Tropical Ecology and Conservation [Monteverde Institute]

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Jason Fisher

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Publication Date

March 2003

Abstract

Colonies of nesting birds are often subdivided into clusters of nests; the causes and effects of these clusters have profound impacts on the breeding strategies of colonial nesting birds. Five colonies of a Neotropical colonial nesting bird, the Chestnut-headed Oropendola (Psarocolius wagleri), were studied to identify trends in male and female intersexual competition, mating strategy, and brood parasitism in relation to colony and cluster size. The results were compared with data from studies by Webster (1994a, b) for another colonial nester, the Montezuma Oropendola (Psarocolius montezuma), which is known to use female-defence polygyny. The comparisons showed that while both the Montezuma and Chestnut-headed Oropendolas use female-defense polygyny, the degree to which a few males are able to monopolize copulations varies with cluster size and synchrony. Montezumas construct nests in larger colonies, more dense clusters, and as a result, males compete to defend nest clusters and copulate at the clusters, female aggression increases with cluster size, and nest parasitism decreases as clusters become larger. For the Chestnut-headed Oropendola, on the other hand, I found that males rarely compete near nests, guard clusters, or copulate near clusters. Female aggression and parasitism showed no significant relationship. Because copulations occur away from nests, and because there is partial synchrony of nest stage for all active nests in a colony, males must focus their breeding attention on one female at a time, which allows multiple males, rather than only a few, to successfully copulate. As a result, cluster sizes are smaller and more spread out. Reproducción en colonias y apiñándose de nidos han ventajas y desventajas. Cinco colonias de un pájaro de los neotrópicos que hace sus nidos en colonias, la Oropendola Cabelicastafia (Psarocolius wagleri), era estudiada para buscar tendencias en la competición intersexual de muchachos y muchachas, la estrategia para aparearse, y parasitismo de nidos en relación al tamaño de colonias y grupos de nidos. Las resultas eran comparadas con información del trabajo de Webster (1994a, b) para otro pájaro con nidos en grupos, la Oropendola de Montezuma (Psarocolius montezuma), la cual usa la poligamia de muchacha-defensa. Las comparaciones muestran que la Oropendola de Montezuma hace colonias más grandes nidos en grupos densos, y por eso los muchachos pegan para proteger los nidos y aparearse cerca de grupos de nidos, la agresividad de muchachas es más grande con grupos más grande de nidos, y el paratismo es menos para grupos más grandes. Para la Oropendola Cabelicastaña, por otro lado, no encontré evidencia que los muchachos pelan cerca de los nidos ni protegen los grupos de nidos ni se aparean cerca de grupos. La agresividad de muchachas y el paratismo no tienen una relación con el tamaño de grupos de nidos. Estas diferencias sugieren que aunque la Oropendola Cabelicastaña usaba la poligamia de muchacha-defensa en el pasado, ahora la especie esta cambiando y esta usando una estrategia diferente para aparearse.

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Student affiliation: Department of Biology, University of Puget Sound Digitized by MVI

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Reports

Holding Location

Monteverde Institute MVI

Identifier

M39-00128

The causes and effects of nest clustering in colonies of the chestnut-headed oropendola (Psarocolius wagleri)

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