Ecology and Management of Shiny Cowbirds (Molothrus bonariensis) and Endangered Yellow-Shouldered Blackbirds (Agelaius xanthomus) in Puerto Rico

Abstract

Historically, the Shiny Cowbird (Molothrus bonariensis; hereafter "cowbird") was confined to South America, Trinidad, and Tobago. Within the past century, cowbirds have expanded their range into the West Indies, bringing them into contact with avian communities that have never experienced brood parasitism. Because of their small populations, high degree of isolation, and lack of experience with cowbirds, West Indian birds are at greater risk of harm from cowbird contact than mainland birds. Cowbird parasitism was the single most important factor in the reduced reproductive output of the endangered Yellow-shouldered Blackbird (Agelaius xanthomus; hereafter "blackbird"), a Puerto Rican endemic. Roost counts estimated that the blackbird population declined in southwestern Puerto Rico from 1,663 individuals (1974-1975) to 266 (1981-1982). Cowbird trapping in upland areas near blackbird nesting areas in mangroves and cowbird feeding areas has proved successful in managing cowbirds and increasing blackbird populations. Removal of cowbird eggs and nestlings from artificial nesting structures has resulted in fewer parasitized blackbird nests. As a result of those measures, cowbird parasitism of blackbirds has fallen from 95% (1973-1983) to <3% (2000-2003) and, judging from roost counts in 2004, the blackbird population has increased to 800 individuals in southwestern Puerto Rico. Cowbird management programs have excellent potential to reduce the adverse effects of cowbird parasitism; however, trapping within nesting areas is not recommended, because it may increase *he probability of capturing and stressing breeding blackbirds. Resumen.Historicamente, Molothrus bonariensis estaba confinado a Sudamerica, Trinidad, y Tobago. A lo largo del siglo pasado, Molothrus bonariensis ha expandido su area a las Indias Orientales, lo que los ha puesto en contacto con comunidades de aves que nunca antes habian experimentado el parasitismo de cria. Debido a su pequeno tamano poblacional, alto grado de aislamiento, y falta de experiencia previa con Molothrus, las aves de las Indias Orientales estan en un mayor riesgo de dano causado por los tordos que las especies continentales. El parasitismo por el Molothrus bonariensis fue el unico y mas importante factor responsable de la reduccion del exito reproductivo de Agelaius xanthomus, una especie en peligro endemica de Puerto Rico. Mediante conteos en dormideros se estimo que la poblacion Agelaius xanthomus disminuyo en el sudeste de Puerto Rico de 1,663 individuos (1974-1975) a 266 (1981-1982). El trampeo Molothrus bonariensis en zonas altas cercanas a las areas de nidificacion de Agelais xanthomus, zonas de manglares y areas de alimentation de M. bonariensis demostro ser exitoso en manejar a los M. bonariensis e incrementar las poblaciones de A. xanthomus. La remocion de huevos y pichones de M. bonariensis de las estructuras de nidificacion artificiales se tradujo en un menor numero de nidos de A. xanthomus parasitados. Como consecuencia de dichas 5E-mail: alexander.cruz@colorado.edu