| Manolis, T., and A. Cruz. 2002. Mating and nest-searching behaviour of Shiny Cowbirds associated with different host species in Trinidad and Tobago. Department of Life Sciences, University of the West Indies, St. Augustine, Occasional Paper 11:155-165. ABSTRACT.--The Shiny Cowbird (Molothrus bonariensis) is a brood parasite of a wide variety of passerine species in South America and the Caribbean region. We examined the mating and nest-searching behaviour of cowbirds parasitising various hosts in different environments in Trinidad and Tobago. Breeding cowbirds congregate at nesting colonies of Yellow-hooded Blackbirds (Agelaius icterocephalus) in marshes in Trinidad in the rainy season. Most female cowbirds visiting territories and nests of blackbirds do so singly or in groups of two or more females. Little evidence of cowbird pair formation was observed at blackbird colonies. We observed a different pattern of cowbird mating behaviour in rural areas of mixed agricultural plots and woodland, where House Wrens (Troglodytes aedon), which have more uniformly dispersed nesting attempts than the colonial blackbirds, are the primary host. At one such site, we documented stable pair formation over a period of a few weeks within a small population of colour-banded cowbirds. Cowbirds also showed a greater likelihood to visit House Wren nests and territories in male-female pairs than for other potential hosts. Combining various lines of evidence (grouping patterns, mating and nest-searching behaviour, and pair formation among colour-banded birds) offers some support for a hypothesis that cowbirds are promiscuous where host (and cowbird) densities are highly clumped (e.g., blackbird colonies), and monogamous where hosts are fairly common but uniformly dispersed (e.g., House Wren territories). Shiny Cowbird mating behaviour seems to show considerable flexibility, both within and between populations. More extensive studies with a greater range of hosts over a broad range of habitats throughout the cowbird�s range are warranted to elucidate the effects of host type and availability, as well as other ecological factors, on its mating behaviour. RESUMEN.--El Tordo Mirlo (Molothrus bonariensis) es una ave que par�sita a una amplia variedad de especies paserinas en Sudam�rica y el Caribe. Examinamos los comportamientos de acoplamiento y b�squeda de nidos del tordo al parasitar varios hu�spedes en ambientes diferentes en Trinidad y Tobago. Durante la �poca de anidaci�n, que coincide con la temporada de lluvia, los tordos se congregan en las colonias reproductivas del Turpial de Agua (Agelaius icterocephalus) en los pantanos de Trinidad. La mayor�a de hembras del tordo que visitan los territorios y nidos del turpial lo hacen solas o en grupos de dos o m�s hembras. Poca evidencia de formaci�n de parejas macho-hembra del tordo fue observada en las colonias del turpial. Observamos en el tordo un patr�n diferente de apareamiento en las �reas rurales (terrenos de agricultura mezclados con bosques) donde el Cucarachero Com�n (Troglodytes aedon), quien distribuye sus intentos de anidaci�n m�s uniformemente dispersos que el turpial, es el hu�sped primario. En uno de dichos lugares documentamos la formaci�n estable de parejas durante un per�odo de semanas en una poblaci�n de tordos anillados. Los tordos tambi�n mostraron una mayor tendencia a visitar en pareja los nidos y territorios del cucarachero que el de otros hu�spedes potenciales. Combinando varias l�neas de evidencia (patrones de agrupamiento, acoplamiento, comportamiento de b�squeda de nidos y formaci�n de parejas entre aves anilladas) hay una tendencia que soporta la hip�tesis que los tordos son promiscuos cuando los densidades de los h�espedes (y tordos) son sumamente agrupadas (e.g., colonias del turpial), y monogamos cuando los hu�spedes son comunes pero uniformemente dispersos (e.g., territorios del cucarachero). El Tordo Mirlo muestra flexibilidad considerable, tanto dentro como entre poblaciones. Estudios m�s intensivos con m�s hu�spedes, sobre una escala m�s amplia de h�bitats y a trav�s de todo el �rea de distribuci� del tordo son necesarios para aclarar los efectos de tipos y disponibilidad de huespedes, as� como otros factores ecol�gicos, en su comportamiento de apareamiento. KEY WORDS.--brood parasitism, mate attendance, mating behaviour, mating systems, Molothrus bonariensis, nest-searching behaviour, Shiny Cowbird |
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| STUDIES IN TRINIDAD AND TOBAGO ORNITHOLOGY HONOURING RICHARD FFRENCH Edited by Floyd E. Hayes and Stanley A. Temple Department of Life Sciences, University of the West Indies, St. Augustine, Occasional Paper 11, 2002 |