The research team, led by Jin Liu and Shaopeng Wang of Peking University together with colleagues at the Senckenberg Biodiversity and Climate Research Centre, the University of Massachusetts Amherst and the University of Queensland, assembled 4,351 population-level estimates of migration timing spanning 549 bird species across the globe. Each timing estimate was paired with corresponding population trend data and with rates of climate change measured at both the breeding and non-breeding grounds of the populations involved. This pairing is what allowed the authors to move beyond the long-standing observation that migration is shifting and ask, for the first time at a global scale, whether those shifts actually help birds survive and reproduce.

The scale of the phenological change is striking. Drawing on evidence that bird migration has been advancing at an accelerating pace over the past century, the study quantified a cumulative advancement of nearly fourteen days between 1970 and 2019. That figure represents an average across hundreds of species and dozens of migratory flyways, and the underlying variation is considerable. Some populations have barely moved their schedules; others have shifted by weeks. The researchers used phylogenetically controlled Bayesian models, specifically MCMCglmm analyses, to estimate change rates while accounting for the fact that related species share evolutionary histories that can confound simple comparisons.

To interpret the patterns, the team coupled their empirical database with a full-annual-cycle theoretical model, a mathematical framework that tracks a migratory population through all four phases of its year: pre-breeding migration, breeding, post-breeding migration and the non-breeding season. The model incorporates daily maximal energy intake rates during breeding and non-breeding stages and coefficients representing energy shortfalls caused by phenological mismatching, the situation in which the timing of one life-history event falls out of sync with the environmental resources it depends on. By simulating how shifts in migration timing propagate through the annual cycle, the model provides a mechanistic bridge between observed behavioural change and observed population trajectories.

The central result is a clean seasonal split. Populations whose pre-breeding, or spring, migration has advanced rapidly tend to show more favourable abundance trends, consistent with the idea that arriving earlier on breeding grounds allows birds to exploit the earlier flush of spring productivity, secure better territories and time their breeding with peak food availability. Competition for early arrival is a well-documented force in migratory birds, and individuals that reach breeding grounds first often enjoy disproportionate reproductive success. In a warming world, where vegetation green-up and insect emergence are shifting earlier, the ability to advance spring migration may determine whether a population can track the moving window of resource abundance.

The autumn picture is the mirror opposite. Rapid advancement of post-breeding migration was associated with detrimental effects on abundance. The authors suggest several non-exclusive explanations rooted in migration ecology. Autumn departure timing governs access to resources on the non-breeding grounds and the condition in which birds arrive there. Leaving breeding areas too early may shorten the period available for moulting, fuelling and juvenile development, imposing carry-over costs that echo through the following seasons. Autumn migration is also shaped by different selective pressures than spring, including wind patterns, precipitation and the demands of the non-breeding period, and the consequences of shifting it are correspondingly different.

Crucially, the net demographic benefit for a population depends on the balance between the two seasons. The study shows that the advantage accrues only when advancement during pre-breeding migration outpaces advancement during post-breeding migration. Species that have shifted both journeys earlier in lockstep gain little, and species whose autumn advancement dominates may actually be losing ground. This finding reframes the familiar narrative that flexible, responsive migrants are the climate-change winners. Flexibility, it turns out, is not uniformly beneficial; its value is season dependent, and the full annual cycle must be considered before declaring a population well adapted.

Body size emerged as another important modulator. The benefit of rapid advancement was more pronounced in large-bodied species, and the analysis found that the rate of pre-breeding migration advancement itself varies with body mass. Larger birds often face different energetic constraints, with greater fuel-storage capacity but also higher absolute energy demands, and their life histories tend to sit at the slower end of the pace-of-life spectrum. The interaction between body size, migratory plasticity and demographic outcome adds a trait-based dimension to the study, suggesting that predictions about which species will cope with continued warming should incorporate morphology as well as behaviour.

The methodological rigour underpinning these conclusions is considerable. Population trends were assessed in two complementary ways, using categorical assessments aligned with the IUCN Red List and continuous rates of population change estimated from time-series abundance databases including the Living Planet Index, BioTIME and TetraDENSITY. Climate exposure was quantified with the Climatic Research Unit Time Series version 4.06 temperature and precipitation data, while vegetation phenology was captured through the PKU GIMMS NDVI product. Species traits came from AVONET and BIRDBASE, distributions from BirdLife DataZone, and phylogenetic structure from BirdTree. The authors also tested for publication bias and used multiple imputation to handle missing covariates, and the full database and analysis code are publicly available, an increasingly important standard for synthetic studies of this kind.

For conservation, the implications are sobering. Migratory species are declining worldwide, and the new results indicate that behavioural flexibility alone will not rescue them. A population can advance its spring migration impressively and still decline if its autumn schedule shifts in ways that undermine survival on the non-breeding grounds or during the journey itself. The authors argue that their findings underscore the importance of full-annual-cycle perspectives for understanding and conserving migratory species under global change, a call that echoes a growing movement in migration ecology to link events across seasons and continents rather than studying breeding grounds in isolation. As warming continues to compress and reshuffle the seasonal calendar, the species that thrive will be those whose entire annual programme, not just their spring departure, remains matched to the environments they traverse.

Subject of Research: Season-dependent effects of advancing bird migration timing on population abundance under climate change

Article Title: Avian migration timing advances linked to season-dependent population benefits

Article References: Liu, J., Xu, W., Fuller, R. A., Mueller, T., Yang, Q., & Wang, S. (2026). Avian migration timing advances linked to season-dependent population benefits. Nature Ecology & Evolution. https://doi.org/10.1038/s41559-026-03198-9

Image Credits: AI Generated

DOI: 10.1038/s41559-026-03198-9

Keywords: avian migration, phenology, climate change, population trends, migratory birds, phenological mismatch, full annual cycle, Nature Ecology & Evolution, bird conservation, spring migration, autumn migration, body mass

The research, published in Ecology and Evolution, combined three kinds of data that are rarely brought together at this scale. Between 2008 and 2022, the team fitted 306 adult Bobolinks with light-level geolocators weighing just 0.65 grams, retrieving 43 of the devices in subsequent years. From these, 29 birds with usable tracks and blood samples formed the core dataset, comprising 20 males and 9 females whose full annual cycles were reconstructed. The geolocators recorded ambient light, allowing researchers to estimate latitude and longitude with average errors of about 33 kilometers north-south and 21 kilometers east-west. Meanwhile, microsatellite markers from blood samples provided a picture of each bird’s genetic background, and historical climate reanalysis data from the European Centre for Medium-Range Weather Forecasts supplied daily temperature, precipitation, and pressure values along every route.

To distill a multi-continent journey into something statistically manageable, the researchers defined nine distinct migration events, from the first consistent southward movement of fall to the arrival back on the breeding grounds in spring. Seven of these events were analyzed directly, each with two response variables: the date of the event and its latitude. The team then asked which factors best explained variation in timing and location, testing composite genetic scores, sex, carryover effects from earlier stages of the annual cycle, and local weather conditions. Genetic relatedness was summarized using a paired ordination approach that combined principal components analyses of individual microsatellite loci through a multiple co-inertia analysis, producing three composite genotype scores that together captured 30 percent of the population’s genetic variation.

The results were clearest at the bookends of migration. The start of fall migration was significantly correlated with a bird’s composite genotype score, which alone accounted for roughly 22 percent of the variation in departure date. Males left earlier than females, departing on average around day 256 of the year compared with day 264 for females, and males also departed from higher latitudes. Genetics resurfaced at the other end of the cycle as well: the timing of winter departure, which marks the start of spring migration, was significantly linked to the first composite genotype score, with genetically similar birds leaving their wintering grounds on similar dates. Even the timing of arrival in South America, after the perilous overwater crossing, showed a genetic correlation, as did the timing of arrival back on the breeding grounds in spring.

By contrast, the evidence for weather as a driver of migration decisions was remarkably thin. Temperature, precipitation, and pressure did appear as significant terms in several models, but on closer inspection these correlations mostly tracked seasonal patterns, such as cooler temperatures at later dates in the North American fall, or reflected underlying differences between the sexes rather than genuine responses to weather. The researchers note that if weather were truly governing migration behavior, they would have expected consistent temperatures within each event or warmer conditions driving earlier departures, in line with broader trends of advancing migration phenology. Instead, most birds avoided flying in rain and experienced little to no precipitation before departing a migration event, suggesting that weather may shape decisions at spatial or temporal scales finer than geolocators can resolve.

One of the study’s most unexpected findings concerns sex. Males and females differed consistently in both timing and location throughout the annual cycle, patterns that had gone undetected in Bobolinks partly because the species’ nonbreeding plumage is not sexually dimorphic. During fall migration, females staged farther north than males and over a narrower range of latitudes, with 78 percent of females using the breeding grounds themselves as their pre-water-crossing staging sites, likely because their larger share of parental care leaves them needing more recovery time before departure. Males, meanwhile, congregated around latitude 38 degrees North, the most commonly used staging region, before launching flights of roughly 1,000 kilometers over water toward the Caribbean. Sexual segregation persisted onto the wintering grounds, where males wintered farther north and over a smaller latitudinal range than females.

Carryover effects proved to be a powerful organizing principle of the annual cycle. Birds that started fall migration early arrived at their pre-water-crossing staging grounds early, and the date of departure from South America strongly predicted the timing of breeding ground arrival, explaining about 32 percent of the variation in that event. Because male Bobolinks are polygynous and early arrival confers advantages in claiming high-quality territories capable of supporting multiple females, these cascading delays carry real fitness consequences. Males departing South America significantly earlier than females, around day 120 versus day 132, fit the well-documented pattern of protandry, which tends to be pronounced in populations with high rates of extra-pair paternity, as is the case in this Vermont population.

The conservation implications are sobering. Bobolinks number roughly 10 million individuals globally, but their population is projected to decline by half by 2069, with grassland habitat loss as the leading threat. If the timing of migration is genetically hardwired, birds may be unable to adjust quickly enough as climate change reshapes the environments they depend on. In Vermont, farmers have already advanced their haying dates by about 10 days in response to a changing climate, yet Bobolinks have not shifted their nest initiation dates, creating a widening phenological mismatch that brings mowing machines into ever-greater conflict with active nests. A migration calendar locked in by inheritance could similarly leave the species vulnerable if food availability and habitat conditions along the route shift faster than microevolution can respond.

The study’s authors caution that geolocator research carries an inherent bias: only birds that survived the full annual cycle were recovered, so the behavior of individuals that perished en route remains invisible. Experiments with captive migrants have shown that food availability can modulate the onset of endogenous migratory urges, meaning environmental conditions may still fine-tune the genetically anchored schedule in ways this study could not detect. Nevertheless, the work stands as one of the first to test genetic and environmental influences on a long-distance songbird’s behavior across its entire annual cycle using neutral genetic markers. The picture that emerges is of a bird whose most consequential migration decisions, when to leave, when to cross the water, and when to return home, are inscribed in its genome, with flexibility reserved mainly for the finer details of the journey.

Subject of Research: Genetic and environmental control of annual-cycle migration behavior in the Bobolink, a long-distance migratory songbird

Article Title: Genetics, Not Weather, Informs Migration Behaviors in a Long‐Distance Migrant Songbird

Article References: McGee, K. E., Travis, S. E., Strong, A. M., & Perlut, N. G. (2026). Genetics, Not Weather, Informs Migration Behaviors in a Long‐Distance Migrant Songbird. Ecology and Evolution, 16(9), Article e74245. https://doi.org/10.1002/ece3.74245

Image Credits: AI Generated

DOI: 10.1002/ece3.74245

Keywords: Bobolink, bird migration, migratory timing, genetics, geolocators, microsatellites, annual cycle, carryover effects, climate change, phenological mismatch, sexual segregation, grassland birds

The research team, led by Dieter J. A. Heylen of the Evolutionary Ecology Group at the University of Antwerp, together with colleagues from the Luxembourg Institute of Health and certified bird ringers in Belgium, examined 284 redwings captured during late-autumn migration in 2022 and 2024. Both years were marked by unusually mild autumn conditions, a detail that proved central to interpreting the results. Of the birds sampled, 72.9 percent were infested with ticks, a rate the authors describe as the highest ever reported for this species anywhere in Europe. The overwhelming majority of the parasites collected were Ixodes ricinus, the castor bean tick whose blood-feeding stages are responsible for transmitting the majority of tick-borne infections to humans and domestic animals across the European continent.

The scale of pathogen carriage among those ticks was equally striking. The team screened 698 ticks removed from the migrating birds and found that 12.9 percent carried Borrelia bacteria. The infections were not confined to a single lineage. Genetic screening identified multiple genospecies of the Borrelia burgdorferi sensu lato complex, including strains associated with birds and others associated with mammals, alongside Borrelia miyamotoi, a relapsing-fever spirochete increasingly recognized as a human pathogen. Perhaps most telling, infected ticks were found on nearly one-third of the birds examined, meaning that a large proportion of the migrating population was effectively functioning as a long-distance courier service for viable, pathogen-laden parasites.

Extrapolating from their field measurements to the size of the European redwing population, the researchers estimate that tens of millions of Borrelia-infected ticks may be transported southward each autumn on the backs of these migrating thrushes. Ticks attached to birds can detach at stopover sites hundreds or thousands of kilometers from where they were acquired, seeding new locations with both the arthropods themselves and the pathogens they carry. Because migratory birds travel enormous distances in short periods, they represent one of the most efficient dispersal mechanisms available to ticks, whose own locomotion over a lifetime rarely exceeds a few meters. The study thus adds a seasonal dimension to what ecologists already understood about birds as tick vectors: the phenomenon does not stop when summer ends.

The timing of the infestations is what gives the work its conceptual force. Migratory birds calibrate the onset and pace of their journeys primarily by photoperiod, the changing length of daylight, which is an astronomical cue immune to year-to-year weather. Tick activity, by contrast, is governed largely by ambient temperature, since ticks are ectotherms whose questing behavior halts when conditions become cold. Historically, late-autumn temperatures across much of western and central Europe fell low enough to push ticks into dormancy well before the main passage of redwings, minimizing the overlap between the two biological calendars. The Belgium study, conducted during two exceptionally mild Novembers, documents what happens when that buffer disappears: host and parasite schedules collide, and the birds depart with an unintended cargo.

This divergence between climatically driven parasite activity and photoperiodically driven migration is a textbook example of a phenological mismatch, and the study’s authors argue that continued climate warming is likely to widen it in the parasites’ favor. Migration timing is comparatively stable, anchored to day length, while the tick activity season expands at both ends of the year as winters warm. Each increment of autumn warming therefore extends the period during which questing ticks can board migrating hosts. The consequence, according to the researchers, is a strengthening seasonal window for long-distance parasite dispersal, one that epidemiological models of Lyme disease risk have largely failed to incorporate because they treat late autumn as an epidemiological dead zone.

The age structure of the infestations added a further layer of biological insight. Birds in their first calendar year, that is, individuals hatched the same summer and undertaking their first migration, carried significantly higher tick loads than older birds. The authors suggest several non-exclusive explanations, including the inexperience of young birds in grooming and preening parasites effectively, behavioral differences in habitat use that bring first-year birds into closer contact with questing ticks, and possible physiological differences in skin characteristics that affect how readily ticks attach and feed. Whatever the mechanism, the pattern means that a disproportionate share of the dispersing parasite burden travels on the youngest cohort, the very individuals that make up the bulk of many migratory populations and that colonize wintering areas before returning north in spring.

The pathogens detected on these birds are of more than academic concern. Borrelia burgdorferi sensu lato encompasses the genospecies responsible for the great majority of Lyme borreliosis cases in Europe, an infection that can produce erythema migrans rashes, neurological complications, arthritis and cardiac involvement when untreated. Borrelia miyamotoi, though historically overshadowed by its Lyme-causing relatives, has been implicated in human cases of a relapsing-fever-like illness and, in immunocompromised patients, severe meningoencephalitis. The identification of both avian-associated and mammalian-associated genospecies on migrating redwings underscores the role of birds in maintaining and redistributing genetically diverse pathogen reservoirs, with consequences for the local disease ecology of every stopover and wintering ground along the flyway.

It is important to note that the study does not claim that migrating birds directly infect humans. Ticks feeding on birds do not transmit Borrelia to their hosts in ways that pose immediate public-health danger at the moment of capture. The risk pathway is indirect: infected ticks detach in new environments, and if those environments support the tick’s life cycle and suitable reservoir hosts, the parasites and their pathogens can establish locally. A southbound redwing that drops an infected larva or nymph in a French hedgerow, an Iberian woodland or a Mediterranean scrubland effectively introduces Borrelia genetic material into a new ecological arena, where subsequent feeding cycles on local wildlife can amplify and maintain the pathogen. The study’s contribution is to show that this introduction channel remains open deep into the season previously considered safe.

The work also carries a methodological message for disease surveillance. Monitoring programs for tick-borne pathogens in Europe typically concentrate their efforts on spring and early summer, when human exposure risk peaks and tick activity is maximal. The Belgian findings argue for extending surveillance into the late-autumn migration window, particularly during warm years, to capture a dispersal event of continental significance. With both sampling years of the study falling among the mildest autumns on record, and with climate projections pointing toward more frequent such conditions, the overlap between southbound migration and active, infected ticks may shift from an anomaly to a routine feature of the European disease landscape. What the redwings reveal, in effect, is that the geography and seasonality of Lyme disease risk are being quietly rewritten at altitude, one migrating thrush and one hitchhiking tick at a time.

Subject of Research: Late-autumn dispersal of Borrelia-infected ticks by southward-migrating redwings and its implications for Lyme disease epidemiology under climate warming

Article Title: Lyme beyond the season: late-autumn dissemination of infected ticks by southward migrating redwings (Turdus iliacus)

Article References: Heylen, D. J. A., Ledegen, I., Grimon, M., Reteng, P., Hübschen, J. M., & Krawczyk, A. I. (2026). Lyme beyond the season: late-autumn dissemination of infected ticks by southward migrating redwings (Turdus iliacus). Parasites & Vectors. https://doi.org/10.1186/s13071-026-07673-x

Image Credits: AI Generated

DOI: 10.1186/s13071-026-07673-x

Keywords: Lyme disease, redwing, Turdus iliacus, Ixodes ricinus, Borrelia, bird migration, tick-borne pathogens, phenological mismatch, climate change, parasite dispersal, Borrelia miyamotoi, seasonal surveillance