Most passerines use fat to fuel migration and pause at stopover sites to rest or refuel. Moreover, during spring migration, en route to breeding grounds, passerines may deposit “excess” fat as either ...insurance against unpredictable environmental conditions or in anticipation of breeding. We analyzed the energetic condition of Hermit Thrush (Catharus guttatus), Swainson's Thrush (C. ustulatus), and Veery (C. fuscescens) during spring and autumn migration at the Braddock Bay Bird Observatory (Rochester, New York, USA). We used path analysis to determine if the “spring fatter” or insurance hypotheses could help explain some of the variation in energetic condition in Catharus thrushes by designing and analyzing biologically plausible models of the potential effects of season, capture date, hour captured, and age on energetic condition during stopover. While path models differed among species and seasons, capture (or arrival) date was the strongest predictor of energetic condition; contrary to the insurance hypothesis condition increased with date during both seasons for all species. Hour of capture predicted much less variation in condition but was consistently positive (when significant). In long-distance migrants (i.e., Swainson's Thrush and Veery), less experienced or young migrants exhibited better condition than adults regardless of arriving later, which was revealed by including a direct path between age and condition and an indirect path mediated via capture date to control for potential differences in arrival timing related to age. Despite being closely related, we found only a few patterns in common among Swainson's Thrush, Hermit Thrush, and Veery. We suspect differences in phenology, flight, morphology, and migratory strategy may play a significant role in the differences among these species. La mayoría de las aves paserinas utilizan la grasa para impulsar la migración y los sitios de escala para descansar o recargarse. Por ejemplo, durante la migración de primavera, camino a las zonas de reproducción, las aves paserinas pueden formar depósitos de grasa ya sea como seguro contra condiciones ambientales impredecibles o en anticipación a la reproducción. Analizamos la condición energética de los zorzales Catharus guttatus, C. ustulatus, y C. fuscescens durante las migraciones de primavera y de otoño en Braddock Bay Bird Observatory (Rochester, New York, USA). Utilizamos análisis de trayectorias para determinar si la hipotésis de “más gordo en primavera” o la hipótesis de aseguramiento podrían explicar parte de la variación en la condición energética de zorzales Catharus al diseñar y analizar modelos biológicamente posibles de efectos potenciales de estacionalidad, fecha de captura, hora de captura y edad en la condición energética durante su escala. Aunque los modelos de trayectoria difirieron entre especies y estaciones, la fecha de captura (o llegada) fue el predictor más importante de condición energética y esta condición se incrementó a lo largo de ambas estaciones para todas las especies. La hora de captura predijo mucha menos variación pero fue consistentemente positiva (cuando fue significativa). En aves migratorias de larga distancia (como los zorzales C. ustulatus y C. fuscescens), las aves migratorias con menos experiencia o más jóvenes mostraban mejor condición que los adultos aunque llegaran más tarde, lo que fue revelado al incluir una trayectoria directa entre la edad y la condición y una trayectoria indirecta mediada por la fecha de captura para controlar por diferencias potenciales en el tiempo de llegada relacionadas a la edad. A pesar de su relación cercana, encontramos solo unos pocos patrones relacionados entre los zorzales C. guttatus, C. ustulatus, y C. fuscescens. Sospechamos que diferencias en fenología, vuelo, morfología y estrategia migratoria pueden jugar un papel importante en las diferencias entre estas especies. Palabras clave: análisis de confirmación de trayectoria, aves canoras, ecología de escala, hipótesis de aseguramiento, hipótesis de “más gordo en primavera”, migración, modelado de ecuaciones estructurales.
Many passerines regularly use flight calls to communicate during migration. However, flight-calling is inconsistent throughout the day, with a markedly lower degree of calling by free-flying ...individuals during the early evening hours and with peaks in calling typically around dawn. Based on these observations, we hypothesized that individual flight call response would vary by time of day, and the likelihood to respond would be lower at dusk than during the daytime. We tested daytime and dusk flight call responsiveness of captive American Redstarts (Setophaga ruticilla) and Magnolia Warblers (S. magnolia) during spring migration at the Braddock Bay Bird Observatory (near Rochester, New York, USA). We played a conspecific flight call sound stimulus for individual birds placed inside a soundproof recording studio and recorded their acoustic responses. In our experiment, American Redstarts and Magnolia Warblers were significantly less likely to give flight call responses at dusk than the individuals tested during the daytime. The almost absent responsiveness to flight calls at dusk suggests that these species either minimally communicate with one another during take-off or do not use flight calls as a source of communication at this time. These results have important implications for detection and quantitative bioacoustics migration monitoring. They also demonstrate the need for additional studies in temporal flight-calling behavior.
Most passerines use fat to fuel migration and pause at stopover sites to rest or refuel. Moreover, during spring migration, en route to breeding grounds, passerines may deposit "excess" fat as either ...insurance against unpredictable environmental conditions or in anticipation of breeding. We analyzed the energetic condition of Hermit Thrush (Catharus guttatus), Swainson's Thrush (C. ustulatus), and Veery (C. fuscescens) during spring and autumn migration at the Braddock Bay Bird Observatory (Rochester, New York, USA). We used path analysis to determine if the "spring fatter" or insurance hypotheses could help explain some of the variation in energetic condition in Catharus thrushes by designing and analyzing biologically plausible models of the potential effects of season, capture date, hour captured, and age on energetic condition during stopover. While path models differed among species and seasons, capture (or arrival) date was the strongest predictor of energetic condition; contrary to the insurance hypothesis condition increased with date during both seasons for all species. Hour of capture predicted much less variation in condition but was consistently positive (when significant). In long-distance migrants (i.e., Swainson's Thrush and Veery), less experienced or young migrants exhibited better condition than adults regardless of arriving later, which was revealed by including a direct path between age and condition and an indirect path mediated via capture date to control for potential differences in arrival timing related to age. Despite being closely related, we found only a few patterns in common among Swainson's Thrush, Hermit Thrush, and Veery. We suspect differences in phenology, flight, morphology, and migratory strategy may play a significant role in the differences among these species.
Many passerines regularly use flight calls to communicate during migration. However, flight-calling is inconsistent throughout the day. with a markedly lower degree of calling by free-flying ...individuals during the early evening hours and with peaks in calling typically around dawn. Based on these observations, we hypothesized that individual flight call response would vary by time of day, and the likelihood to respond would be lower at dusk than during the daytime. We tested daytime and dusk flight call responsiveness of captive American Redstarts (Setophaga ruticilla) and Magnolia Warblers (S. magnolia) during spring migration at the Braddock Bay Bird Observatory (near Rochester, New York, USA). We played a conspecific flight call sound stimulus for individual birds placed inside a soundproof recording studio and recorded their acoustic responses. In our experiment. American Redstarts and Magnolia Warblers were significantly less likely to give flight call responses at dusk than the individuals tested during the daytime. The almost absent responsiveness to flight calls at dusk suggests that these species either minimally communicate with one another during take-off or do not use llight calls as a source of communication at this time. These results have important implications for detection and quantitative bioacoustics migration monitoring. They also demonstrate the need for additional studies in temporal flight-calling behavior.
