The Brown-headed Cowbird, Molothrus ater, is a brood parasite
(Kilner, 2003). Female cowbirds lay their eggs in nests of more than 240 host
bird 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 generalist
parasites, cowbird eggs are typically not mimetic. Yet only 10% of host species
reject M. ater eggs (Abernathy & Peer,
2014). A female cowbird may produce more than 40 eggs during a breeding season (Goguen,
Curson, & Mathews, 2011). Host nests parasitized by multiple cowbirds are a
result 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 nest

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Unlike other brood-parasitic species, cowbird
young often cohabit the nest with the host offspring rather than killing nest
mates (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 food
resources. The size of the host young is a factor in determining the benefits
and costs of cohabitation. Kilner
(2003) found that M. ater fare best
with intermediate-sized host young in a clutch size of 1-2 nestlings. Small
host nest mates may starve when competing for provisions with the larger cowbird
young, decreasing the clutch size and provisioning rate by host parent. In the
presence of large host young, M. ater may
be out-competed for provisions (Kilner, 2003). 

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