Susan Heath is the Gulf Coast Bird Observatory’s director of conservation research. The Gulf Coast Bird Observatory (GCBO) is a nonprofit organization whose mission is to protect birds and their habitats throughout the Gulf Coast and into their wintering grounds in Central and South America.
Because cowbirds are native to the U.S., they are protected under the Migratory Bird Treaty Act and in most instances it is unlawful to use lethal control without a permit, including the removal of their eggs from a nest. However, unpermitted control of cowbirds is occasionally permissible under special circumstances outlined in the Migratory Bird Treaty Act. Additionally, in some states, such as Michigan and Texas, permits can be obtained to trap cowbirds to protect endangered species like Kirtlands Warbler, Golden-cheeked Warbler, and Black-capped Vireo. Please check with your states wildlife management agency for local regulations.
The Brown-headed Cowbird (Molothrus ater) is a brood parasite, meaning that it lays its eggs in nests of other species. A female cowbird quietly searches for female birds of other species that are actively laying eggs. Once she has found a suitable host, the cowbird will sneak onto the resident birds nest when it is away, usually damage or remove one (or more) egg, and replace that egg with one (or more) of her own. The foster parents then unknowingly raise the young cowbirds, usually at the expense of their own offspring (watch a cowbird fledgling being fed by an Eastern Phoebe and a cowbird removing an Eastern Kingbird nestling from a nest at 0:45). Cowbird eggs require a shorter incubation period than most other songbirds and thus usually hatch first. Cowbird nestlings also grow large very quickly. These advantages allow them to command the most food from their foster parents, usually resulting in reduced nesting success of the host species.
Originally from the United States, brown-headed cowbirds favor agricultural, urban, and suburban settings with easily accessible grain or cattle-disturbed soil. They used to follow herds of bison and eat the insects that the animals’ hooves kicked up. It’s unclear if they adopted their breeding strategy as a result of having to relocate regularly to stay up with the bison herds, or if their breeding strategy allowed them the flexibility to do so. This species has benefited greatly from the expansion of agricultural areas and the removal of forest cover, which have increased overall habitat and allowed cowbirds to access new host species that do not have defensive mechanisms against nest parasitism. Although it is evident that cowbirds have profited from forest fragmentation, it is less certain how much of a role they have played in the population-level declines of many forest birds.
Due to its ability to parasitize over 220 different species of North American birds, the cowbird is not exclusively dependent on one host species, allowing its effects to be felt by numerous populations. While cowbirds have been linked to the decline of rare species like Black-capped Vireo and Kirtland’s Warbler, habitat loss and fragmentation probably contribute significantly more to songbird declines. This is demonstrated by the fact that the endangered Kirtland’s Warbler did not see a rise in population on its own; rather, the warblers did not recover until cowbird control was paired with habitat management for nascent Jack Pine forests.
Some species, such as the Yellow Warbler, can recognize cowbird eggs and will reject them or build a new nest on top of them. Those species which accept cowbird eggs either do not notice the new eggs, or as new evidence suggests, accept them as a defense against total nest destruction. Cowbirds may punish egg-rejectors by destroying the entire nest, whereas it is possible for egg-acceptors to raise some of their own young in addition to the cowbird young (see Birdscope 2008).
FAQ
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