Every so often I cheat and post an old article that appeared on my previous, now defunct blog. I saw some cowbirds at the park where I've been birding a lot over the past few weeks, so decided to recycle this one. A fellow-Lower Mainlander, Wanderin' Weeta, also recently wrote about cowbirds and provided links to further information on these birds.
A few days ago, for the first time this year, I heard a distinctive bird song, a short bubbling, followed by a squeaky hinge -- the call of the male Brown-headed Cowbird. This species, slightly smaller than a robin, has a disproportionate influence on the survival of other species. It is a dirty trickster, a layabout, a cruel opportunist, at least by human standards. The male is sleek and handsome, James Bond in a tux, glossy black with a brown head. The female is a dowdy grey-brown, and for a reason. She skulks through the forest, searching egg-bearing nests of other species, and then, when the coast is clear, swoops down, very quickly lays an egg, removes one of the originals, and skedaddles. The nest owners, often smaller species, dutifully incubate the intruder’s egg along with their own, and raise the cowbird chick. This is known as brood parasitism – the reliance on another species to raise your progeny, mistakenly thinking them their own. Cowbird eggs usually hatch first, and the changelings are more demanding when parents arrive with food, so eat more and grow faster than the real offspring. Some, or all, of the true nestlings starve, or are forcibly ejected from the nest by the cowbird chick. In mid-summer, it is not unusual to see diminutive warbler adults frantically stuffing insects down the gullets of squawking, much larger cowbird fledglings.
Historically, cowbirds were associated with the bison herds of the central plains. They would dine on the seeds of prairie grasses and the insects stirred up by the bison, but would also scoot into nearby woods to lay their eggs in the nests of forest birds. Only those potential hosts nesting adjacent to grasslands were targeted, and among those, some had evolved defensive tactics, either recognizing and ejecting cowbird eggs from their nests, or covering up cowbird eggs with additional nesting material, or abandoning their nests and building new ones. Through these countermeasures, cowbird numbers remained in check.
Then came the great manipulation of the continent’s woodlands and prairies, the clearing of deciduous forests in the east, and conversion of scrubland to agricultural field, particularly grain crops, in the west. These changes favoured cowbirds, who spread in both directions, encountering birds that had never before dealt with a brood parasite. Naive species were raising increasing numbers of cowbird offspring, and cowbird populations expanded further.
Cowbirds were first known in significant numbers in B.C. in the southern mountains and interior early in the 1900s, and only became common in the Lower Mainland after the 1950s. It is suspected that the birds in this region came north from the U.S., after moving west across that country. Now cowbirds range throughout the province. As listed in The Birds of British Columbia, cowbirds are known to have parasitized the nests of at least 83 species, with varying rates of success, depending on the host.
Elsewhere, a few species have been greatly harmed by cowbirds. Kirtland’s Warbler, which has a small nesting range in northern Michigan, was pushed near the brink of extinction, in part due to cowbird parasitism. Precipitous decline in the least Bell’s Vireo in southern California was also partly attributable to cowbirds. However, it is important to note that in both cases the original habitats of the hosts had been so severely reduced and fragmented that there was almost nowhere left to hide from a brood parasite.
The Brown-headed Cowbird is a cheat, but is also a historical component of an ecosystem. It is because human activities have so dramatically altered the natural landscape of North America that this small bird can have such influence on the survival of other species.
Campbell, R.W., N.K. Dawe, I. McTaggart-Cowan, J.M. Cooper, G.W. Kaiser, A.C. Stewart and M.C.E. McNall. 2001. The Birds of British Columbia. Vol. 4. UBC Press. 8 million pp.