Egg removal by brown-headed cowbirds: a field test of the host incubation efficiency hypothesis

Article Abstract:

The parasitic brown-headed cowbird preys on more than 200 North American bird species but the significance of its egg-removal strategy is ill understood. One hypothesis suggests that host egg size and number is linked to the incubation efficiency of the parasitic egg. A new study designed to test the theory examines the incubation of parasitic eggs by red-winged blackbirds and chipping sparrows. It is concluded that the incubation efficiency theory does not explain host-egg removal by brown-headed cowbirds.

Observations, Parasites, Host-parasite relationships

Brood parasitism by brown-headed cowbirds on brown thrashers: frequency and rates of rejection

Article Abstract:

The rate of parasitism by brown-headed cowbirds, Molothrus ater, on brown thrashers', Toxostoma rufum, nests is significantly higher than reported. The parasitism rate averaged 12% in a study period covering 1984 to 1986. Experimental parasitism studies were conducted to determine whether this high rate is brought about by an increased parasitism of cowbirds or a reduced rejection by brown thrashers. Results indicate that the low rejection rate of thrashers contributes to the high rate of parasitism.

Fecundity and egg-laying patterns of captive yearling Brown-headed Cowbirds

Article Abstract:

A study was conducted on the Brown-headed Cowbird (Molothrus ater) to determine the individual fecundity, clutch sizes and egg-laying patterns of juvenile females. Twenty-five juvenile females were caught and kept under surveillance throughout the breeding season in pens containing nests with real or mock eggs. It was found that most females laid eggs at random and not in sets or clutches and could lay as many as 30 eggs in their first season.

Bird breeding, Captive wild birds