GRIFFITH WILDLIFE BIOLOGY

Cowbird trap	Cowbird trap in riparian habitat

GWB is the recognized leader in the design and implementation of brown-headed cowbird control programs. Jane and John Griffith co-authored the Brown-headed Cowbird Trapping Protocol, a document used by the California Department of Fish & Game and U.S Fish and Wildlife Service for permitting purposes.

Brown-headed cowbirds are about the size of a red-winged blackbird. They are brood parasites – they never make their own nests or raise their own young. Instead, cowbirds lay their eggs in the nests of other birds (hosts), who then raise the cowbird chick as their own. This is fine for blackbirds and other large host species, who can raise a cowbird with little or no harm to their own young. However, when a cowbird parasitizes a small species like the vireo, flycatcher, or gnatcatcher, these smaller hosts are able to raise only the cowbird and none of their own young – a short route to extinction.

Because female cowbirds do not invest time or energy into nesting or caring for young, they can lay one egg every other day for the entire nesting season – 40 to 60 eggs each year. Each egg laid in the nest of a small host eliminates up to 4 songbird young.

Since cowbirds first appeared in California about 1900, the amount of native (host) habitat has greatly decreased, while the amount of cowbird habitat (range and agricultural land, golf courses, parks, lawns) has greatly increased. This combination of severe habitat loss (upwards of 90% for riparian and coastal sage birds) and increasing cowbird parasitism has resulted in the extirpation or endangered status of many small hosts.

Cowbird control has been very successful. For example, the number of vireo pairs at MCB Camp Pendleton has increased from less than 20 to almost 1, 000 since cowbird trapping began in 1983. Prior to trapping, parasitism rates approached 50% at Pendleton (up to 100% at other sites). With trapping, the rate dropped to less than 5%. No cowbird eggs have been found in vireo nests at Camp Pendleton since 1990.

At most sites with fairly narrow linear riparian habitat, one cowbird trap per 1-2 km is sufficient to reduce or eliminate parasitism among a targeted host. Each trap is placed near or within the host habitat or at a nearby cowbird foraging site, and baited with seed, water, perches, shade, and 2 male and 3 female live decoy cowbirds. On top the trap is a 1 3/8" slot through which cowbirds can drop in but cannot fly up and out. Per agency regulations and the GWB trapping protocol, each trap is checked daily for the standard 1 April - 30 June trapping period. This period can be modified depending upon the breeding chronology of the host species protected.

GWB has designed and performed cowbird control projects since 1986. We wrote the industry-standard protocol, and have operated more traps in more locations than any other entity.
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