Brood parasitism is one of the darker sides of the avian world. It is a strategy used by many bird species to conserve energy during the breeding season. Rather than raising young themselves, brood parasites will lay their eggs in the nests of other birds. The most well known of these absentee parents, at least in North America, is the Brown-headed Cowbird. Their beige, brown-spotted eggs which stand out compared to the regular eggs in nests are somewhat iconic to nest watchers, but not for any good reason. When Brown-headed Cowbird chicks hatch they quickly outcompete with the other nestlings for food, causing those nestlings to die. Brown-headed Cowbirds often lay their eggs in the nests of warblers, which manifests in photos of fledglings being fed by adopted parent half their size. In these images it is blatantly obvious that the fledgling is not the same species as the parent, it isn’t even trying to hide it, but effort is not something that you would expect from a species who has others raise their young. Brown-headed Cowbirds are able to get away with this because many of the species they predate on have not developed strategies to deal with cowbird eggs. Brown-headed Cowbirds are a relatively new species in the United States, having spread from the prairies to all f the lower 48 due to human deforestation. Other brood parasites, such as the Common Cuckoo have had to go to extra lengths to hide their eggs, such as having the color match those in their victims, because their victims are used to them.
Obviously, the idea of a species who lays their eggs in other species nests and outcompetes with them may ring an alarm bell with conservationists, and they would be right. The Brown-headed Cowbird played a large role in the decline of the Kirtland’s Warbler, and until recently the species was reliant upon “Cowbird traps”, cages which lure in flocks of Cowbirds for their removal. At the species lowest point, there were only 204 known male Kirtland’s Warbler and 70% of its nests had Cowbird eggs. Kirtland’s Warbler almost collapsed because they had little experience with Cowbirds, which were new to the state of Michigan. The spread of Cowbirds can be linked back to human actions, specifically deforestation, which was the other reason that Kirtland’s Warbler populations fell. Cowbirds originally were found in prairies and fed with Buffalo (hence the name cowbird). This means they prefer open spaces, and so when laying eggs they will do so in nests that are at the edge of a forest. However, when a forest is fragmented to make way for suburban houses and farming, more of that forest becomes accessible to Cowbirds. Not only do vulnerable species lose nesting habitat, but the habitat that is left has been penetrated by parasites.
Despite their darker nature, Brown-headed Cowbirds are genuinely fun birds to see out in the field and are quite incredible. They can be fun photography subjects, even as adults. Despite its drabness, I like Cowbird plumage. One cool fact I learned while writing this piece is that Brown-headed Cowbirds lay eggs with a seasonal sex bias. What this means is that they tend to have more female offspring at the beginning fo the breeding season (May) and more male offspring towards the end (July). The reason for this trend is unknown and it has not been observed in other brood parasites (although the study that found this trend used a larger data sample than studies on other species). All in all, despite being parasitic and somewhat invasive, Brown-headed Cowbirds are an incredible and somewhat under appreciated North American bird.
Croston, R. & Hauber, M. E. (2010) The Ecology of Avian Brood Parasitism. Nature Education Knowledge 3(10):56
Sen, Ananya. “Cowbirds Change Their Eggs’ Sex Ratio Based on Breeding Time.” Illinois News Bureau, Illinois News Bureau, 24 June 2020, news.illinois.edu/view/6367/809736.
Service, U.S. Fish and Wildlife. “USFWS: Questions and Answers about the Kirtland’s Warbler Delisting Proposal.” Official Web Page of the U S Fish and Wildlife Service, www.fws.gov/midwest/endangered/birds/Kirtland/ProposedDelistingFAQsKIWA.html.