The Brown-headed Cowbird, Molothrus ater, is a brood parasite(Kilner, 2003). Female cowbirds lay their eggs in nests of more than 240 hostbird species (Abernathy & Peer, 2014).

Cowbird young depend on the host “parent”for incubation and provisioning. Host responses, such as egg rejection,acceptance, or nest abandonment, to parasitic eggs vary intraspecifically and interspecifically. As generalistparasites, cowbird eggs are typically not mimetic. Yet only 10% of host speciesreject M. ater eggs (Abernathy & Peer,2014).

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A female cowbird may produce more than 40 eggs during a breeding season (Goguen,Curson, & Mathews, 2011). Host nests parasitized by multiple cowbirds are aresult of the high fecundity of M. ater.Cowbird eggs incubate for a shorter period than most host eggs, granting M. ater a competitive advantage over nestmates.Unlike other brood-parasitic species, cowbirdyoung often cohabit the nest with the host offspring rather than killing nestmates (Kilner, 2003).

The benefit to the cowbird young is an increased likelihood of provisioning by the host parent,despite the cost of increased competition with host nestlings for foodresources. The size of the host young is a factor in determining the benefitsand costs of cohabitation. Kilner(2003) found that M. ater fare bestwith intermediate-sized host young in a clutch size of 1-2 nestlings.

Smallhost nest mates may starve when competing for provisions with the larger cowbirdyoung, decreasing the clutch size and provisioning rate by host parent. In thepresence of large host young, M. ater maybe out-competed for provisions (Kilner, 2003). 

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