Nicholas James Bayly

Identifying when and where animals experience the highest mortality risk during migration is critical for understanding their long-term population dynamics. To estimate risk by latitude during pre-breeding (spring) migration for the steeply declining Setophaga striata (Blackpoll Warbler), we fitted 96 birds with radio transmitters on their nonbreeding grounds in Colombia and tracked their migration via the Motus Wildlife Tracking System. The resulting detection histories (82% of birds were detected at least once between Colombia and their boreal breeding grounds) were analyzed using a continuous-time multistate survival model, providing estimates of daily mortality risk in each of 4 regions (states) corresponding to discrete latitudinal bands. Hazard rates varied among regions, with the southeastern United States (including the Florida peninsula, ∼27–40°N) showing the highest instantaneous daily hazard (median = 0.027, 95% CRI: 0.001–0.143), ∼5 times higher than the region including northern Colombia and the Caribbean Sea (∼5–27°N; median = 0.006, 95% CRI: 0.000–0.024) and twice as high as the mid-latitude region (40°N to the southern edge of the breeding grounds; median = 0.012, 95% CRI: 0.000–0.064). Overall pre-breeding migration survival was estimated at 0.70 (95% CRI: 0.44–0.90), which is similar to estimates for other long-distance migratory birds. These results provide one of the first spatially explicit estimates of latitudinal mortality risk during migration and the first insights into regional variation in survival rates for S. striata. Our findings also highlighted the potential of Motus to be used as a tool for assessing survival in migratory species across broad spatial scales. Expanding tagging efforts and the coverage of the Motus array could provide much-needed information on the vital rates of migratory species of conservation concern during vulnerable stages of their annual cycle.

Cardellina canadensis (Canada Warbler) displays flexible space use during the nonbreeding season that is only weakly associated with habitat characteristics, while higher local precipitation is linked to increased survival. Understanding the relationships between fitness, space use, climate, and land-use is key to assessing population vulnerability under future precipitation and land-use scenarios. We examined these relationships by estimating home ranges of 29 C. canadensis using manual radio-telemetry in forests across 3 biogeographic regions of the Colombian Andes: the western slope of the Cauca Valley, the eastern slope of the Magdalena Valley and the eastern slope of the Eastern Andes. We assessed the effect of precipitation, habitat quality, and habitat availability on home-range sizes and estimated weekly and nonbreeding survival. Home-range sizes were weakly associated with habitat variables. Individuals in areas with higher vegetation greenness (normalized difference vegetation index [NDVI]) tended to have smaller home ranges, although support for this pattern was limited and dependent on site-level effects. Variation in home range among sites suggests that broader regional or site-level factors influence space use during the nonbreeding season. Survival was consistently high, averaging 98% monthly and 92% from January through April. Notably, individuals experienced higher survival when precipitation prior to initial capture was higher. Together, our findings indicate that nonbreeding C. canadensis exhibit flexible space use and that home-range size is only weakly linked to the habitat variables we measured. Furthermore, increased local precipitation enhances survivorship during the nonbreeding season, underscoring the importance of climatic factors during this period. Overall, our findings provide valuable insights into the adaptive behaviors of migratory birds during the nonbreeding season and highlight the critical role of environmental factors in their survival. This knowledge provides insight for conservation strategies to protect the species under changing precipitation and land-use conditions.

Despite recent avian research describing major migratory pathways and stopover regions, many species-specific migratory pathways and stopover locations remain unidentified and uncharacterized. We selected 4 Nearctic–Neotropical migrant bird species experiencing population declines and for which targeted conservation is limited during migratory periods: Cardellina canadensis (Canada Warbler), Setophaga cerulea (Cerulean Warbler), Vermivora chrysoptera (Golden-winged Warbler), and Hylocichla mustelina (Wood Thrush). In three phases, we delineated species-specific migratory pathways with movement tracking data and eBird Status and Trends products (Phase I), identified species-specific stopover landscapes within migratory pathways with eBird Status and Trends products (Phase II), and characterized multi-species stopover hotspots within stopover landscapes with land use/land cover covariates (Phase III). Phase I revealed that 3 species (S. cerulea, V. chrysoptera, and H. mustelina) crossed the Gulf of Mexico via a more easterly pathway during post-breeding migration and a more westerly pathway during pre-breeding migration. In contrast, C. canadensis went around the Gulf of Mexico, while also avoiding the core Prairie Pothole Region and remaining in forested landscapes within the aspen parkland ecotone of southern Canada. In Phase II, forested landscapes predicted to support higher stopover use included major river valleys, the Appalachian Mountains, the northeastern United States, southern Ontario in Canada, and the Central American Highlands. In Phase III, we selected 2 case studies for characterization of predicted stopover habitat use and protected area status. Likely stopover areas near Huntsville, Alabama, USA, occurred in deciduous broadleaf forests away from developed land, while in northern Nicaragua near Ocotal, likely stopover areas occurred in premontane and montane tropical forests. Opportunities for conservation action were presented in both case studies, where all stopover use areas contained larger proportions of unprotected land. Moving forward, our flexible three-phase analytical approach provides the tools to support single- and multi-species conservation strategies.

La expansión de la energía eólica en La Guajira, principal región de desarrollo eólico en Colombia, coincide con un corredor de migración de aves que concentra más de 150 especies. Sin embargo, la información local sobre todos los aspectos de su ecología migratoria, como la fenología, es limitada, lo que restringe la evaluación de riesgos de colisión. Entre 2016 y 2018 registramos aves migratorias mediante censos estandarizados y capturas al suroeste de La Guajira durante la migración pos-reproductiva. En total, registramos 19.697 indivuduos pertenecientes a 39 especies, con una alta concentración de especies de interés para la conservación como Spiza americana, Dolichonyx oryzivorus y Setophaga striata. El análisis reveló dos periodos de mayor abundancia: uno de alta intensidad en octubre y otro moderado-alto en septiembre. Una evaluación de alturas de vuelo, indica un solapamiento con el rango de barrido de los aerogeneradores, lo que incrementa el riesgo de colisión durante la migración. Estos hallazgos sugieren la necesidad de medidas integradas de mitigación que incluyan apagados programados en picos migratorios, monitoreo con tecnologías de detección y predicción, atención particular a especies vulnerables o en declive, y una planeación espacial que evite áreas prioritarias para la conservación, complementadas con estudios estandarizados en ambas temporadas migratorias. El estudio aporta información de base para la planificación eólica en Colombia, en un contexto de transición energética acelerada y bajo limitada disponibilidad de evaluaciones locales.

Avian species have inspired numerous studies on ornamentation, exploring its role in intra- and intersexual selection as a marker for individual quality. Male birds have been the focus of most studies of plumage coloration despite the fact that females also display ornamentation. Coloration added to feathers during their growth, particularly carotenoid-based pigments, are derived from food resources. As weather patterns affect food availability and abundance, there may be a link between weather conditions during molt and feather coloration the subsequent breeding season. We collected 474 breast feather samples from 329 unique female Protonotaria citrea (Prothonotary Warbler) and 436 crown feather samples from 310 females across 13 years (2010–2023) in eastern Virginia, USA, to assess relationships between temperature and precipitation during molt and breast and crown feather reflectance metrics the following year. We considered conditions in both the breeding and nonbreeding seasons as possible drivers of crown reflectance because at least some feathers are replaced during a previously undescribed limited prealternate molt. Breast and crown violet-blue chroma was lower (therefore higher carotenoid concentration) and breast yellow intensity was higher following cooler and wetter conditions during the late summer molt and not the winter prealternate molt. The negative impact of warmer temperatures on breast yellow intensity was greater in younger females. These results provide insight into how climate conditions affect female plumage coloration, which has previously been linked to reproductive success, and may aid in understanding the impacts of climate change on coloration as an honest signal of individual condition.

Seasonal migrations between temperate and tropical latitudes have evolved repeatedly across the tree of life, but we know little about how tropical environments have shaped this life history strategy. In this first aeroecological radar study in South America, we tested whether tropical atmospheric stability leads to a more continuous pace of nocturnal bird migration across a season than the episodic pulses of migration in temperate regions. We estimated and compared the volume, flight directions and heights of birds detected by weather radars and acoustic sensors in three regions of Colombia across four years and assessed the effects of atmospheric conditions. We found that stable tropical winds have distinct effects on migration timing depending on geographic context. Within the Andes mountains, supportive winds explain the altitudes at which birds fly, but not the volume of birds aloft on a given night, resulting in lower nightly variation in migration intensity. However, lapses in prevailing headwinds east of the Andes drove higher variation in migration intensity similar to temperate latitudes. With a reduced relationship between forecastable weather and the magnitude of nightly migration, predicting high volume migration events to target for conservation initiatives may be more challenging in tropical climates.

Declines in migratory landbird populations require targeted research to identify the population-limiting factors acting throughout the annual cycle. Tracking studies provide data on migration timing and speed, which have been used to describe carryover effects of nonbreeding habitat use. To explore how variation in precipitation and forest cover among nonbreeding sites influenced departure dates and migration speed in Vermivora chrysoptera (Golden-winged Warbler), we fitted 69 birds with radio transmitters in Costa Rica and tracked them via the Motus Wildlife Tracking System. Capture sites covered a broad gradient of precipitation and forest cover and were separated by <100 km. Warblers departed Costa Rica during a 33-day window (26 March to 27 April, n = 46), and 23 birds were subsequently detected at Motus stations in North America, primarily in a narrow migration pathway through the Midwestern United States. Individual variation in departure date was explained by adults departing before immatures, males before females, and birds occupying drier (lower annual precipitation) and less forested sites departing 10 days earlier, on average, than those in wetter and more forested sites. Following departure, later-departing birds migrated faster allowing them to reach latitudes between 36° and 44° in fewer days and thereby catch up to and, in 28% of paired birds, overtake the earlier-departing bird. Our findings contrast with the general expectation that earlier departures lead to earlier arrivals along migration routes, instead providing evidence for alternative migratory strategies covering a spectrum of outcomes from earlier departures leading to earlier arrivals through to later departures leading to earlier arrivals. We present hypotheses for why these alternative strategies might exist within an optimal migration theory framework. Future studies in Costa Rica should examine additional proxies for habitat quality, to corroborate whether faster migration speeds from wetter more forested habitats are linked to higher habitat quality.

Vermivora chrysoptera (Golden-winged Warbler) is one of the most striking but also one of the most vulnerable migratory warblers on Earth. To understand its nonbreeding ecology in northern South America, where the steeply declining Appalachian-breeding population is known to spend the nonbreeding season, we conducted 2,534 surveys between 2018 and 2023 in isolated massifs and the Andes of Colombia. Our goal was to evaluate the distribution, detection rates, elevational range, habitat preferences, and microhabitat associations of the species. We obtained 54 detections, with a very low detection rate (2.13%) compared to rates reported from Central America (~10%). Detections covered a wide range of elevations (634–2,551 m above sea level [m.a.s.l.]), with support for a mid-elevation peak at around 1,750 m.a.s.l. Importantly, detection elevation was negatively correlated with latitude such that peak abundances occurred at higher elevations closer to the equator. Most detections were in mature or secondary forests (72%), but birds also occurred along forest edges (17%) and in agroforestry systems (11%), such as shade-grown coffee. We found weak evidence for a female preference for lower elevations as well as more frequent use of agroforestry systems relative to males. Logistic regressions provided some support for a preference for forest over agriculture in both sexes as well as a higher detection probability at points with intermediate canopy height and canopy cover. Our results highlight the low density at which the species occurs across the northern Andes of Colombia and other isolated massifs. The association with forested habitats and the relatively narrow elevation range occupied by most birds implies that focusing conservation efforts on protecting or restoring forests at elevations where detection rates peaked would likely be an effective strategy for ensuring that nonbreeding habitat remains for Appalachian-breeding V. chrysoptera.

El paisaje de Colombia ha sido alterado radicalmente debido al reemplazo de una buena parte de la cobertura de la vegetación original por agroecosistemas (López-Hernández et al., 2005; Viloria 2009). Como resultado de estos cambios, los agroecosistemas ocupan un área cada vez más grande y, por lo tanto, es necesario involucrarlos en estrategias para la conservación de la biodiversidad. Uno de los grupos más afectado por estos cambios son las aves migratorias, debido a la disminución de hábitat natural disponible en las últimas décadas

Setophaga striata (Blackpoll Warbler), one of the few boreal-breeding migratory birds that migrates to the Orinoco and Amazon regions, is experiencing steep population declines. However, knowledge of the species’ distribution and habitat use during the nonbreeding season is limited. Here, we explore how stationary nonbreeding season (December to April) occupancy probabilities vary across regional and landscape scales, and with vegetation structure for S. striata in eastern Colombia. By running single-species, single-season hierarchical occupancy models and N-mixture models, we evaluated how regional (annual precipitation, nonbreeding season precipitation, driest month, elevation, and net primary productivity), landscape (habitat type and % forest in the landscape), and vegetation structure variables influenced occupancy by S. striata. In the regional analysis, occupancy was positively influenced by stationary nonbreeding precipitation and net primary productivity, and was predicted to peak at elevations between 500 and 1,000 m. At the landscape scale, occupancy and abundance were higher in agroforestry systems (in order of magnitude: shade-grown cacao, citrus plantations and silvopastures) than in forested habitats. In relation to vegetation structure, occupancy was negatively correlated with a principal component containing shrub density, canopy height, and canopy cover. Within the Orinoco region, a spatial prediction based on our results supported high occupancy rates in moister areas along and close to the Andean foothills and where the Orinoco grasslands transition into Amazonian forests. Our results highlight the importance of landscapes containing agroforestry systems as stationary nonbreeding habitats for S. striata in Colombia’s Orinoco region and the importance of promoting agroforestry systems, like shade-grown cacao, in areas with higher nonbreeding season precipitation and net primary productivity values in order to increase habitat availability for the species.