Brown-Headed Cowbird Male

Day 209: Another of my "backyard birds," Brown-Headed Cowbirds exhibit sexual dimorphism, i.e., the males (shown above) are coloured differently than the females. The female is a nondescript brownish-grey, identifiable as a Cowbird most easily when she strikes the pose typical of the species: beak pointed at the sky, the spine in an almost perfectly straight line from the back of the head to the tail. For want of better terms, it makes them look pin-headed and silly, but Cowbirds are not as brainless as they may look. Cowbirds are brood-parasites, which is to say that they do not build their own nests but lay their eggs in the nests of other bird species. Because their eggs (sometimes as many as three dozen in a season!) develop more rapidly than those of their unwitting hosts, the young frequently outcompete the less-developed offspring of the nest-building species. The good news is that some hosts recognize Cowbird eggs when they see them, and may start a new nest on top of the Cowbird eggs, puncture them, or pitch them out of the nest altogether. So how do young Cowbirds know that in fact they are Cowbirds, and not, say, Warblers? Studies have suggested that they recognize both the call and the appearance of adult Cowbirds in just a few weeks from hatching, and may even examine themselves for comparison to adults, as was demonstrated when both young Cowbirds and adults were artifically marked. The young birds, seeing a distinctive marking on themselves, were drawn to similarly marked adults. That said, Brown-Headed Cowbirds are gifted vocal mimics as well. I spent some time trying to figure out how a whinnying horse had gotten fifty feet up an old Doug-fir at my previous home.