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The Indian subcontinent is the primary wintering ground and stopover site for migratory shorebirds to refuel along the Central Asian and South Asian Flyways. Despite the conservation importance of this region for migratory shorebirds, we lack information on the distribution and abundance of over-summering shorebirds—migrants that remain on their wintering grounds during the breeding season—to evaluate the impacts of anthropogenic change on this poorly understood life history strategy. We conducted weekly surveys of over-summering shorebirds at mudflats, mangroves, and sand beaches at the Kadalundi-Vallikkunnu Community Reserve on the southwest coast of India, 2005–2018. We examined long-term patterns of over-summering shorebird abundance, species composition, and habitat use and estimated proportional changes in over-summering abundance of each shorebird species documented at this globally important site. Over the 14 yr study, we documented 7 over-summering species, including Lesser Sand Plover (Charadrius mongolus), Whimbrel (Numenius phaeopus), Greater Sand Plover (Charadrius leschenaultia), Kentish Plover (Charadrius alexandrinus), Common Sandpiper (Actitis hypoleucos), Ruddy Turnstone (Arenaria interpres), and Pacific Golden-Plover (Pluvialis fulva). Mean abundance and species richness were higher at mudflats than at mangroves and sand beaches, with fewer individuals and species observed over-summering after 2010. Mudflats also had the highest species diversity and evenness compared to mangroves and sand beaches. Management plans that aim to restore vulnerable mudflats and mangroves and to reduce anthropogenic threats such as sand mining and waste dumping are needed to prevent the loss of important over-summering, foraging habitat for migratory shorebirds in southern India and along the Central Asian and South Asian Flyways.

Numerous studies have correlated the advancement of lay date in birds with warming climate trends, yet the fitness effects associated with this phenological response have been examined in only a small number of species. Most of these species--primarily insectivorous cavity nesters in Europe--exhibit fitness declines associated with increasing asynchrony with prey. Here, we use 25 years of demographic data, collected from 1986 to 2010, to examine the effects of spring temperature on breeding initiation date, double brooding, and annual fecundity in a Nearctic-Neotropical migratory songbird, the black-throated blue warbler (Setophaga caerulescens). Data were collected from birds breeding at the Hubbard Brook Experimental Forest, New Hampshire, USA, where long-term trends toward warmer springs have been recorded. We found that black-throated blue warblers initiated breeding earlier in warmer springs, that early breeders were more likely to attempt a second brood than those starting later in the season, and that double brooding and lay date were linked to higher annual fecundity. Accordingly, we found selection favored earlier breeding in most years. However, in contrast to studies of several other long-distance migratory species in Europe, this selection pressure was not stronger in warmer springs, indicating that these warblers were able to adjust mean lay date appropriately to substantial inter-annual variation in spring temperature. Our results suggest that this North American migratory songbird might not experience the same fecundity declines as songbirds that are unable to adjust their timing of breeding in pace with spring temperatures.

Many studies of sexual selection assume that individuals have equal mating opportunities and that differences in mating success result from variation in sexual traits. However, the inability of sexual traits to explain variation in male mating success suggests that other factors moderate the strength of sexual selection. Extrapair paternity is common in vertebrates and can contribute to variation in mating success and thus serves as a model for understanding the operation of sexual selection. We developed a spatially explicit, multifactor model of all possible female-male pairings to test the hypothesis that ecological (food availability) and social (breeding density, breeding distance, and the social mate’s nest stage) factors influence an individual’s opportunity for extrapair paternity in a socially monogamous bird, the black-throated blue warbler, Setophaga caerulescens. A male’s probability of siring extrapair young decreased with increasing distance to females, breeding density, and food availability. Males on food-poor territories were more likely to sire extrapair young, and these offspring were produced farther from the male’s territory relative to males on food-abundant territories. Moreover, males sired extrapair young mostly during their social mates’ incubation stage, especially males on food-abundant territories. This study demonstrates how ecological and social conditions constrain the spatial and temporal opportunities for extrapair paternity that affect variation in mating success and the strength of sexual selection in socially monogamous species.

Changes in leaf phenology from warming spring and autumn temperatures have lengthened the temperate zone growing “green” season and breeding window for migratory birds in North America. However, the fitness benefits of an extended breeding season will depend, in part, on whether species have sufficient dietary flexibility to accommodate seasonal changes in prey availability. We used fecal DNA metabarcoding to test the hypothesis that seasonal changes in the diets of the insectivorous, migratory black‐throated blue warbler (Setophaga caerulescens) track changes in the availability of arthropod prey at the Hubbard Brook Experimental Forest, New Hampshire, USA. We examined changes across the breeding season and along an elevation gradient encompassing a 2‐week difference in green season length. From 98 fecal samples, we identified 395 taxa from 17 arthropod orders; 242 were identified to species, with Cecrita guttivitta (saddled prominent moth), Theridion frondeum (eastern long‐legged cobweaver), and Philodromus rufus (white‐striped running crab spider) occurring at the highest frequency. We found significant differences in diet composition between survey periods and weak differences among elevation zones. Variance in diet composition was highest late in the season, and diet richness and diversity were highest early in the season. Diet composition was associated with changes in prey availability surveyed over the green season. However, several taxa occurred in diets more or less than expected relative to their frequency of occurrence from survey data, suggesting that prey selection or avoidance sometimes accompanies opportunistic foraging. This study demonstrates that black‐throated blue warblers exhibit diet flexibility and track seasonal changes in prey availability, which has implications for migratory bird responses to climate‐induced changes in insect communities with longer green seasons. Changes in seasonality have extended the breeding window for migratory birds in North America with earlier springs and later autumns that affect insect availability. DNA metabarcoding and insect surveys revealed that migratory, insectivorous black‐throated blue warblers exhibit diet flexibility in response to seasonal pulses in arthropod prey availability. They also express prey preferences, especially earlier in the season when birds are vulnerable to changes in insect availability.

Sex allocation theory predicts that females should bias their offspring sex ratios when the fitness benefits of producing sons or daughters differ depending on rearing environment. The Trivers-Willard hypothesis proposes that whether females produce more sons or daughters depends on food availability via both intrinsic maternal condition and differing reproductive potential (typically from mating system structure) for sons versus daughters. However, tests of its key predictions are often based on untested, implicit assumptions that are difficult to quantify, especially in migratory animals. In a 5-year study, we manipulated food availability in low- and high-elevation forest to test the Trivers-Willard hypothesis in the migratory black-throated blue warbler (Setophaga caerulescens). We found that the population-wide offspring sex ratio was significantly male-biased (population mean: 0.58), which was driven by an overproduction of sons in high-elevation forest (high-quality habitat mean: 0.59). Yet, we found no effect of food availability on offspring sex ratio from either natural variation or supplemental feeding. Sex-specific developmental costs did not differ for sons and daughters reared under low and high food availability. These results suggest that female black-throated blue warblers do not manipulate offspring sex ratios in response to food availability and are not consistent with the predictions of the Trivers-Willard hypothesis. This study highlights challenges of examining mechanisms driving patterns in offspring sex allocation in migratory species for which both the costs of rearing and relative fitness benefits of sons and daughters cannot be tracked into adulthood.SignificanceBirds can optimize their fitness return on parental investment by biasing offspring sex ratios. When the costs and benefits of raising sons or daughters differ under low and high food availability, females could produce either more sons or daughters depending on those conditions. Black-throated blue warbler offspring sex ratios were male biased in high quality habitat. However, we found no evidence that food-supplemented females or females on territories with higher caterpillar abundance produced more sons than daughters as predicted by the Trivers-Willard hypothesis. The relative costs of producing sons or daughters did not differ under low and high food availability, but we could not directly examine differences in relative benefits. Findings indicate that this migratory bird does not adjust sex ratios in response to food availability and highlights the need for evaluating future fitness benefits of sons and daughters in migratory species.

Incubation of an empty nest is a rarely detected phenomenon in birds but has been documented in a diversity of species. The causes of this breeding anomaly are unclear, but previous accounts have occurred in regions with elevated air pollution. We provide a detailed account of a female Black-throated Blue Warbler (Setophaga caerulescens) incubating an empty nest for 8 d at the Hubbard Brook Experimental Forest, New Hampshire, USA, May–June 2023, and explore possible explanations based on a literature review of this breeding anomaly. Our observation coincided with the presence of smoke from widespread wildfires in Quebec, Canada. The fires were caused by record temperatures and dry conditions. Wildfire smoke increased air particle pollution. We found 231 published accounts of 11 species incubating empty nests. Cavity-nesters in Europe comprised 45% of species (5 of 11) and 96% of accounts (225 of 231). Although many accounts lacked a clear explanation, the most common factors hypothesized to explain incubating empty nests were exposure to environmental pollution (81% of accounts, 167 of 206), calcium deficiency from environmental acidification (19% of accounts, 40 of 206), and disease and/or infertility (4% of accounts, 8 of 206). Our synthesis of published observations highlights the need for more data on individuals expressing this unusual behavior. Such data would improve our understanding of how egg laying and reproductive success respond to intrinsic and environmental factors, including wildfire emissions, which are forecasted to increase with climate change. La incubación de un nido vacío es un fenómeno poco detectado en aves pero ha sido documentado en diversas especies. Las causas de esta anomalía reproductiva no son claras pero se han dado reportes previos en regiones con alta contaminación aérea. Proporcionamos un reporte detallado de una hembra de reinita Setophaga caerulescens incubando un nido vacío por 8 días en Hubbard Brook Experimental Forest, New Hampshire, Estados Unidos, de mayo a junio 2023 y exploramos posibles explicaciones basadas en una revisión de literatura de reproducción anómala. Nuestra observación coincide con la presencia de humo de extensos incendios forestales en Quebec, Canadá. Los incendios fueron causados por temperaturas récord y condiciones de sequía. El humo de los incendios forestales incrementó la contaminación por partículas en el aire. Encontramos 231 reportes publicados de 11 especies de aves incubando nidos vacíos. Las especies que anidaban en cavidades en Europa comprendían el 45% de las especies (5 de 11) y 96% de los reportes (225 de 231). Aunque muchos reportes no incluían una explicación clara, las hipótesis más comunes que explicaban la incubación de nidos vacíos incluían los siguientes factores la exposición a contaminación ambiental (81% de los reportes, 167 de 206), deficiencia de calcio por acidificación ambiental (19% de reportes, 40 de 206) y enfermedad y/o infertilidad (4% de los casos, 8 de 206). Nuestra síntesis de las observaciones publicadas resalta la necesidad de más datos de individuos expresando este comportamiento inusual. Estos datos mejorarían nuestra comprensión de cómo la puesta de huevos y el éxito reprodcutivo responden a factores intrínsecos y ambientales, incluyendo emisiones de incendios forestales, que se pronostican en aumento con el cambio climático. Palabras clave: calidad de aire, comportamiento anormal, contaminación del aire, fracaso reproductivo, humo de incendio forestal, incubación de nido vacío.

Incubation of an empty nest is a rarely detected phenomenon in birds but has been documented in a diversity of species. The causes of this breeding anomaly are unclear, but previous accounts have occurred in regions with elevated air pollution. We provide a detailed account of a female Black-throated Blue Warbler (Setophaga caerulescens) incubating an empty nest for 8 d at the Hubbard Brook Experimental Forest, New Hampshire, USA, May-June 2023, and explore possible explanations based on a literature review of this breeding anomaly. Our observation coincided with the presence of smoke from widespread wildfires in Quebec, Canada. The fires were caused by record temperatures and dry conditions. Wildfire smoke increased air particle pollution. We found 231 published accounts of 11 species incubating empty nests. Cavity-nesters in Europe comprised 45% of species (5 of 11) and 96% of accounts (225 of 231). Although many accounts lacked a clear explanation, the most common factors hypothesized to explain incubating empty nests were exposure to environmental pollution (81% of accounts, 167 of 206), calcium deficiency from environmental acidification (19% of accounts, 40 of 206), and disease and/or infertility (4% of accounts, 8 of 206). Our synthesis of published observations highlights the need for more data on individuals expressing this unusual behavior. Such data would improve our understanding of how egg laying and reproductive success respond to intrinsic and environmental factors, including wildfire emissions, which are forecasted to increase with climate change. Received 22 February 2024. Accepted 25 June 2024.

In socially monogamous species, male reproductive success consists of “within-pair” offspring produced with their socially paired mate(s), and “extra-pair” offspring produced with additional females throughout the population. Both reproductive pathways offer distinct opportunities for selection in wild populations, as each is composed of separate components of mate attraction, female fecundity, and paternity allocation. Identifying key sources of variance and covariance among these components is a crucial step toward understanding the reproductive strategies that males use to maximize fitness both annually and over their lifetimes. We use 16 years of complete reproductive data from a population of black-throated blue warblers (Setophaga caerulescens) to partition variance in male annual and lifetime reproductive success, and thereby identify if the opportunity for selection varies over the lifetimes of individual males and what reproductive strategies likely favor maximum lifetime fitness. The majority of variance in male reproduction was attributable to within-pair success, but the specific effects of individual components of variance differed between total annual and total lifetime reproductive success. Positive overall lifetime covariance between within-pair and extra-pair components indicates that males able to maximize within-pair success, particularly with double-brooding females, likely achieve higher overall lifetime fitness via both within-pair and extra-pair reproductive pathways.

Understanding the demographic drivers of range contractions is important for predicting species’ responses to climate change; however, few studies have examined the effects of climate change on survival and recruitment across species’ ranges. We show that climate change can drive trailing edge range contractions through the effects on apparent survival, and potentially recruitment, in a migratory songbird. We assessed the demographic drivers of trailing edge range contractions using a long-term demography dataset for the black-throated blue warbler (Setophaga caerulescens) collected across elevational climate gradients at the trailing edge and core of the breeding range. We used a Bayesian hierarchical model to estimate the effect of climate change on apparent survival and recruitment and to forecast population viability at study plots through 2040. The trailing edge population at the low-elevation plot became locally extinct by 2017. The local population at the mid-elevation plot at the trailing edge gradually declined and is predicted to become extirpated by 2040. Population declines were associated with warming temperatures at the mid-elevation plot, although results were more equivocal at the low-elevation plot where we had fewer years of data. Population density was stable or increasing at the range core, although warming temperatures are predicted to cause population declines by 2040 at the low-elevation plot. This result suggests that even populations within the geographic core of the range are vulnerable to climate change. The demographic drivers of local population declines varied between study plots, but warming temperatures were frequently associated with declining rates of population growth and apparent survival. Declining apparent survival in our study system is likely to be associated with increased adult emigration away from poor-quality habitats. Our results suggest that demographic responses to warming temperatures are complex and dependent on local conditions and geographic range position, but spatial variation in population declines is consistent with the climate-mediated range shift hypothesis. Local populations of black-throated blue warblers near the warm-edge range boundary at low latitudes and low elevations are likely to be the most vulnerable to climate change, potentially leading to local extirpation and range contractions.

The Indian subcontinent is the primary wintering ground and stopover site for migratory shorebirds to refuel along the Central Asian and South Asian Flyways. Despite the conservation importance of this region for migratory shorebirds, we lack information on the distribution and abundance of over-summering shorebirds–migrants that remain on their wintering grounds during the breeding season–to evaluate the impacts of anthropogenic change on this poorly understood life history strategy. We conducted weekly surveys of over-summering shorebirds at mudflats, mangroves, and sand beaches at the Kadalundi-Vallikkunnu Community Reserve on the southwest coast of India, 2005-2018. We examined long-term patterns of over-summering shorebird abundance, species composition, and habitat use and estimated proportional changes in over-summering abundance of each shorebird species documented at this globally important site. Over the 14 yr study, we documented 7 over-summering species, including Lesser Sand Plover (Charadrius mongolus), Whimbrel (Numenius phaeopus), Greater Sand Plover (Charadrius leschenaultia), Kentish Plover (Charadrius alexandrinus), Common Sandpiper (Actitis hypoleucos), Ruddy Turnstone (Arenaria interpres), and Pacific Golden-Plover (Pluvialis fulva). Mean abundance and species richness were higher at mudflats than at mangroves and sand beaches, with fewer individuals and species observed over-summering after 2010. Mudflats also had the highest species diversity and evenness compared to mangroves and sand beaches. Management plans that aim to restore vulnerable mudflats and mangroves and to reduce anthropogenic threats such as sand mining and waste dumping are needed to prevent the loss of important over-summering, foraging habitat for migratory shorebirds in southern India and along the Central Asian and South Asian Flyways. Received 23 January 2019. Accepted 24 November 2019.