Brown-headed cowbird parasitism of neotropical migratory songbirds in riparian areas along the lower Colorado river

Populations of several riparian-obligate, neotropical migratory songbirds have declined in recent years, partly due to brood parasitism by brown-headed cowbirds (Molothrus ater). In 1994 and 1995, 1 measured parasitism rates and nesting success of 4 riparian-obligate, neotropical migrants breeding in the lower Colorado River valley. Because vegetation characteristics of nest sites may influence cowbird parasitism and host nesting success, I also measured vegetation attributes associated with nest-site selection. Parasitism rates were 40-90%, and reproductive success was significantly lower in parasitized nests. Foliage cover may influence host nest-site selection and prevent cowbird nest discovery. The patchy distribution of native tree species may have aided cowbird nest discovery by providing elevated survey perches. Saltcedar (Tamarix ramosissima) was an important understory component providing foliage cover at nests. Understanding vegetation characteristics associated with host and cowbird nest-site selection should be useful in riparian restoration projects and cowbird control.hydrology collectio