SDRP - Did You Know?

The San Dieguito River Park
18372 Sycamore Creek Rd.
Escondido, CA 92025
Phone: (858) 674-2270
Fax: (858) 674-2280

The River Park has converted from a print-based distribution system to a web-based system. If you are interested in receiving e-mail notices when the quarterly activity schedule is posted to the website, and news of special events occurring in the Park, click above to send us an e-mail.

DID YOU KNOW?
by Shawna Anderson
Spring 2003

CAPTURING COWBIRDS

Have you ever seen a cage near our trail in San Pasqual Valley? No, it’s not an aviary for hummingbirds. It’s a trap for a harmful parasitic bird called the brown-headed cowbird ( Molothrusater). These small blackbirds are known in the San Pasqual Valley (and other riparian areas in the county) as “brood parasites” that victimize other bird species including the endangered least Bell’s vireo, a small, migratory songbird that makes its home in riparian habitats. Brood parasitic birds lay their eggs in other bird nests for the host bird to incubate and raise, often at the expense of the host-bird’s own eggs or hatchlings. They can lay an egg in less than 30 seconds while the host species is away, sometimes ejecting the existing eggs before laying their own eggs. The cowbird eggs usually hatch sooner than the host eggs and the young demand and receive food at a faster rate than the smaller hatchlings, which may eventually die from neglect. A sad story and a common result of the decreasing range of habitat for all bird species, including predator species, which makes predatory behavior a greater threat to vulnerable species like the vireo.

The San Dieguito River Park is participating in a five-year program to trap cowbirds in the valley. These birds tend to congregate at dairies and feed lots where the source of food (insects and seed) is plentiful. The River Park’s two cowbird traps are installed near the San Dieguito River among agricultural operations and are monitored by avian experts and ranger staff. Food and water are kept in the traps, which are checked daily. Last year (Summer 2002), 40 cowbirds were caught. It’s no fun, but the birds are then euthanized. The cooperation of two agricultural lessees in the valley, Witman Ranch and Verger Dairy, contribute to the success of the program. The River Park’s rangers and volunteers are being trained to care for the trap and correctly identify cowbirds. Studies have shown that removing cowbirds has benefited endangered songbirds by increasing their nesting success. So, if you see someone tampering with a trap – please let us know.