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Declining numbers of Blackpoll Warblers (Setophaga striata) have been documented at long-term migration monitoring sites as well as in breeding areas. However, the \"loop migration\" of Blackpoll Warblers makes it difficult to ascribe population change at migration monitoring sites to specific breeding populations. Individuals from all populations across the breeding range of Blackpoll Warblers concentrate in fall along the Atlantic coastline of eastern North America prior to initiating a transoceanic flight to wintering areas. In spring, Blackpoll Warblers return along a different route, moving north into the southeastern United States where birds from eastern and western breeding populations then diverge during migration to reach their respective breeding areas. To monitor breeding populations outside of breeding areas and identify factors potentially affecting those populations, we must be able to identify where birds captured during migration breed and map seasonal variation in population-specific flyways. To \"map\" population-specific migration movements of Blackpoll Warblers, we used feather deuterium ( delta super(2)H sub(f)) values and a spatially explicit model to assign molt origins of 289 Blackpoll Warblers moving through sites in the Gulf of Maine (GOM) region and at three locations further west and south (northern Great Lakes area, Pennsylvania, and Florida). The assignment method was validated with feather samples from 35 birds captured during the breeding season at Churchill, Manitoba, Canada. As predicted, the spatial pattern of movement within and between seasons reflected \"loop migration.\" Blackpoll Warblers captured during fall migration in the GOM region included birds from across their breeding range, whereas birds captured during the spring were exclusively from northeastern populations. During fall migration, Blackpoll Warblers captured at two sites west of the GOM were from breeding areas further northwest than those from western Canada that were captured in the GOM. Blackpoll Warblers captured in eastern Florida during spring migration were assigned exclusively to breeding areas in the northeast, suggesting that eastern and western populations diverge soon after entering the United States. Finally, most Blackpoll Warblers sampled at Manomet Bird Observatory originated from breeding populations in Alaska and western Canada that have shown a similar (70-90%) decline over the same period. Our results, therefore, not only document the \"loop migration\" pattern of Blackpoll Warblers, but, by mapping patterns of connectivity between breeding and non-breeding areas, may help target conservation efforts for breeding populations of Blackpoll Warblers where most needed.

Conservation of migratory animals requires information about seasonal survival rates. Identifying factors that limit populations, and the portions of the annual cycle in which they occur, are critical for recognizing and reducing potential threats. However, such data are lacking for virtually all migratory taxa. We investigated patterns and environmental correlates of annual, oversummer, overwinter, and migratory survival for adult male Kirtland’s warblers (Setophaga kirtlandii), an endangered, long-distance migratory songbird. We used Cormack–Jolly–Seber models to analyze two mark–recapture datasets: 2006–2011 on Michigan breeding grounds, and 2003–2010 on Bahamian wintering grounds. The mean annual survival probability was 0.58 ± 0.12 SE. Monthly survival probabilities during the summer and winter stationary periods were relatively high (0.963 ± 0.005 SE and 0.977 ± 0.002 SE, respectively). Monthly survival probability during migratory periods was substantially lower (0.879 ± 0.05 SE), accounting for ~ 44 % of all annual mortality. March rainfall in the Bahamas was the best-supported predictor of annual survival probability and was positively correlated with apparent annual survival in the subsequent year, suggesting that the effects of winter precipitation carried over to influence survival probability of individuals in later seasons. Projection modeling revealed that a decrease in Bahamas March rainfall > 12.4 % from its current mean could result in negative population growth in this species. Collectively, our results suggest that increased drought during the non-breeding season, which is predicted to occur under multiple climate change scenarios, could have important consequences on the annual survival and population growth rate of Kirtland’s warbler and other Neotropical–Nearctic migratory bird species.

Speciation is one of the most important processes in biology, yet the study of the genomic changes underlying this process is in its infancy. North American warbler species Setophaga townsendi and Setophaga occidentalis hybridize in a stable hybrid zone, following a period of geographic separation. Genomic differentiation accumulated during geographic isolation can be homogenized by introgression at secondary contact, whereas genetic regions that cause low hybrid fitness can be shielded from such introgression. Here, we examined the genomic underpinning of speciation by investigating (1) the genetic basis of divergent pigmentation traits between species, (2) variation in differentiation across the genome, and (3) the evidence for selection maintaining differentiation in the pigmentation genes. Using tens of thousands of single nucleotide polymorphisms (SNPs) genotyped in hundreds of individuals within and near the hybrid zone, genome‐wide association mapping revealed a single SNP associated with cheek, crown, breast coloration, and flank streaking, reflecting pleiotropy (one gene affecting multiple traits) or close physical linkage of different genes affecting different traits. This SNP is within an intron of the RALY gene, hence we refer to it as the RALY SNP. We then examined between‐species genomic differentiation, using both genotyping‐by‐sequencing and whole genome sequencing. We found that the RALY SNP is within one of the highest peaks of differentiation, which contains three genes known to influence pigmentation: ASIP, EIF2S2, and RALY (the ASIP‐RALY gene block). Heterozygotes at this gene block are likely of reduced fitness, as the geographic cline of the RALY SNP has been narrow over two decades. Together, these results reflect at least one barrier to gene flow within this narrow (∼200 kb) genomic region that modulates plumage difference between species. Despite extensive gene flow between species across the genome, this study provides evidence that selection on a phenotype‐associated genomic region maintains a stable species boundary.

Migratory birds are physiologically challenged by intense exercise while fasting during flights that may last hours to days. Exercise‐induced oxidative stress could compromise flight performance by inducing mitochondrial dysfunction in the flight muscle. Endurance flight is partially fuelled by the catabolism of lean tissues, but how this catabolism is partitioned between different organs and muscles has not been previously studied under controlled conditions. We hypothesized that simulated migratory flight would result in dysfunction of flight muscle mitochondria, and selective catabolism of lean tissues. We predicted that simulated migratory flight would cause reduced mitochondrial oxidative phosphorylation capacity while increasing emission of reactive oxygen species (ROS) and that lean tissue mass catabolism would preferentially occur in digestive organs not needed in flight. We measured mitochondrial function, muscle morphology and the wet masses of organs and muscles following 8‐hour wind tunnel flights in blackpoll warblers Setophaga striata, which use multi‐day nonstop flights as part of their migration strategy. In contrast to our predictions, we found that simulated migratory flight did not alter mitochondrial fatty acid oxidation capacity or ROS emission. However, flight and fasting increased whole‐animal lean mass catabolism and were associated with reductions in the masses of liver, gizzard and proventriculus, but masses of tissues in the flight apparatus (pectoralis, heart, lungs) were unaffected. Pectoralis muscle fiber morphology was also unchanged over the tested flight duration. Our findings indicate that mitochondrial function in blackpoll warblers is robust against damage induced by simulated migratory flight, and energy deprivation is sufficient for organ catabolism.

When prioritizing populations for conservation of a given species, it is unclear whether the distribution of standing genetic variation can be used as a suitable proxy for the distribution of useful adaptive genetic variation. We tested whether using genome-wide and putatively adaptive genetic variation give similar prioritization results. We identified adaptive loci via their association with either environmental factors or phenotypic traits using two genomic data sets: yellow warblers (Setophaga petechia) across North America and lodgepole pines (Pinus contorta) in western Canada. We measured pairwise differentiation among populations using a principal components analysis and used a phylogenetic approach (NeighborNet networks) coupled with a measure of evolutionary distinctiveness (Shapley value) to attribute a priority rank to each population. Overall, we found that prioritization rankings using adaptive variation alone were not significantly divergent from rankings based on genome-wide genetic variation. Our testing framework might be of immediate use to conservation practitioners collecting next-generation sequencing data, and we call for further investigation in other species. Our results suggest that we may not need to pursue the contingent step of identifying adaptive variation in species of concern before prioritizing populations, i.e. a naive approach of using genome-wide genetic variation might be a suitable proxy for identifying local adaptation.

The importance of understanding the geographic distribution of the full annual cycle of migratory birds has been increasingly highlighted over the past several decades. However, the difficulty of tracking small birds between breeding and wintering areas has hindered progress in this area. To learn more about Kirtland’s warbler Setophaga kirtlandii movement patterns throughout the annual cycle, we deployed archival light-level geolocators across their breeding range in Michigan. We recovered devices from 27 males and analyzed light-level data within a Bayesian framework. We found that most males wintered in the central Bahamas and exhibited a loop migration pattern. In both fall and spring, departure date was the strongest predictor of arrival date, but in spring, stopover duration and migration distance were also important. Though stopover strategies varied, males spent the majority of their spring migration at stopover sites, several of which were located just before or after large ecological barriers. We argue that loop migration is likely a response to seasonal variation in prevailing winds. By documenting a tight link between spring departure and arrival dates, we provide a plausible mechanism for previously documented carry-over effects of winter rainfall on reproductive success in this species. The migratory periods remain the least understood periods for all birds, but by describing Kirtland’s warbler migration routes and timing, and identifying locations of stopover sites, we have begun the process of better understanding the dynamics of their full annual cycle. Moreover, we have provided managers with valuable information on which to base future conservation and research priorities.

Understanding how animals will adapt to climate change requires understanding how climate variables influence their biology year round, and how events in different seasons interact. Migratory birds may be especially vulnerable because of the wide range of geographic areas that they depend on throughout the annual cycle. We examined the potential effects of non-breeding season climate change on the breeding biology of Kirtland's Warbler (Setophaga kirtlandii, formerly Dendroica kirtlandii), an endangered songbird that breeds in the Northern Lower Peninsula of Michigan and winters in the Bahamas. Our objectives were to determine whether spring arrival dates on the breeding grounds correlate with late winter rainfall in non-breeding areas and whether this has consequences for reproductive success. We used data on spring arrival dates and number of fledglings per year, sampling many individuals across multiple years, to show that males arrive on breeding grounds later following drier winters. There was a strong male age * rainfall interaction, which indicates that first-time breeders were much more sensitive to changes in rainfall than experienced adults. Regardless of age, however, drier winters and delayed arrival and nest initiation were significantly associated with fewer offspring fledged. These results are important because the Caribbean region is currently experiencing a significant drying trend, and climate change models predict that the severity of this drought will continue to increase. Any resulting adjustments to the timing of migration could constrain spring arrival dates and limit reproductive success for the endangered Kirtland's Warbler, as well as other Neotropical migrants wintering in the Caribbean. Comprender cómo los animales se adaptan al cambio climático requiere entender cómo las variables climáticas afectan su biología a lo largo del año, y cómo interactúan los eventos sucedidos en diferentes temporadas. Las aves migratorias pueden ser especialmente vulnerables dado el amplio espectro de áreas geográficas de las que ellas dependen a lo largo del ciclo anual. Examinamos los efectos potenciales del cambio climático en las temporadas no reproductivas sobre la biología reproductiva de Setophaga kirtlandii (antes Dendroica kirtlandii), un ave canora amenazadaque se reproduce en el norte de la peninsula inferior de Michigan y pasa el invierno en las Bahamas. Nuestros objetivos fueron determinar si las fechas de llegada a las áreas de reproducción en primavera se correlacionan con la precipitación del final del invierno en la áreas no reproductivas, y si lo anterior tiene consecuencias sobre el éxito reproductivo. Usamos datos de las fechas de llegada en primavera y el número de volantones por año, muestreando múltiples individuos a lo largo de múltiples años, y logramos demostrar que los machos llegan a las áreas de reproducción más tarde luego de inviernos más secos. Hubo una interaccián fuerte entre la edad de los machos y la precipitacián, de modo que los que se reproducían por primera vez fueron mucho más sensibles a los cambios en la precipitación que los adultos más experimentados. Sin embargo, independientemente de la edad, los inviernos más secos y la llegada e iniciación tardía de la anidación estuvieron significativamente asociados con un menor número de crías emplumadas. Estos resultados son importantes porque la región Caribe actualmente está experimentando una tendencia hacia la aridez y los modelos de cambio climático predicen que la severidad de la sequía continuará incrementándose. Cualquier desajuste en la sincronización de la migración que se produzca como consecuencia de lo anterior podría restringir las fechas de arribo en la primavera y limitar el éxito reproductivo de S. kirtlandii, así como de otros migrantes neotropicales que pasan el invierno en el Caribe.

Migratory birds that experience poor overwintering conditions are often late to arrive at the breeding grounds, which is known to depress individual fitness. Despite the importance of this carryover effect, few studies have investigated how individuals can modify migratory behaviors en route to reduce delays on arrival and whether accelerating migration incurs survival costs. To examine this, we used Motus Wildlife Tracking System to track individual American redstarts (Setophaga ruticilla) as they migrated from wintering grounds in Southwest Jamaica through Florida en route to their breeding areas. We leveraged long-term data on spring departure timing and breeding latitude to quantify the relative departure dates (early vs. delayed) of tagged individuals, which we then related to individual migration rates and apparent annual survival. Compared to those initiating migration earlier, individuals that departed relatively late (10-day delay) migrated at a 43% faster rate, which decreased their annual survival by 6.3%. Our results are consistent with the hypothesis that spring migrants use speed to compensate for departure delays despite incurring survival costs. This compensatory behavior may potentially underly differential survival during spring migration and may be particularly widespread across short-lived migratory birds generally considered time-constrained.

The Yellow Warbler (Setophaga petechia) is a Neotropical migratory passerine that breeds in riparian and other wetland habitats across North America. Yellow Warblers use several woody plants as substrates to build nests on, including both native and invasive species. We monitored Yellow Warbler nests from 2018 to 2021 in east-central Illinois, USA, and observed similar numbers of nests in invasive and native plant substrates. We also documented qualitatively lower nest mortality for nests placed in invasive plants compared to native plants. The patterns documented here could suggest that for Yellow Warblers, invasive plant encroachment in riparian habitat may not have strong negative repercussions on their reproductive success. La reinita amarilla (Setophaga petechia) es un paserino migratorio neotropical que se reproduce en hábitats riparios y en otros humedales en Norteamérica. Las reinitas amarillas usan varias plantas leñosas como sustrato sobre los cuales construir sus nidos, incluyendo tanto especies nativas como invasoras. Monitoreamos nidos de reinita amarilla del 2018 al 2021 en la región centro-este de Illinois, Estados Unidos, y observamos números similares de nidos en sustratos de plantas nativas y no nativas. También documentamos una mortalidad de nidos cualitativamente menor para nidos colocados sobre plantas invasoras que sobre plantas nativas. Los patrones documentados aquí podrían sugerir que para la reinita amarilla, la llegada de plantas invasoras en hábitats riparios podría no tener fuertes repercusiones negativas en su éxito reproductivo. Palabras clave: especies invasivas, sustrato del nido, sobrevivencia del nido.

Oak (Quercus spp.) woodlands and savannas throughout the Mid-South, USA have become closed-canopy forests through succession in the absence of fire, contributing to declines in disturbance-dependent avian species. Restoration could ameliorate these trends, but effects of such management practices on avian communities of the Mid-South are poorly understood. We implemented an experiment on the Cumberland Plateau, Tennessee with treatments involving fall (Oct) or spring (Mar) fire with woodland (14 m2/ha) or savanna (7 m2/ha) residual basal area and unmanaged control stands (20 ha each). We conducted fixed-radius point counts 3 times annually (2010–2012) and analyzed trends in naïve occupancy for 41 species. For 20 species with adequate data (10 early-, 10 late-successional), we performed multiple-season, robust-design occupancy modeling and accounted for detection bias. We then used mixed-model polynomial regression to define relationships between occupancy and measures of live and dead basal area, midstory density, and herbaceous groundcover. Restoration substantially altered forest structural characteristics but did not affect the occupancy of most late-successional species. In contrast, the presence of early-successional species increased as live basal area decreased and herbaceous groundcover increased. Only 3 of 41 species (hooded warbler [Setophaga citrina], ovenbird [Seiurus aurocapilla], and worm-eating warbler [Helmitheros vermivorum]) exhibited reduced occupancy as woodland and savanna conditions were approached. The presence of all other species, including 13 regionally and 2 continentally declining, remained constant or increased as restoration progressed. Woodland conditions of 10 m2/ha live basal area and 20% herbaceous groundcover maximized occupancy of early-successional species with little consequences for late-successional species. Proceeding further toward savanna conditions negatively affected species associated with lower strata of mature-forests but further increased the presence of some early-successional species. Midstory density and dead basal area were poor predictors of occupancy and thus may be necessary but not sufficient in promoting early-successional species. Our results suggest implementing oak woodland and savanna management in the Mid-South could benefit disturbance-dependent birds of high conservation priority with minimal negative impacts on the presence of late-successional species.