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Resource Selection by Juvenile Swainson's Thrushes during the Postfledging Period

Jennifer D. White, Thomas Gardali, Frank R. Thompson III and John Faaborg
The Condor
Vol. 107, No. 2 (May, 2005), pp. 388-401
Stable URL: http://www.jstor.org/stable/4096519
Page Count: 14
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Resource Selection by Juvenile Swainson's Thrushes during the Postfledging Period
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Abstract

Resource-selection studies of passerine birds during the breeding season have mainly been limited to understanding those factors important to nesting. However, little is known about what resources are selected by juveniles that are no longer dependent on their parents. The postfledging period may be a critical part of the breeding season for independent juveniles because they must avoid predators and learn to forage on a changing resource base. We used radio-telemetry to study postfledging habitat use and resource selection of juvenile Swainson's Thrushes (Catharus ustulatus) in coastal California from 2000 to 2002. We generated population-level contours (50% and 95% fixed-kernel) to describe habitat use by independent juveniles, and we determined juvenile resource selection by comparing vegetation characteristics at sites used by juveniles versus random sites. Juvenile Swainson's Thrushes used mixed-hardwood forest and coastal scrub during the postfledging period as well as riparian vegetation used by nesting adults. The most parsimonious predictors of resource selection were fruit abundance variables, suggesting that postfledging habitat selection by the Swainson's Thrush is best explained by the optimal-foraging hypothesis. We suggest that juvenile thrushes can track food resources in a habitat mosaic and use vegetation types distinct from what is traditionally considered Swainson's Thrush breeding habitat. /// Los estudios de selección de recursos en aves paserinas durante la estación reproductiva, se han centrado principalmente en entender los factores importantes para la anidación. Sin embargo, se sabe poco acerca de los recursos seleccionados por los juveniles que ya no dependen de sus padres. Para los juveniles independientes, el periodo de emancipación podría ser una parte crítica de la estación reproductiva en la que deben evitar depredadores y aprender a forrajear sobre una base de recursos fluctuante. Por medio de telemetría, estudiamos el uso de hábitat y selección de recursos en juveniles del zorzal Catharus ustulatus en la costa de California entre los años 2000 y 2002. A nivel de población, generamos polígonos (de 50% y 95% de "kernel" fijo) para describir la utilización de hábitat por juveniles independientes, y para determinar la selección de recursos comparamos las características de la vegetatión de los sitios utilizados con la de sitios aleatorios pareados. Los juveniles utilizaron bosques mixtos y matorral costero durante el periodo de emancipación, así como la vegetación riparia utilizada por adultos nidificantes. Las variables con las predicciones más parsimoniosas del uso de recursos fueron las relacionadas con la abundancia de frutos, lo que sugiere que la selección de hábitat de emancipación por C. ustulatus es explicada en mejores términos por la hipótesis de forrajeo óptimo. Sugerimos que los zorzales juveniles pueden encontrar recursos alimenticios en mosaicos de hábitat y usar tipos de vegetación distintos a los que tradicionalmente se consideran como hábitat de anidación.

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