Individual variation in orientation promotes a 3000‐km latitudinal change in wintering grounds in a long‐distance migratory raptor

Migrating juvenile birds rely on endogenous information in choosing the direction in which to fly, but such input may be overridden by social interactions with experienced individuals. We tagged seven juvenile Short‐toed Eagles Circaetus gallicus with GPS transmitters in southern Italy. This trans‐Saharan migrant flies mainly by soaring and is therefore not well adapted to performing long water crossings. Five of the seven tagged juveniles used the longer but apparently safer route towards the Strait of Gibraltar, and two migrated along a southerly trajectory and subsequently spent the winter in Sicily, apparently forced to do so by the 150‐km‐wide Sicily Channel. One of these individuals took the longer route the following autumn. These results, combined with long‐term (15 years) visual field observations involving thousands of individuals, suggest that inexperienced Short‐toed Eagles may learn their migratory routes from experienced adults, whereas some of them migrate south in response to an innate orientation instinct. Transport costs, inherited information and geography apparently interact, forcing some Short‐toed Eagles to winter 3000 km to the north of the majority of their conspecifics.     

Around the Mediterranean: an extreme example of loop migration in a long‐distance migratory passerine

An important issue in migration research is how small‐bodied passerines pass over vast geographical barriers; in European–African avian migration, these are represented by the Mediterranean Sea and the Sahara Desert. Eastern (passing eastern Mediterranean), central (passing Apennine Peninsula) and western (via western Mediterranean) major migration flyways are distinguished for European migratory birds. The autumn and spring migration routes may differ (loop migration) and there could be a certain level of individual flexibility in how individuals navigate themselves during a single migration cycle. We used light‐level loggers to map migration routes of barn swallows Hirundo rustica breeding in the centre of a wide putative contact zone between the northeastern and southernwestern European populations that differ in migration flyways utilised and wintering grounds. Our data documented high variation in migration patterns and wintering sites of tracked birds (n = 19 individuals) from a single breeding colony, with evidence for loop migration in all but one of the tracked swallows. In general, two migratory strategies were distinguished. In the first, birds wintering in a belt stretching from southcentral to southern Africa that used an eastern route for both the spring and autumn migration, then shifted their spring migration eastwards (anti‐clockwise loops, n = 12). In the second, birds used an eastern or central route to their wintering grounds in central Africa, shifting the spring migration route westward (clockwise loops, n = 7). In addition, we observed an extremely wide clockwise loop migration encompassing the entire Mediterranean, with one individual utilising both the eastern (autumn) and western (spring) migratory flyway during a single annual migration cycle. Further investigation is needed to ascertain whether clockwise migratory loops encircling the entire Mediterranean also occur other small long‐distance passerine species.     

The large‐scale drivers of population declines in a long‐distance migratory shorebird

Migratory species can travel tens of thousands of kilometers each year, spending different parts of their annual cycle in geographically distinct locations. Understanding the drivers of population change is vital for conserving migratory species, yet the challenge of collecting data over entire geographic ranges has hindered attempts to identify the processes leading to observed population changes. Here, we use remotely sensed environmental data and bird count data to investigate the factors driving variability in abundance in two subspecies of a long‐distance migratory shorebird, the bar‐tailed godwit Limosa lapponica. We compiled a spatially and temporally explicit dataset of three environmental variables to identify the conditions experienced by each subspecies in each stage of their annual cycle (breeding, non‐breeding and staging). We used a Bayesian N‐mixture model to analyze 18 years of monthly count data from 21 sites across Australia and New Zealand in relation to the remote sensing data. We found that the abundance of one subspecies L. l. menzbieri in their non‐breeding range was related to climate conditions in breeding grounds, and detected sustained population declines between 1995 and 2012 in both subspecies (L. l. menzbieri, –6.7% and L. l. baueri, –2.1% year^–1). To investigate the possible causes of the declines, we quantified changes in habitat extent at 22 migratory staging sites in the Yellow Sea, East Asia, over a 25‐year period and found –1.7% and –1.2% year^–1 loss of habitat at staging sites used by L. l. menzbieri and L. l baueri, respectively. Our results highlight the need to identify environmental and anthropogenic drivers of population change across all stages of migration to allow the formulation of effective conservation strategies across entire migratory ranges.     

Spatial and temporal variation in population trends in a long‐distance migratory bird

Aim Over the past three decades, evidence has been growing that many Afro‐Palaearctic migratory bird populations have suffered sustained and severe declines. As causes of these declines exist across both the breeding and non‐breeding season, identifying potential drivers of population change is complex. In order to explore the roles of changes in regional and local environmental conditions on population change, we examine spatial and temporal variation in population trajectories of one of Europe’s most abundant Afro‐Palaearctic summer migrants, the willow warbler, Phylloscopus trochilus. Location Britain and Ireland. Methods We use national survey data from Britain and Ireland (BBS: BTO/RSPB/JNCC Breeding Bird Survey and CBS: BWI/NPWS/Heritage Council Countryside Breeding Survey) from 1994 to 2006 to model the spatial and temporal variation in willow warbler population trends. Results Across Britain and Ireland, population trends follow a gradient from sharp declines in the south and east of England to shallow declines and/or slight increases in parts of north and west England, across Scotland and Ireland. Decreasing the spatial scale of analysis reveals variation in both the rate and spatial extent of population change within central England and the majority of Scotland. The rates of population change also vary temporally; declines in the south of England are shallower now than at the start of the time series, whereas populations further north in Britain have undergone periods of increase and decline. Main conclusion These patterns suggest that regional‐scale drivers, such as changing climatic conditions, and local‐scale processes, such as habitat change, are interacting to produce spatially variable population trends. We discuss the potential mechanisms underlying these interactions and the challenges in addressing such changes at scales relevant to migratory species.     

Inter‐annual variability and long‐term trends in breeding success in a declining population of migratory swans

Population declines among migratory Arctic‐breeding birds are a growing concern for conservationists. To inform the conservation of these declining populations, we need to understand how demographic rates such as breeding success are influenced by combinations of extrinsic and intrinsic factors. In this study we examined inter‐annual variation and long‐term trends in two aspects of the breeding success of a migratory herbivore, the Bewick's swan Cygnus columbianus bewickii, which is currently undergoing a population decline: 1) the percentage of young within the wintering population and 2) mean brood size. We used an information‐theoretic approach to test how these two measures of productivity were influenced over a 26 yr period by 12 potential explanatory variables, encompassing both environmental (e.g. temperature) and intrinsic (e.g. pair‐bond duration) factors. Swan productivity exhibited sensitivity to both types of explanatory variable. Fewer young were observed on the wintering grounds in years in which the breeding period (May to September) was colder and predator (Arctic fox) abundance was higher. The percentage of young within the wintering population also showed negative density‐dependence. Inter‐annual variance in mean swan brood size was best explained by a model comprised of the negative degree days during the swan breeding period, mean pair‐bond duration of all paired swans (i.e. mean pair duration), and an interaction between these two variables. In particular, mean pair duration had a strong positive effect on mean brood size. However, we found no long‐term directional trend in either measure of breeding success, despite the recent decline in the NW European population. Our results highlight that inter‐annual variability in breeding success is sensitive to the combined effects of both intrinsic and extrinsic factors.     

Finish with a sprint: Evidence for time‐selected last leg of migration in a long‐distance migratory songbird

Under time‐selected migration, birds should choose a strategy for outcompeting rivals over securing access to prime resources at the final destination. Thus, migration can be viewed as a race among individuals where winners are arriving first when conditions are suitable. The sprint migration hypothesis predicts that individuals shift from maximum sustained speed to a final burst of sprint to shorten the transition from migration to breeding (Alerstam, 2006). In this study, we test the hypothesis of a final sprint migration in a long‐distance Afro‐Palearctic migrant, the collared flycatcher Ficedula albicollis, during autumn and spring, and compare migration strategies between the seasons. In both seasons, collared flycatchers evidently exhibited sprint migration by increasing their overall speed over the last leg of migration after the Sahara crossing. This phenomenon was more pronounced in spring, contributing to overall faster spring migration and possibly highlighting higher importance for early arrival at the breeding grounds. In both seasons and particularly in spring, late departing individuals flew at a faster rate, partially being able to catch up with their early departing conspecifics. Differential fueling strategies may play an important role in determining migration speed, especially during the early stages of the migration, and might explain the observed differences in migration speeds between late and early departing individuals. Our findings suggest competition for early arrival at the breeding and at the nonbreeding destinations alike. Sprint migration might be an appropriate strategy to gain advantage over conspecifics and settle in prime territories as well as to cope with the increasingly earlier springs at high latitudes.     

Route optimisation and solving Zermelo's navigation problem during long distance migration in cross flows

The optimum path to follow when subjected to cross flows was first considered over 80 years ago by the German mathematician Ernst Zermelo, in the context of a boat being displaced by ocean currents, and has become known as the ‘Zermelo navigation problem’. However, the ability of migrating animals to solve this problem has received limited consideration, even though wind and ocean currents cause the lateral displacement of flyers and swimmers, respectively, particularly during long‐distance journeys of 1000s of kilometres. Here, we examine this problem by combining long‐distance, open‐ocean marine turtle movements (obtained via long‐term GPS tracking of sea turtles moving 1000s of km), with a high resolution basin‐wide physical ocean model to estimate ocean currents. We provide a robust mathematical framework to demonstrate that, while turtles eventually arrive at their target site, they do not follow the optimum (Zermelo's) route. Even though adult marine turtles regularly complete incredible long‐distance migrations, these vertebrates primarily rely on course corrections when entering neritic waters during the final stages of migration. Our work introduces a new perspective in the analysis of wildlife tracking datasets, with different animal groups potentially exhibiting different levels of complexity in goal attainment during migration.     

“Closer‐to‐home” strategy benefits juvenile survival in a long‐distance migratory bird

Human‐induced changes in the climate and environment that occur at an unprecedented speed are challenging the existence of migratory species. Faced with these new challenges, species with diverse and flexible migratory behaviors may suffer less from population decline, as they may be better at responding to these changes by altering their migratory behavior. At the individual level, variations in migratory behavior may lead to differences in fitness and subsequently influence the population's demographic dynamics. Using lifetime GPS bio‐logging data from 169 white storks (Ciconia ciconia), we explore whether the recently shortened migration distance of storks affects their survival during different stages of their juvenile life. We also explore how other variations in migratory decisions (i.e., time, destination), movement activity (measured using overall body dynamic acceleration), and early life conditions influence juvenile survival. We observed that their first autumn migration was the riskiest period for juvenile white storks. Individuals that migrated shorter distances and fledged earlier experienced lower mortality risks. In addition, higher movement activity and overwintering “closer‐to‐home” (with 84.21% of the tracked individuals stayed Europe or North Africa) were associated with higher survival. Our study shows how avian migrants can change life history decisions over only a few decades, and thus it helps us to understand and predict how migrants respond to the rapidly changing world.     

Broad wintering range and intercontinental migratory divide within a core population of the near‐threatened pallid harrier

Aim To identify the migration routes and wintering grounds of the core populations of the near‐threatened pallid harrier, Circus macrourus, and highlight conservation needs associated with these phases of the annual cycle. Location Breeding area: north‐central Kazakhstan; Wintering areas: Sahel belt (Burkina Faso to Ethiopia) and north‐west India. Methods We used ring recovery data from Kazakhstan and satellite tracking data from 2007 to 2008 on six adults breeding in north‐central Kazakhstan to determine migration routes and locate wintering areas. In addition, one first‐year male was tagged in winter 2007–2008 in India. Results Data evidenced an intercontinental migratory divide within the core pallid harrier population, with birds wintering in either Africa or India. The six individuals tagged in north‐central Kazakhstan followed a similar route (west of the Caspian Sea and Middle East) towards east Africa, before spreading along the Sahel belt to winter either in Sudan, Ethiopia, Niger or Burkina Faso. Spring migration followed a shorter, more direct route, with marked interindividual variation. The bird tagged in India spent the summer in central Kazakhstan. Half of the signal losses (either because of failure or bird mortality) occurred on the wintering areas and during migration. Main conclusions Our study shows that birds from one breeding area may winter over a strikingly broad range within and across continents. The intercontinental migratory divide of pallid harriers suggests the coexistence of distinct migratory strategies within the core breeding population, a characteristic most likely shared by a number of threatened species in central Asia. Conservation strategies for species like the pallid harrier, therefore, require considering very large spatial scales with possibly area‐specific conservation issues. We highlight urgent research priorities to effectively inform the conservation of these species.     

Impacts of nest predators and weather on reproductive success and population limitation in a long‐distance migratory songbird

Although avian nesting success is much studied, little is known about the relative importance of the factors that contribute to annual reproductive success and population limitation, especially for long‐distance migratory songbird species. We combined a field experiment limiting access to nests by mammalian predators with modeling of long‐term field data of American redstarts (Parulidae: Setophaga ruticilla) to assess the effects of multiple environmental variables on breeding success and population limitation. Experimental treatment (baffles placed around tree boles beneath active nests; n = 71) increased nesting success of this single‐brooded species significantly (77 vs 50% in controls; n = 343), demonstrating that scansorial mammals, primarily red squirrels Tamiasciurus hudsonicus and eastern chipmunks Tamias striatus, reduced reproductive success. Based on unbaffled nests (n = 466), daily nest survival varied annually, and was positively influenced by May temperature and negatively by sciurid nest predator abundance. Daily nest survival was also influenced positively by June rainfall, and declined with nest age but not with calendar date. Since nest failure was overwhelmingly caused by nest predation, these significant climate and nest‐age effects in our models are indirect, likely influencing nest predator and/or nesting bird behaviors that in turn influenced nest predation. Redstart population density had no effect on nesting success, after accounting for other factors. Annual reproductive success accounted for 34% of the variability in annual population change in redstarts in our study area. Our findings document 1) breeding season population limitation in this species, 2) a link between tree masting and bird population dynamics via mammal population fluctuations, 3) the independent contributions of summer versus winter population processes in a migratory species, and 4) the potential complexity of climate‐biotic interactions.     

Large‐scale geographic variation in iridescent structural ornaments of a long‐distance migratory bird

Iridescent colours produced during moult likely play an important role in pair formation in birds. We sought to quantify geographic variation in such colouration in a duck species, Eurasian teal Anas crecca, in winter (when mating occurs) to evaluate whether this variation reflects birds’ breeding origins or differential individual migration strategies in both males and females. We combined information on feather production region and individual attributes (body size, sex and age) of Eurasian teal from 82 wintering sites in France. Feather production region (moult site or natal origin) was inferred using feather deuterium values (δD_f). We performed spectral measurements to evaluate speculum colour and brightness contrasts for 1052 teal collected over four years. Colouration differed strongly among wintering regions, with birds wintering in eastern France exhibiting higher colour contrast than those wintering in the west. Body size and colouration were positively related. There were no differences in cohort‐specific δD_f values between separate wintering regions in France, indicating that within a winter quarter teal originated from areas across the entire breeding range. Overall, patterns of spatial variation in feather colouration were related most closely to body size which was consistent with predictions of a differential migration hypothesis, with larger and more colour‐contrasting birds wintering closer to their breeding grounds. Because moult speed is also known to affect colour production, early breeders or individuals that skipped reproduction may have invested more or earlier in their feather quality to gain potential advantages in monopolizing future mates.     

Form,function and consequences of density dependence in a long‐distance migratory bird

The density dependence of demographic parameters and its implications for population regulation have long been recognized. Recent work has revealed potential effects of density on mating systems and sexual selection, but few studies concurrently assess the consequences of density on both demography and sexual selection. Such an approach is important because population processes and individual behaviors can interact to influence population growth and evolutionary trajectories. In this study, we tested the density dependence of breeding success, extra‐pair paternity, and the opportunity for sexual selection in a population of American redstarts Setophaga ruticilla using two different measures of density. To evaluate temporal patterns, we analyzed annual territory density, based on the total number of territories at our study site each year. To evaluate spatial patterns, we analyzed local territory density within years, based on the number of territories surrounding a focal territory. Greater annual density was associated with fewer offspring fledged per female, a reduced mean population rate of fledging success, and a lower relative contribution of extra‐pair paternity to male fitness. Greater local density was associated with fewer offspring fledged, reduced fledgling success, higher rates of nest loss, and higher rates of paternity loss on focal territories. Interestingly, greater local density was also associated with greater nestling mass on focal territories, which could imply that more densely‐packed territories contain superior resources. Overall, our results suggest that the effects of crowding via greater territory density reduce fecundity through increased nest predation, rather than reduced food availability, and increase rates of extra‐pair paternity. Thus, the selective pressures faced by individuals and their reproductive behaviors are likely to differ based on the annual and local density they experience, which may then feed back into population demography.     

Adaptive fault bar distribution in a long‐distance migratory,aerial forager passerine?

Fault bars are translucent bands produced by stressful events during feather formation. They weaken feathers and increase their probability of breakage, and thus could compromise bird fitness by lowering flight performance. It has been recently suggested ('fault bar allocation hypothesis') that birds could have evolved adaptive mechanisms for reducing fault bar load on the feathers with the highest function during flight. We tested this hypothesis by studying first-year individuals of the long-distance migratory, aerial forager barn swallow Hirundo rustica . We predicted that fault bars should be less abundant on the outermost wing and tail feathers, but more frequent on the tail than on the outermost wing feathers. Accordingly, we found that fault bars occurred more often in tertials than in primaries or secondaries. Tail feathers had fewer fault bars than tertials, but more than primaries. Within the tail, the distribution pattern of fault bars was W-shaped, with the highest fault bar load occurring on the streamers and on the two central feathers. Because streamers are the most important tail feathers for flight performance, this finding seems to contradict the 'fault bar allocation hypothesis'. However, flight performance is much less sensitive to changes in the shape of the tail than of the wings, which could explain why evolutionary forces have not counteracted the increase of fault bars associated with feather elongation during the recent evolution of streamers in the tail of hirundines.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 85 , 455–461.     

Longitudinal data reveal ontogenetic changes in the wing morphology of a long‐distance migratory bird

In migratory bird species, juveniles normally have shorter and more rounded wings than adults. The causes of this age‐specific difference in wing morphology, however, are largely unknown. Here, we used longitudinal data collected over 3 years from a Pied Flycatcher Ficedula hypoleuca population to assess whether age‐related differences in wing morphology are a consequence of ontogenetic changes or of selection favouring birds with longer and more pointed wings. Our study provides evidence of ontogenetic changes in wing length and shape, whereby birds grow longer and more pointed wings as they grow older. Age‐dependent variation is likely to be adaptive and may partly explain age differences in spring migration phenology and breeding success.     

Increase in protandry over time in a long‐distance migratory bird

Protandry is a widespread life‐history phenomenon describing how males precede females at the site or state of reproduction. In migratory birds, protandry has an important influence on individual fitness, the migratory syndrome, and phenological response to climate change. Despite its significance, accurate analyses on the dynamics of protandry using data sets collected at the breeding site, are lacking. Basing our study on records collected during two time periods, 1979 to 1988 and 2006 to 2016, we aim to investigate protandry dynamics over 38 years in a breeding population of willow warblers (Phylloscopus trochilus). Change in the timing of arrival was analyzed in males and females, and protandry (number of days between male and female arrival) was investigated both at population level and within breeding pairs. Our results show advancement in the arrival time at the breeding site in both sexes, but male arrival has advanced to a greater extent, leading to an increase in protandry both at the population level and within breeding pairs. We did not observe any change in sex ratio that could explain the protandry increase, but pronounced temperature change has occurred and been reported in the breeding area and along the migratory route. Typically, natural selection opposes too early arrival in males, but given warmer springs, this counteracting force may be relaxing, enabling an increase in protandry. We discuss whether our results suggest that climate change has induced sex‐specific effects, if these could be evolutionary and whether the timing of important life‐history stages such as arrival at the breeding site may change at different rates in males and females following environmental shifts.     

Early arrival is not associated with more extra‐pair fertilizations in a long‐distance migratory bird

When assessing the benefits of early arrival date of migratory birds, a hidden and often ignored component of males’ fitness is the higher chance of early‐arriving birds to obtain extra‐pair fertilizations. Here we investigated how extra‐pair paternity might affect the relationship between male arrival date and number of fertilizations in a model study system, the European pied flycatcher Ficedula hypoleuca. For this purpose, we sampled and genotyped breeding pairs, unpaired males and offspring (including embryos from unhatched eggs when possible) of a Dutch pied flycatcher population. Detailed information on arrival date of males, egg laying date of their social mates and nest success was also recorded. Early‐arriving males had early‐laying females and males with early‐laying females had a higher probability of siring extra‐pair eggs and obtain more fertilizations. However, male arrival date alone did not correlate with the probability to gain extra‐pair paternity and neither to the amount of fertilized eggs. Both early‐ and late‐arriving males had a higher probability of losing paternity in their own nest compared to birds with an intermediate arrival date. Finally, late‐arriving males were more likely to remain unpaired but, interestingly, a few of these birds obtained paternity via extra‐pair copulations. Because earlier arrival date did not lead to more extra‐pair fertilizations and because such relationship seems to be driven mainly by the female's laying date, we conclude that the contribution of extra‐pair paternity to the overall fitness benefits of early male arrival date is relatively small.     

Effects of blood parasite infections on spatiotemporal migration patterns and activity budgets in a long‐distance migratory passerine

How blood parasite infections influence the migration of hosts remains a lively debated issue as past studies found negative, positive, or no response to infections. This particularly applies to small birds, for which monitoring of detailed migration behavior over a whole annual cycle has been technically unachievable so far. Here, we investigate how bird migration is influenced by parasite infections. To this end, we tracked great reed warblers (Acrocephalus arundinaceus) with multisensor loggers, characterized general migration patterns as well as detailed flight bout durations, resting times and flight heights, and related these to the genus and intensity of their avian haemosporidian infections. We found migration distances to be shorter and the onset of autumn migration to be delayed with increasing intensity of blood parasite infection, in particular for birds with Plasmodium and mixed‐genus infections. Additionally, the durations of migratory flight bout were prolonged for infected compared to uninfected birds. But since severely infected birds and particularly birds with mixed‐genus infections had shorter resting times, initial delays seemed to be compensated for and the timing in other periods of the annual cycle was not compromised by infection. Overall, our multisensor logger approach revealed that avian blood parasites have mostly subtle effects on migratory performance and that effects can occur in specific periods of the year only.     

Density‐independent predation affects migrants and residents equally in a declining partially migratory elk population

Migration is expected to benefit individuals through exposure to higher quality forage and reducing predation rates more than non‐migratory conspecifics. Previous studies of partially migratory ungulates (with migrant and resident individuals) have focused on bottom–up factors regulating resident and migrant segments, yet differential predation between strategies could also be a density‐dependent regulatory mechanism. Our study tested for density‐dependence in mortality, as well as mechanisms of ­bottom–up or top–down regulation in the resident and migrant portions of the partially migratory Ya Ha Tinda elk population. We tested for density dependence in adult female and juvenile survival rates, and then discriminated between predator‐ and food‐regulation hypotheses by testing for density‐dependence amongst mortality causes for adult female elk. Notably, the population declined almost 70% from near previously published estimates of carrying capacity over 10 years, providing ideal conditions to test for density dependence. In contrast to predictions, we found only weak support for density dependence in adult survival and juvenile survival. We also found few differences between migrant and resident elk in adult or juvenile survival, though juvenile survival differences were biologically significant. Predation by humans and grizzly bears was density dependent, but similar between migratory strategies. Predation by wolves was the leading known cause of mortality, yet remained constant with declining elk density equally for both migrant and resident elk, indicating wolf predation was density‐independent. Instead of being strongly regulated by food or predation, we found adult female survival was driven by density‐independent predation and climatic factors. The few differences between migratory strategies suggest equivalent fitness payoffs for migrants and residents. This population is being limited by density‐independent predation leading to declines of both migratory strategies. Our results challenge classical predator–prey theory, and call for better integration between predator–prey and migration theory.