Catherine Ortega, an assistant professor of biology at Fort Lewis, hopes to reduce the number of large-scale cowbird eradication campaigns by shedding light on the ecological and evolutionary role that the ubiquitous brown bird plays in her new reference book, Cowbirds and Other Brood Parasites, published this fall by Arizona University Press in Tucson. In 1996, the U.S. Justice Department ruled that federal agencies no longer had to obtain a permit from the U.S. Fish and Wildlife Service to trap and kill cowbirds. As a result, wildlife managers charged with protecting endangered songbird species have often conducted cowbird eradication campaigns without thoroughly investigating causes of songbird declines, determining whether cowbird parasitism was, indeed, the cause, or investigating possible long-term ecological effects of their eradication campaigns. One cowbird eradication campaign that Ortega investigated was based on data from only five nests. We have songbird populations that are declining, but we don't always understand why they're declining.
Cowbirds are only one source of nesting mortality.
Habitat loss and predation may be more important causes of songbird declines than cowbird parasitism. In some cases, destroying cowbird populations may actually be doing more harm than good, said Ortega. Cowbirds may have evolved with the great bison herds of the Miocene. They followed the herds on the prairies, eating the bugs and seeds the herds stirred up as they roamed. Because their food source moved daily, cowbirds developed a reproduction system in which they would leave their eggs to be raised by nearby host bird species while they migrated with the bison.