that wing shape is influenced by foraging as well as migratory flight is discussed. We conclude that flight efficiency may be improved through different morphological adaptations in birds with different flight mechanisms.     

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.     

Sex and age-specific differences in wing pointedness and wing length in blackcaps Sylvia atricapilla migrating through the southern Baltic coast

The blackcap Sylvia atricapilla shows a complex migratory pattern and is a suitable species for the studies of morphological migratory syndrome, including adaptations of wing shape to different migratory performance. Obligate migrants of this species that breed in northern, central, and Eastern Europe differ by migration distance and some cover shorter distance to the wintering grounds in the southern part of Europe/North Africa or the British Isles, although others migrate to sub-Saharan Africa. Based on ˃40 years of ringing data on blackcaps captured during autumn migration in the Southern Baltic region, we studied age- and sex-related correlations in wing pointedness and wing length of obligate blackcap migrants to understand the differences in migratory behavior of this species. Even though the recoveries of blackcaps were scarce, we reported some evidence that individuals which differ in migration distance differed also in wing length. We found that wing pointedness significantly increased with an increasing wing length of migrating birds, and adults had longer and more pointed wings than juvenile birds. This indicates stronger antipredator adaptation in juvenile blackcaps than selection on flight efficiency, which is particularly important during migration. Moreover, we documented more pronounced differences in wing length between adult and juvenile males and females. Such differences in wing length may enhance a faster speed of adult male blackcaps along the spring migration route and may be adaptive when taking into account climatic effects, which favor earlier arrival from migration to the breeding grounds.     

Resting metabolic rate in migratory and non‐migratory geese following range expansion: go south,go low

While many species suffer from human activities, some like geese benefit and may show range expansions. In some cases geese (partially) gave up migration and started breeding at wintering and stopover grounds. Range expansion may be facilitated and accompanied by physiological changes, especially when associated with changes in migratory behaviour. Interspecific comparisons found that migratory tendency is associated with a higher basal or resting metabolic rate (RMR). We compared RMR of individuals belonging to a migratory and a sedentary colony of barnacle geese Branta leucopsis. The migratory colony is situated in the traditional arctic breeding grounds (Russia), whereas the sedentary colony has recently been established in the now shared wintering area (the Netherlands). We measured RMR by oxygen consumption () during two ontogenetic phases (juvenile growth and adult wing moult). We also investigated juvenile growth rates and adult body mass dynamics. Mass‐independent was 13.6% lower in goslings from the sedentary colony than in goslings from the migratory colony. Similarly, in adult geese, mass‐independent was 15.5% lower in sedentary than in migratory conspecifics. Goslings in the Netherlands grew 36.2% slower than goslings in Russia, while we found no differences in body dimensions in adults. Adult geese from both colonies commenced wing moult with similar body stores, but whereas Russian barnacle geese maintained this level throughout moult, body stores in geese from the Netherlands fell, being 8.5% lower half‐way through the moult. We propose that the colony differences in resting metabolic rate, growth rate and body mass dynamics during moult can be explained by environmental and behavioural differences. The less stringent time constraints combined with poorer foraging opportunities allow for a smaller ‘metabolic machinery’ in non‐migratory geese. Our analysis suggests that range expansion may be associated with changes in physiology, especially when paired with changes in migratory tendency.     

Contrasting Patterns of Genetic Differentiation among Blackcaps (Sylvia atricapilla) with Divergent Migratory Orientations in Europe

Migratory divides are thought to facilitate behavioral, ecological, and genetic divergence among populations with different migratory routes. However, it is currently contentious how much genetic divergence is needed to maintain distinct migratory behavior across migratory divides. Here we investigate patterns of neutral genetic differentiation among Blackcap (Sylvia atricapilla) populations with different migratory strategies across Europe. We compare the level of genetic divergence of populations migrating to southwestern (SW) or southeastern (SE) wintering areas with birds wintering in the British Isles following a recently established northwesterly (NW) migration route. The migratory divide between SW and SE wintering areas can be interpreted as a result of a re-colonization process after the last glaciation. Thus we predicted greater levels of genetic differentiation among the SW/SE populations. However, a lack of genetic differentiation was found between SW and SE populations, suggesting that interbreeding likely occurs among Blackcaps with different migratory orientations across a large area; therefore the SW/SE migratory divide can be seen as diffuse, broad band and is, at best, a weak isolating barrier. Conversely, weak, albeit significant genetic differentiation was evident between NW and SW migrants breeding sympatrically in southern Germany, suggesting a stronger isolating mechanism may be acting in this population. Populations located within/near the SW/SE contact zone were the least genetically divergent from NW migrants, confirming NW migrants likely originated from within the contact zone. Significant isolation-by-distance was found among eastern Blackcap populations (i.e. SE migrants), but not among western populations (i.e. NW and SW migrants), revealing different patterns of genetic divergence among Blackcap populations in Europe. We discuss possible explanations for the genetic structure of European Blackcaps and how gene flow influences the persistence of divergent migratory behaviors.     

Do Healthy Monarchs Migrate Farther? Tracking Natal Origins of Parasitized vs. Uninfected Monarch Butterflies Overwintering in Mexico

Long-distance migration can lower parasite prevalence if strenuous journeys remove infected animals from wild populations. We examined wild monarch butterflies (Danaus plexippus) to investigate the potential costs of the protozoan Ophryocystis elektroscirrha on migratory success. We collected monarchs from two wintering sites in central Mexico to compare infection status with hydrogen isotope (δ ²H) measurements as an indicator of latitude of origin at the start of fall migration. On average, uninfected monarchs had lower δ ²H values than parasitized butterflies, indicating that uninfected butterflies originated from more northerly latitudes and travelled farther distances to reach Mexico. Within the infected class, monarchs with higher quantitative spore loads originated from more southerly latitudes, indicating that heavily infected monarchs originating from farther north are less likely to reach Mexico. We ruled out the alternative explanation that lower latitudes give rise to more infected monarchs prior to the onset of migration using citizen science data to examine regional differences in parasite prevalence during the summer breeding season. We also found a positive association between monarch wing area and estimated distance flown. Collectively, these results emphasize that seasonal migrations can help lower infection levels in wild animal populations. Our findings, combined with recent declines in the numbers of migratory monarchs wintering in Mexico and observations of sedentary (winter breeding) monarch populations in the southern U.S., suggest that shifts from migratory to sedentary behavior will likely lead to greater infection prevalence for North American monarchs.     

Habitat distribution of migratory and sedentary blackcaps Sylvia atricapilla wintering in southern Iberia: a morphological and biogeochemical approach

In migratory species, the way in which conspecifics from different breeding populations are distributed during the non‐breeding period is important from and ecological, evolutionary and conservation perspective, but such knowledge is still limited for most species. Migratory and sedentary blackcaps Sylvia atricapilla wintering in southern Spain can occupy two habitat types: forests and shrublands. According to earlier studies, blackcaps prefer forests over shrublands, and residents remain nearly restricted to forests. However, whether migrants with different breeding origin occupy the two habitats differently is unknown. We used morphological and biogeochemical data (hydrogen isotope ratios measured on feathers: δ²H_f), which show variation along the breeding range of the species, to answer this question. Isotope analyses supported the reliability of morphology as a method for distinguishing between migratory and sedentary blackcaps in sympatry, showing that sedentary individuals are rare in shrublands while migratory ones are abundant in both habitat types. However, migratory blackcaps scored similar δ²H_f values in forests and shrublands, and neither did vary in structural size or flight morphology between habitats. Our study suggests that migrants from a wide range of breeding origins end up mixing between forests and shrublands, which may explain the patterns of variation in space and time in the abundance of blackcaps in this area, and supports the view that inequalities may arise among migrants with the same origin but wintering in different habitats. Such inequalities might carry over into other stages of blackcaps’ life cycle contributing to the regulation of its migratory populations.     

Migratory shorebird adheres to Bergmann's Rule by responding to environmental conditions through the annual lifecycle

The inverse relationship between body size and environmental temperature is a widespread ecogeographic pattern. However, the underlying forces that produce this pattern are unclear in many taxa. Expectations are particularly unclear for migratory species, as individuals may escape environmental extremes and reorient themselves along the environmental gradient. In addition, some aspects of body size are largely fixed while others are environmentally flexible and may vary seasonally. Here, we used a long‐term dataset that tracked multiple populations of the migratory piping plover Charadrius melodus across their breeding and non‐breeding ranges to investigate ecogeographic patterns of phenotypically flexible (body mass) and fixed (wing length) size traits in relation to latitude (Bergmann's Rule), environmental temperature (heat conservation hypothesis), and migratory distance. We found that body mass was correlated with both latitude and temperature across the breeding and non‐breeding ranges, which is consistent with predictions of Bergmann's Rule and heat conservation. However, wing length was correlated with latitude and temperature only on the breeding range. This discrepancy resulted from low migratory connectivity across seasons and the tendency for individuals with longer wings to migrate farther than those with shorter wings. Ultimately, these results suggest that wing length may be driven more by conditions experienced during the breeding season or tradeoffs related to migration, whereas body mass is modified by environmental conditions experienced throughout the annual lifecycle.     

Body Size Variations in Caribou Ecotypes and Relationships With Demography

ABSTRACT In many vertebrates size is one of the most influential and variable individual characteristics and a strong determinant of reproductive success. Body size is generally density dependent and decreases when intraspecific competition increases. Frequent and long-distance movements increase energy expenditures and, therefore, may also influence body size, particularly in highly mobile species. Caribou (Rangifer tarandus, also known as reindeer) exhibit tremendous variation in size and movements and thus represent an excellent candidate species to test the relationships between body size, population size, and movements. We analyzed body measurements of adult female caribou from 7 herds of the Québec-Labrador Peninsula, Canada, and we related their morphology to population size, movements, and annual ranges. The herds represented 3 ecotypes (migratory, montane, and sedentary). Ecotypes and herds differed in size (length), shape (roundness), and movements. The sedentary ecotype was larger and moved 4 to 7 times less than the migratory ecotype in the 1990s. At the start of a demographic growth period in the early 1960s, migratory caribou from the Rivière-George (hereafter George) herd had longer mandibles than caribou of the sedentary ecotype. Mandible length in the George herd declined in the 1980s after rapid population growth, while individuals performed extensive movements and the herd's annual range increased. Migratory caribou then became shorter than sedentary caribou. After the George herd decline in the 1990s, mandible length increased again near levels of the 1980s. Caribou from the migratory Rivière-aux-Feuilles herd later showed a similar decline in mandible length during a period of population growth, associated with longer movements and increasing annual range. We hypothesize that the density-dependent effect observed on body size might have been exerted through summer habitat degradation and movement variations during herd growth. Our study has 2 important implications for caribou management: the distinctiveness of different populations and ecotypes, and the correlations between population trajectories and changes in body condition and habitat.     

Population differentiation of migratory directions in birds: comparison between ringing results and orientation behaviour of hand-raised migrants

Summary Blackcaps (Sylvia atricapilla) from five areas in Central Europe were hand-raised. Their autumn migratory orientation was tested in funnel-shaped cages. Their directional choices were compared to recoveries of conspecifics ringed during the breeding season in the same areas, which are situated on a transect across a migratory divide between southeastward and southwestward migrating populations. Results from ringing and orientation tests were in good agreement with respect to mean direction and dispersion of flight directions. An exception is the area around Linz (NW Austria), right on the migratory divide, where ringing yielded a strong scatter, but hand-raised birds chose westerly directions. The recent establishment of a novel migration route toward the British Isles was reflected in both data sets: in southern Germany the percentage of northwestward migrants is 6.8% according to orientation tests and 11.8% according to ringing recoveries. Testing the orientation of young passerines in captivity can yield valuable information about population differentiation of migratory behaviour. It is more efficient than ringing in this respect, because it circumvents the low recovery rates and is free of biases affecting ringing data. In the blackcap, geographic differentiation of migratory directions occurs on a finer scale than previously recognized and can change significantly within 2–3 decades.