Several lines of evidence indicate declines in American Redstart (Setophaga ruticilla) populations in northeastern NorthAmerica. I used data on historical changes in forest characteristics to test the hypothesis that these declines are a consequence of forest maturation and subsequent loss of the early successional habitats preferred by redstarts. Comparison of Breeding Bird Survey data on redstart abundance with those on habitat change in Vermont and New Hampshire revealed two major patterns. In three regions, redstart abundance was positively correlated with the amount of early successional habitat, whereas in two others it was positively correlated with total forest area. A mathematical model incorporating historical patterns of habitat change generated population trajectories that were reasonable approximations of the Breeding Bird Survey data for five of six regions when parameterized to reflect the higher suitability of early successional habitats. For this model to approximate observed trends, redstarts had to occur at higher densities in early successional habitats and experience higher nest success and juvenile survival there than in mature forest. In particular, unless nest success in early successional habitats was higher than published values for mature forest, simulated populations declined toward extinction. These results suggest that mature forests may frequently function as sink habitats for American Redstarts and that recent redstart population declines have resulted from losses of early successional habitat.Evidencia de la Importancia del Hábitat de Sucesión Temprana para Setophaga ruticilla Mediante un Modelo Poblacional de Paisaje Resumen: Varias líneas de evidencia indican disminuciones en las poblaciones de Setophaga ruticilla en el noreste de Norte América. Utilicé datos de cambios históricos en las características del bosque para evaluar la hipótesis de que estas disminuciones son una consecuencia de la maduración del bosque y la pérdida subsecuente de los hábitats de sucesión tempranos preferidos por S. ruticilla. Comparaciones de datos de S. ruticilla del Programa de Observaciones de Anidación de Aves con datos de cambio de hábitat en Vermont y New Hampshire revelan dos patrones principales. En tres regiones la abundancia de S. ruticilla estuvo positivamente correlacionada con la cantidad de hábitat sucesional temprano, mientras que en otras dos, estuvo positivamente correlacionada con el área total del bosque. Un modelo matemático que incorpora los patrones históricos de cambio del hábitat generó trayectorias poblacionales que resultaron ser aproximaciones razonables a los datos obtenidos de las observaciones de anidación para cinco de seis regiones en las que se modeló para reflejar la máxima viabilidad de hábitats sucesionales tempranos. Según este modelo, para que S. ruticilla se aproxime a las tendencias observadas, la especie tiene que ocurrir en grandes densidades en los hábi-tats sucesionales tempranos y experimentar un éxito de anidación mayor en estos lugares que en los bosques...
To investigate the possible relationship between habitat changes and population declines in the American Redstart (Setophaga ruticilla), I studied redstart population structure in habitats spanning a successional gradient in northern hardwoods forests of the White Mountains, New Hampshire during the summers of 1992 to 1994. Early successional stands, in general, supported higher densities and a greater percentage of older (ASY) males than did mature stands. Among mature forests, redstart densities and proportions of ASY males were lower in stands with greater representation of conifers. Mating success varied among habitats, primarily for yearling males, which were more likely to obtain a mate in the early successional habitats and less likely to do so in coniferous habitats. Territories were significantly smaller in early successional habitats. The combination of high densities, high proportions of older males, high mating success in yearlings, and small territory size supports the hypothesis that early successional habitats are more suitable for redstarts than are older forests. Early successional habitats may be more suitable due to higher prey availability and/ or reduced predation on nests, although conclusive data are not available for either of these variables. If reproductive success follows a similar pattern, then the loss of early successional habitats as a result of reforestation in the breeding areas could be a major cause of population declines of American Redstarts in New England.
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