Species-Specific Management (SSM)

Cowbirds

The Eastern cowbird , Molothrus ater ater, seems to have increased in numbers and range within recent decades. Once believed to be a part of (and commensal with) the mid-continent bison ecosystem, the small blackbird has expanded its range and reached pest proportions. It is known for its nest parasitism. It lays its eggs in the nests of other birds and the nestlings are raised by substitute parents. Concern has been expressed about the reduced survival rates of warblers (notably the Cerulean warbler) and other species affected by this practice.

They eat plant seeds and grains and less than 25 % of their diet is insects.

The birds have become notable users of the nests of birds at field and pasture edges. In wildlife clearings within forests, they are believed to parasitize nests of thrushes and warblers, birds of importance often found at such edges.

Their effects are notable when they "work" from fields into woodlots or small forest patches. The effects are less when they work from the gap, the small clearcut. The edge length of 20 roughly-square, one-acre, group-selection cuts is 16,697 feet. This is correlated with the attack zone said to be that of the cowbird. The edge of a 20-acre roughly square forest clearcut is 3734, a rsatio of about 0.22. Wildlife managers need area-related studies of the densities of the song birds that can be affected by the cowbird, the probable parasitism rate, the value of the birds (and thus the loss-potential, and the status of the population relative to the needs of observers or other resource users. The nesting birds of the "interior forest" may not be those abundantly parasitized or for which greatly expanding populations are desired.

The cowbirds winter in the southern states of the US, are migratory, and thus are a federally-protected "blackbird" and control practices are limited. Few people will seek to increase them. Pest control operations seeking to control blackbird flocks damaging crops or cattle feeding operations should be contacted. Mist netting has been used in some areas. Only clear demonstration of harmful effects to a measured wildlife resource is likely to allow control authorization.

General blackbird control descriptions are available in PDF format from the Internet.

Submitted by Robert H. Giles, Jr.

This Web site is maintained by R. H. Giles, Jr.
Last revision January 17, 2000.