Rusty Blackbird: Mysteries of a Species in Decline

Abstract

Abstract. The Rusty Blackbird (Euphagus carolinus) breeds across the boreal forest zone of North America and winters throughout the eastern United States. Over the past four decades, the North American Breeding Bird Survey and the Christmas Bird Count have shown high rates of population decline ranging from approximately 5 to 12% per year. Regional surveys suggest declines and range retractions in the southern boreal zone. Analyses of historical accounts suggest that the Rusty Blackbird's abundance has been dropping steadily for over a century. A number of hypotheses have been proposed to explain the decline. The species relies on wooded wetlands throughout the year, so loss and degradation of these habitats-particularly in the winter range-is a prime suspect. Blackbird-control programs may have contributed. In recent decades, habitat disturbance, global warming, and environmental contamination in the boreal zone may have taken their toll on breeding populations. In 2005, the International Rusty Blackbird Working Group was formed to develop research efforts toward understanding the mysterious decline. This special section presents the group's research findings-the first on the species' use of breeding and winter habitat, reproductive success, parasite prevalence, patterns of molt, and migratory connectivity. Data on the levels of methylmercury in tissues and the role of timber management on reproductive success are intriguing. We outline research needed for assessment of the roles of various factors in causing the decline of the Rusty Blackbird. Resumen. Euphagus carolinus es una especie distribuida a lo largo de la zona de los bosques boreales de América del Norte durante la época reproductiva y que pasa el invierno a lo largo del este de los Estados Unidos. Durante las últimas cuatro décadas, el Censo Norteamericano de Aves Reproductivas y el Conteo de Aves de Navidad muestran altas tasas de disminución de poblaciones en un rango aproximado que va del 5 al 12% anual. Los censos regionales sugieren disminuciones de población y retracciones de distribución en la zona boreal meridional. Los análisis de conteos históricos sugieren que la abundancia de E. carolinus ha venido disminuyendo sostenidamente por más de un siglo. Se propusieron varias hipótesis para explicar la disminución. Esta especie depende de humedales arbolados a lo largo del año, por lo tanto la pérdida y degradación de estos habitatsparticularmente en el área de distribución de invierno-representan la principal sospechosa. Los programas de control de E. carolinus pueden haber contribuido. En décadas recientes, la alteración de hábitat, el calentamiento global y la contaminación ambiental en la zona boreal pueden haber tenido un grave efecto en las poblaciones reproductivas. En el 2005 se formó el Grupo Internacional de Trabajo de E. carolinus (IRBWG por sus siglas en inglés) para desarrollar esfuerzos de investigación para lograr entender la misteriosa disminución. Esta sección especial presenta los resultados de las investigaciones del IRBWG-el p...

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