Moon snails' mysterious egg cases

Brown-headed cowbirds are terribly problematic

Posted May 1, 2018 at 3:24 PM

Updated May 1, 2018 at 3:24 PM

Brown-headed cowbirds have shown up at my bird feeder a couple times over the last few weeks. Fleeting visits, they were there and then gone. I have always liked them because they seem so exotic - brown heads contrasting with their glossy black bodies (this is true of the males -- the females are plain brown).

These are a problematic bird. Here is a typical sentiment (ohiohistorycentral.org): “The brown-headed cowbird is one of the most hated native songbirds. The females are brood parasites. This means that rather than building a nest and incubating their own eggs, they will lay an egg, almost daily, in the nests of other songbirds. The cowbirds generally hatch first and are larger and more aggressive than the nestlings that are supposed to be there. As a result, these young quickly die of starvation and populations of approximately 150 species of songbirds such as vireos, warblers, buntings and sparrows have declined.” Frank Chapman, founder of the Christmas Bird Count described the mating songs of the males as “guttural bubblings produced with apparently nauseous effort.”

One of the most hated! Guttural bubblings!! This is a strong sentiment for a bird just trying to get by. All of the above is true, but there is so much more to this story. Instead of raising their own young, brown-headed cowbirds lay their eggs in the nests of other birds. This is a good strategy -- instead of putting energy into raising young, the female cowbird puts her energy into laying eggs -- sometimes more than three dozen eggs per season. They have been observed laying eggs in at least 220 different host species. In addition to out-competing their nest mates for food they’ll even throw other eggs out of the nest or smother the other hatchlings. This sounds like a simple strategy for success -- but it isn’t, the female cowbirds must time their laying very precisely and the young must be able to adapt to a variety of host parents and somehow learn they are brown-headed cowbirds instead of the host species.

The female employs a number of strategies to make sure everything goes well. According to Seth Inman (of Cornell Ornithology Lab) the females can only successfully parasitize another nest “by silently watching potential host activity. They must have several nests under observation, so they can lay eggs at daily intervals. A female cowbird’s average laying time is 41 seconds, compared to most songbirds that range from 20.7 to 103 minutes. Timing is key because if a female lays too soon, her host parents might desert the nest or build a new nest floor on top of the egg (Yellow Warblers often do this); too late, and the cowbird offspring will be at a disadvantage with their foster siblings. The female cowbird needs to get in, lay the egg, and get out before the parasitized parents come home to see her at work.”

Another big question is how do young brown-headed cowbirds know they are cowbirds? There appears to be a critical period of time - the first few months of a young cowbirds life - in which it has to recognize itself as a cowbird versus its host species. A study by Mark Hauber, a behavioral ecologist at the University of California Berkeley, has found that six-day-old cowbird chicks can identify sounds produced by adult cowbirds and also look at their own appearance and compare this with other cowbirds they encounter - this is evidence of self-awareness! The researchers also found that adult brown-headed cowbirds visit their offspring while they are still with their host species and teach them cowbird-specific behaviors. This is critical for young cowbirds identifying themselves as cowbirds. When juvenile cowbirds were kept in a cage with canaries for their first winter they adopted the behaviors of their host species - singing canary songs and courting the canaries instead of cowbirds.

Brown-headed cowbirds are native to grasslands - but have expanded into agricultural, urban and suburban habitats - wherever they can get grain, or wherever cattle are disturbing the soil. Before human intervention, they followed herds of bison, they ate insects kicked up by bison hooves. One thought is that they became brood parasites because they were constantly on the move, chasing bison, with no time to care for their young. They have expanded their range because of human development in the North East - fragmentation of forests, increase in agricultural lands, more lawns and birdfeeders. We can only blame ourselves if they are causing the decline of other bird species. So, while I can’t celebrate their impact on other bird species, I can celebrate their tenacity and all of those wonderful adaptations that have helped them succeed in a human-dominated world.