Seasonal migration strategies can vary with life history demands and environmental context, yet such contrasts remain poorly documented for many shorebirds along the East Asian-Australasian Flyway (EAAF). We used high-resolution GPS-GSM tracking of 11 adult Greater Sand Plovers (Charadrius leschenaultii) tagged in the Beibu Gulf, China, during 2021-2022 to quantify and compare their spring (northbound; 12 tracks) and autumn (southbound; 8 tracks) migrations between nonbreeding areas in the northern South China Sea and breeding grounds in Mongolia. Spring movements followed predominantly inland corridors and were relatively direct, with journeys of approximately 3300 km completed in about 2 weeks. During spring migration, birds spent just over half of the total migration period at generally short but more frequent stopovers. In contrast, autumn movements occurred mainly along coastal pathways and covered around 4100 km over nearly 4 weeks. Across seasons, tracked individuals converged on the Beibu Gulf, which functioned both as an important stopover bottleneck and as a nonbreeding endpoint for part of the tracked population. These tracking data indicate seasonal differences in route use, stopover allocation, and habitat use, with spring movements showing a higher travel-to-stopover ratio and autumn movements involving a greater proportion of time at stopovers, suggesting greater refueling investment. Given the limited sample size and number of complete annual tracks, these inferences should be treated as preliminary. Nevertheless, the results highlight the potential importance of conserving both inland wetlands and coastal intertidal habitats along the EAAF.

The continued loss and degradation of wetlands pose major challenges to global waterbird conservation. In response, large-scale wetland restoration projects have been widely implemented worldwide, yet their long-term ecological effectiveness has not been sufficiently evaluated. Here, we assessed the long-term impacts of wetland restoration on waterbird communities at Chongming Dongtan Wetland, China, using 17 years of monitoring data spanning pre-restoration, restoration, and post-restoration phases. Our results suggest that the Ecological Control of Spartina alterniflora and Improvement of Bird Habitats substantially enhanced waterbird diversity, with both species richness and total abundance increasing significantly after restoration. Restored artificial wetlands supported particularly high abundances of waterbirds, confirming their role as critical supplementary habitats alongside natural tidal flats. Notably, different waterbird guilds exhibited pronounced seasonal shifts in habitat use: the Anatidae predominated during the wintering period, whereas Waders dominated during spring and autumn migrations, and the degree of reliance on artificial versus natural wetlands varied markedly between guilds and across seasonal cycles. Beyond local effects, we detected a clear spillover effect, whereby increases in waterbird abundance and species richness were also observed in adjacent non-restored natural intertidal mudflats following restoration. In addition, several threatened and nationally protected species were recorded exclusively during the post-restoration phase, indicating improved habitat suitability for conservation-priority taxa. Overall, our findings highlight that wetland restoration can generate both local and landscape-scale biodiversity benefits, emphasizing the importance of incorporating habitat heterogeneity, seasonal habitat requirements, and spillover effects into coastal wetland restoration and management strategies.

ABSTRACTMigration patterns in birds are shaped by ecological and physiological constraints that differ between sexes, seasons and among breeding regions, resulting in complex temporal and spatial strategies. Understanding this variation is crucial for revealing how individuals optimise their annual cycles. As very little is known about the individual migratory behaviour of pied avocets (Recurvirostra avosetta), this study investigated how spatiotemporal migration patterns of avocets breeding in northern Europe vary seasonally, sexually, and spatially. We captured 122 adults on their nests in three countries using walk‐in traps and equipped them with lightweight (6–9 g) solar‐powered GPS loggers. Avocets spent an extended post‐breeding period (on average until early November) in the Wadden Sea before commencing autumn migration. While most individuals followed the East Atlantic Flyway, four deviated via an inland route across the Alps to Italian wintering sites. On average, avocets travelled 1503 ± 996 km within 13.3 ± 19.8 days, resulting in a mean migration speed of 470.8 ± 554.0 km day−1. The main wintering range was concentrated in France and Portugal, with the Tejo Estuary being the most frequently used site. Migration tactics differed between seasons and sexes: autumn migration lasted longer and involved fewer but extended stopovers, whereas spring migration was faster, with more but shorter stopovers. Females migrated significantly further (1928 km vs. 1277 km in males) and wintered at more southern latitudes, while males remained longer in the Wadden Sea in autumn and tended to arrive at breeding sites earlier in spring. Avocets breeding in the Wadden Sea tended to winter further south than those from the Baltic Sea (Poland), most of which first moved to the Wadden Sea before continuing migration. Our findings highlight the central role of the Wadden Sea as a key fuelling area in the annual cycle of pied avocets from northern Europe.

Climate warming is altering thermal regimes across freshwater and marine ecosystems, with important consequences for fish life-history schedules, particularly during early life stages that span connected habitats. We characterized long-term variation in multiple early life-history traits and examined how thermal factors relate to shifts in the early life-history schedule of amphidromous ayu Plecoglossus altivelis. Individuals initiating spring upstream migration earlier were hatched earlier and were larger in body size across brood years, consistent with established patterns. In contrast, age at migration showed no consistent relationship with migration timing, indicating that variation in marine residence duration is not solely determined by migration phenology but is influenced by conditions experienced during the marine phase. Over the past three decades, age at spring migration declined significantly, indicating progressive shortening of the marine residence period. This decline was closely associated with elevated winter seawater temperature, whereas riverine thermal indicators representing the onset and termination of marine residence showed no comparable relationship. Although delayed autumn reproduction has been reported under warming river conditions, enhanced winter growth in the marine environment may offset these effects, resulting in little long-term change in migration timing. Overall, this study provides evidence that climate-driven changes in coastal marine thermal conditions are associated with shifts in marine residence duration and early life-history schedules in amphidromous fishes, highlighting the importance of integrated freshwater-marine thermal regimes under ongoing climate warming.

Elucidating the spatial-temporal characteristics and influencing factors of migration activities is the basis for understanding the life cycle of migratory birds and for making conservation measures. Short-distance migratory birds experience less timing pressure during migration; they tend to migrate under suitable environmental conditions and thus the migration decision might be strongly influenced by external environmental conditions. To test this hypothesis, we deployed GPS tags on Far Eastern Oystercatchers ( Haematopus ostralegus osculans ) in the northern Yellow Sea and tracked their annual migration. The tagged oystercatchers, including five adults and seven immatures (second/third-year birds), mainly wintered along the west coast of the Yellow Sea and bred inland in northeastern China. The migration distance between breeding and wintering sites was 1640.7 ± 260.4 km in spring and 1668.7 ± 306.1 km in autumn. Compared with adults, immatures departed from the wintering sites and arrived at the breeding sites later, while there were no significant differences between adults and immatures in the dates of departure from breeding sites and arrival at wintering sites. Migration initiation generally occurred around sunset. Wind conditions consistently provided tailwinds on departure days from wintering, breeding, and stopover sites during both seasons, with wind assistance exceeding the 5- or 10-day pre-departure average. During autumn migration, the temperature on departure days at stopover sites was lower than the mean temperature over the preceding 5 or 10 days. The Yalu Jiang estuary was the main stopover site during both spring and autumn migrations. The stopover duration in autumn (adults: 118.3 ± 8.4 days; immatures: 130.4 ± 5.1 days) was the longest among the studied shorebirds, likely due to moulting of flight feathers at the stopover site. Our results indicate that the migratory decisions of oystercatchers are strongly influenced by wind conditions during the whole annual cycle, whereas they are also affected by temperature when departing from stopover sites in autumn. The extended stopover at the Yalu Jiang estuary in autumn highlights its importance for the species.

Wetlands have suffered severe losses due to land-use and climate change, strongly contributing to the recent biodiversity crisis. This is especially true for urban areas. However, urban stormwater ponds, which have recently been constructed with increasing frequency, can represent valuable habitats for both flora and fauna. While previous studies have highlighted the importance of stormwater ponds as habitats for various plant and animal taxa, their role as resting and wintering habitats for birds has received little attention so far. In this study, we aimed to identify the importance of such stormwater ponds as resting and wintering habitats for the declining jack snipe ( Lymnocryptes minimus ) and common snipe ( Gallinago gallinago ). Our study in the oceanic north-west of Germany revealed that relatively small urban stormwater ponds with large semi-aquatic zones were important habitats for both species. In the study year, spring migration ended earlier and autumn migration started later than usual in both species. Climatic conditions during the migration/wintering period had a clear impact on the phenology of the snipes. In the climate models, higher amounts of precipitation prior to the survey fostered their densities. In the habitat models, the key drivers of snipe density were (i) the distance to woodland and (ii) the cover of sparse low-growing vegetation in the semi-aquatic zone. Both had a positive effect. Overall, a high habitat quality was the key for high snipe densities in the stormwater ponds. More precisely, food availability, shelter against predators and absence of anthropogenic disturbance (only jack snipe) drove snipe densities in the stormwater ponds. Based on our findings, newly created stormwater ponds should exhibit low slope angles (1: 15–20), large semi-aquatic zones and regularly be managed. • Small urban stormwater ponds with large semi-aquatic zones were important habitats. • Spring migration ended earlier and autumn migration started later than usual. • Higher amounts of precipitation prior to the survey fostered snipe densities. • High habitat quality was identified as the key driver of snipe densities. • Low slope angles, large semi-aquatic zones and regular management are required.

BackgroundEuropean Common Terns’ migration routes along the eastern Atlantic flyway and wintering areas in western and southern Africa were identified in ringing studies. Recent tracking studies revealed a migratory divide and the use of an eastern African migration route and wintering areas in the Mozambique Channel.Methods71 Common Terns breeding in a colony at the German Baltic coast were tracked with light-level geolocators. Linear Models were used to test influence of sex, year, migration route, and wintering area on timing of migration, and duration of migration and stopovers.ResultsA total of 58 individuals used the western African migration route, while 13 individuals used the eastern route. 45 individuals wintered at the southern African coast, while 12 individuals wintered in the Gulf of Guinea, and 13 individuals wintered in the Mozambique channel. Migration route and wintering area were identified as the most important factors influencing the duration of migration and stopovers, with birds using the eastern route showing significantly longer stopovers and duration of migration. Sex was identified to especially impact the start of migration, with females starting both autumn and spring migration significantly earlier than males. Arrival in the breeding area was only affected by the year, but this might be an effect of very low sample size in some years. Birds wintering in western Africa showed a tendency to a longer spring migration, while those wintering in eastern Africa tended to longer autumn migration.ConclusionThe eastern African migration route and wintering area seem to be much more common than ringing had indicated. Route and sex explain a substantial part of the observed variation in timing and duration of migration, but many factors such as annual variation in weather, wind, food availability as well as individual variation in fitness and breeding success are likely to affect timing of migration and duration of migration and stopovers. Comparison of duration of autumn and spring migration indicates that contradicting results reported in tracking studies from different European breeding areas might be explained by wintering area rather than breeding area, but are pronounced in birds with shorter migration distance.Supplementary InformationThe online version contains supplementary material available at 10.1186/s40462-026-00666-6.

Migratory species often repeat spatio-temporal patterns within their annual cycle. Although this may help to promote knowledge about local features and site quality, stereotyped behaviours may also create an ecological trap by preventing the flexibility required to adjust to environmental changes. Using a long-term international dataset, this study assesses 24 spatial and temporal parameters describing the repeatability of the entire migratory cycle in 94 individuals of the migratory near-threatened Eurasian curlew (Numenius arquata) that were tracked for up to 7 consecutive years using high-resolution GPS tags. Twenty-two parameters show significant repeatability, with the highest repeatability for use of the same breeding and wintering sites, indicating consistent faithfulness. All migration and stopover parameters during spring migration are also significantly repeatable, with lower repeatability for autumn migration, likely related to variable breeding success. The location of migration routes varies between consecutive years, but intra-individual similarity is significantly greater than inter-individual similarity. While the potential of adaptations to long-term environmental changes needs further studies (preferably including several cohorts of individuals) there are indications of a potentially maladaptive behaviour to short-term changes that should be carefully observed by site managers to conserve this near-threatened species.

Billions of birds migrate annually, triggered by endogenous behaviors but also by ecoclimatic drivers which are shifting with climate change. These dynamics play out over huge spatiotemporal scales, making monitoring of phenology challenging with traditional biodiversity survey approaches. In this study, over a complete spring migration season (April through June), we collected 37,429 hours of audio from 28 networked sensors in forests across Norway using a nationwide passive acoustic monitoring (PAM) system. We applied an open-source detection algorithm to automatically classify bird vocalizations; through expert validation we found the algorithm classified 57 species (14 full migrants) with at least 80% precision. Using these automated detections, we developed regional arrival curves for three common migratory passerines: Willow Warbler, Common Chiffchaff, and Spotted Flycatcher. We then demonstrate that PAM detections can be used to train audio species distribution models that map how species vocalization probability changes across Norway during spring migration. Lastly, we discuss how PAM can complement existing manual surveys to support the design and implementation of effective policy and conservation measures.

Long-distance migratory songbirds cross ecological barriers such as deserts and seas as part of their journeys. Yet, the behavioural adaptations used to perform these crossings remain poorly understood. Using accelerometer data from multisensor loggers carried by free-flying thrush nightingales (Luscinia luscinia), a nocturnal migratory passerine, we quantified fine-scale activity patterns during the entire migratory journey, including desert crossings during both spring and autumn migration. When migrating over ecological barriers, the birds performed consecutive nocturnal migratory flights, followed by daytime layovers when the birds remained inactive until the next sunset, when resuming migratory flights. The absence of diurnal locomotion while crossing desert environments suggests that thrush nightingales do not refuel while stopping within the barriers but instead rely on energy reserves accumulated prior to barrier entry. This multi-day behavioural inactivity highlights physiological and energetic constraints faced by small migratory birds when crossing ecological barriers and illustrates the careful allocation of energy reserves prior to the barrier crossing to endure several consecutive migratory flights without refuelling. Our results provide new detailed insights into how long-distance migrants cross ecological barriers, improving our understanding of behavioural adaptations in bird migration ecology.

Climate change is now being seen as one of the major factors of ecological change, which affects wildlife distribution, migration patterns, and the reproductive behaviors of various ecosystems. The paper will examine interactive effects of climate change on nature, species distribution, migration dynamics and reproductive behavior, using the long-term climate data (1980–2024), the species occurrence facts, the migration follow-ups and the evidence of reproductive phenology in several taxa of wildlife. Species distribution modeling revealed considerable poleward and altitudinal changes, terrestrial species with a range shift distance of 1725 km/decade and montane species with an altitude shift of 11 19 m/decade. Sensitivity of the climate-sensitive groups to habitat decrease was more evident with the amphibians, which have shrub habitats reduced by 17-22%. Migration measures demonstrated not only progression of spring migration by 6-11 days but also decreased length of migration paths ( some 220 km ) and times in stopover (3-4 days). Earlier breeding indicated through the reproductive assessments by 512 days but with reduced reproductive performance with a 115% decline in nesting success along with a 15.8% decrease in juvenile survivability through phenological disjunction with food. The comparison with related literature affirms that there is good correspondence with the global trends in addition to pointing out the increased disruption of the past decades. In general, these findings indicate that climate change is a multidimensional stressor, as it can simultaneously impair habitats availability, movement behavior, and reproductive success and increase long-term risks to population stability and biodiversity conservation among the wildlife.

ABSTRACT Climate factors shape the spring phenology of migrant birds in Europe. We examined carry-over effects of temperature and rainfall in four regions of Europe on the spring migration timing of a medium-distance migrant, Eurasian Wren, at the southern Baltic coast over 60 years. We modelled the effects of these climate factors on the timing of the start (10%), median (50%) and end (90%) of wrens’ spring passage at the bird ringing station Hel (N Poland) during 1964–2023. The dates of start and median of spring passage have not changed, but the end shifted earlier by 5.7 days over these 60 years. The start, median and end dates of wren passage on the Baltic coast were early after warm autumn, winter or spring, but delayed after high autumn and winter rainfall in central and northern Europe. Analogous climate conditions in southern and south-western Europe had the opposite effect on wrens’ timing of passage. High spring rainfall in central and southern Europe delayed spring passage at Hel. High rainfall in early summer in northern Europe was related to early wren passage the following spring, but such conditions in late summer resulted in a delay of spring passage. We assume that temperature and rainfall in these regions influence the timing of wren spring passage through their effect on food availability, individual fitness, migration distance and proportion of immatures among migrants. We suggest that varied humidity, which caused different insect abundance for wrens, explains the varied effects of rainfall in various regions of Europe on the timing of wren spring passage at Hel. We show that wrens’ spring migration phenology in the Baltic region is shaped by a combination of carry-over effects of many environmental factors. They affect wrens in regions of Europe where they stay previously, even months before their spring migration.

We used geolocators recording ambient light, conductivity and wet/dry conditions to study the migration of Little Ringed Plovers Thinornis dubius breeding in the Netherlands. Little Ringed Plovers breeding in western Europe mostly migrate in the autumn towards the inner Niger delta in western Africa, mainly using stopover sites around the Mediterranean Sea. Conductivity data indicate that these autumn stopover sites are saline or brackish habitats, suggesting that saline coastal marshes or manmade saline aquatic habitats (saltworks) are important stopover habitats for Little Ringed Plovers. The timing of the spring migration coincides with the spring equinox (preventing accurate geolocator fixes); therefore we could not locate stopover sites in spring. However, we were able to use the conductivity data to infer the timing of the spring stopover sites and migration. We found that migrating Little Ringed Plovers spent 15% (females) or 17% (males) of their time in saline aquatic habitats.

Despite the obvious shift in spring bird migration to earlier dates, the mechanism behind this process remains unclear, partly due to inappropriate climate predictors. Information on the influence of climate is not geographically representative, as it comes almost exclusively from the marginal parts of the continents. The objective of this study is to investigate the mechanism for determining the timing of spring terrestrial bird migration based on a new concept of environmental phenology and phenological navigation, using data from Central Siberia as an example. A literature review suggests that migrants follow the phenology of food resources, the dependence on which intensifies towards the end of migration with increasing seasonality of temperature conditions. We propose the concept of environmental phenology as a mediator between the influence of climate and the response of migrants. We defined the environmental phenology index as the average weighted temperature of the preceding days with weights decreasing exponentially in retrospect. Using original data from the first arrival dates of 145 species in Central Siberia from 1976 to 2025, we demonstrated the effectiveness of the environmental phenology index in predicting arrival dates. This indicator reflects the accumulation of climate information by numerous ongoing phenological processes. Due to its high autocorrelation, it predicts future foraging conditions. Despite significant warming at the study site (0.60 degrees per decade), bird arrival dates in the studied community responded to warming with a slight shift towards earlier dates: on average, by -0.91 days per degree annually and by -0.86 days per decade. These estimates are lower than those known for other regions due to the pronounced continental climate and the steep rise in spring temperatures (1degree in 3 days). We believe the compensated share of this impact (the response to impact ratio) to be a more adequate measure of the organism's response to the impact. Based on the study results, we propose a mechanism of phenological navigation, in which a migrant at each stopover chooses whether to move on or wait for changes based on the relationship between environmental phenology and day length. This simple scheme opens new possibilities for comparing the phenological responses of different groups of organisms. The proposed index of environmental phenology is theoretically and practically substantiated, expands the understanding of the mechanism of phenological navigation and can be used to solve similar phenological issues.

Body size is frequently invoked to explain within-population variation in migratory behaviour because of its far-reaching implications for physiological and ecological processes. Here, we investigated how autumn body size influenced spring river-to-sea migration probability and timing in juvenile brown trout (Salmo trutta) in the river Nybroån, Sweden. To achieve this, we used a combination of individual-based tracking and trapping during two consecutive years. We report a non-linear relationship between autumn body size and probability of spring migration, with migration likelihood increasing steeply up to a body length of ~110 mm before declining with further increasing body size. Among migrants, larger individuals migrated earlier than smaller conspecifics. Our results highlight that autumn body size may be used as a reliable predictor of migratory behaviours in juvenile brown trout and underscore the utility of using recruitment surveys to forecast life-history transitions and suggest that probabilistic reaction norms based on body size can support monitoring and conservation strategies in migratory fish populations.

Long‐distance migrants face numerous challenges during their migration, with the spring migration timing being particularly important, as early arrival at the breeding grounds often enhances breeding success. Both innate and environmental factors influence migration timing in birds, and the earlier arrival of males than females is common among songbirds breeding in temperate zones. The great reed warbler Acrocephalus arundinaceus males departed from the wintering grounds 10.5 days earlier than females. Interestingly, the oldest males (≥ 4‐year‐olds) departed significantly earlier than younger adult males (2‐ and 3‐year‐olds), suggesting that accumulated experience and/or age‐related hormonal changes may underlie this pattern. No significant differences were observed among female age groups. In contrast, we found no significant age effect for the duration of spring migration in either males or females. Males arrived 9.9 days earlier than females at the breeding grounds. Similar to departure dates, the oldest males (≥ 4‐year‐olds) arrived significantly earlier than middle‐aged males (3‐year‐olds), while no such age‐dependent relationship was observed in females. In both males and females, we found positive relationships between the departure date from the wintering grounds and the arrival date at the breeding grounds with a slope of β = 0.59, showing that spring migration duration becomes shorter as the spring season progresses. In addition, for a given departure date from the wintering grounds, males arrived 3.9 days earlier than females, but no significant differences were observed between age groups either in males or females. Our findings have important implications for understanding how long‐distance migratory birds might adjust their endogenous migration programs based on individual qualities, and highlight the potential role of learning and experience in these processes. This further emphasizes how adaptations to ongoing climate change might depend not only on individual quality, but also on plasticity and the age structure of the population.

Timing of migration is seasonal and tied to changing photoperiod; however, individual birds rely extensively on external environmental cues, such as weather, to assess likely flight conditions and make fine‐scale decisions about migratory departures. Considerable variation may exist among individuals in how they respond to specific cues, in turn producing intraspecific variability in aspects of migration timing and flight distances. We were interested in how external environmental cues (e.g. weather, lunar cycles) were associated with departure decisions in American Woodcock Scolopax minor , and whether there was consistent variation in these decisions among individuals based on their age and sex. Between 2017 and 2020, we captured and attached satellite transmitters to 304 American Woodcock throughout eastern North America, and obtained location data as they migrated during both autumn and spring. We used conditional logistic regression and general linear models in a case–control study design, which allowed us to evaluate individual American Woodcock selection for potential cues at departure. We further explored how individual use of specific wind conditions influenced the distance covered during migratory flights. American Woodcock responded to barometric pressure, moon illumination, temperature, presence of tailwinds and wind speed when making departure decisions, but selection varied by season, age and sex. In autumn Woodcock responded to tailwind assistance, and departed under lower temperatures and barometric pressure, but young birds making their first migration were more responsive to temperature and less responsive to wind assistance than adult birds. During spring migration, female American Woodcock showed a greater selection for wind assistance and higher barometric pressure, whereas males showed stronger response to higher temperatures and greater moon illumination. American Woodcock that departed using tailwinds generally had longer flight distances, which we assume reflected more efficient flight associated with wind assistance. Adults were able to better capitalize on more favourable flight conditions during autumn migration, whereas decision‐making during spring migration was instead driven by sex‐specific motivations that were likely to be associated with breeding.

Climate change in Europe can influence the predator-prey interactions, a scarcely studied topic in birds. We examined relationships between the spring passage timing of the Eurasian Sparrowhawk, a generalist peak predator, and its main bird prey species (Song Thrush, Eurasian Blackbird, Common Chaffinch, Great Tit, European Robin). All species were ringed daily (26 March-15 May) in 1982-2021 at Hel Peninsula (N Poland). In 2024, we collected pluckings to identify Sparrowhawks' prey. Sparrowhawks' daily migration dynamics and those for the Song Thrush, Robin and Chaffinch were correlated. Sparrowhawks' median dates of passage showed no multi-year trend, but large variation, correlated with those of its prey and with temperatures. Adult females and males migrated through Hel early after warm February and March at wintering grounds. Young males migrated early during warm Aprils at Hel. Medians were correlated between adult males and Robins, adult females and Blackbirds, and young females and Song Thrush. This suggests that Sparrowhawks adjust their migration timing each spring to the availability of these three prey species, with some sex differences, as males are smaller than females. Identifying changes in predator-prey dynamics in response to climate change helps us understand its effect on forest ecosystems.

Understanding songbird movements throughout their annual cycles is critical to inform conservation planning decisions and management priorities for migratory species. From 2017 to 2020, we attached miniaturized light-level geolocators to 309 male golden-cheeked warblers (Setophaga chrysoparia; hereafter warbler) at five study sites located across the species’ breeding range in Texas. From 2018 to 2021, we recovered data from 61 geolocators and used these data to determine the locations and timing of warbler movements during their non-breeding seasons and to quantify migratory connectivity and migration networks for the species. Mean duration of fall migration (37 d ±19 d) was longer than spring migration (27 d ±14 d), and, on average, second-year warblers initiated fall migration 10 d ±13 d earlier than after-second-year warblers. Migration initiation dates during spring, as well as duration and distance of fall and spring migrations, were similar for both age classes. Overall, 80% of warblers migrated along the Sierra Madre Oriental and Mexico’s Gulf coast between their breeding grounds in Texas and wintering grounds in southern Mexico and Central America during fall and spring; 20% migrated across or eastward around the Gulf of Mexico, primarily in fall. Overall, stopover duration varied along the warbler’s migration route (range = 3–61 d), with the longest stopovers in Texas and along the coast of Tamaulipas and Veracruz in Mexico. The highest percentages of stationary overwintering locations occurred in Chiapas, Mexico (27%), Guatemala (25%), and Honduras (17%), and many warblers used more than one location during winter. Some warblers used areas outside the predicted winter range; one extralimital location was confirmed during field-based surveys in Oaxaca, Mexico. Our results suggested that warblers exhibit weak migratory connectivity and likely disperse widely across the winter range. Given the narrow migration pathway used by most warblers and confined winter range in montane forests, continued and increased conservation along the warbler’s migration route and on the wintering grounds could help ensure population persistence and simultaneously protect co-occurring species.

Identifying the drivers of wildlife immunity is critical for assessing stressor impacts and zoonotic risks. However, such studies are limited by logistical challenges of wildlife research and lack of species-specific reagents. We adapt flow cytometry, typically confined to laboratory settings, to field settings to profile cellular immunity with small blood volumes and extended sample holding times. We apply these methods to analyze immune cell seasonality in migratory Mexican free-tailed bats (Tadarida brasiliensis). We confirmed four antibodies recognizing CD3, CD79a, MHCII, and CD11b that were originally validated in Egyptian fruit bats (Rousettus aegyptiacus), allowing us to quantify T and B cells, macrophages, and neutrophils, respectively. Flow cytometry outperformed hematology in quantifying leukocyte profiles and revealed pronounced immune cell seasonality. Adaptive cells steadily increased between spring and fall migration. Neutrophils were most abundant during the reproductive period and decreased during migrations, whereas B cells were most abundant after reproduction and before fall migration; granulocytes as a whole, macrophages, and T cells had no seasonality. Females had more B cells than males but did not differ in other cells. Our findings lay the groundwork for applying flow cytometry to field studies of wildlife and provide important insights into the seasonality of bat immunity.