Birds Uncategorized

Brown-headed Cowbirds Shake Bad Reputation – Sort of…

Brown-headed cowbirds are brood parasites, meaning that females lay eggs in the nests of other species. Despite having a reputation for being bad parents, brown-headed cowbirds apparently aren’t that bad.

Brown-headed cowbirds have earned their reputation as bad parents — after all they are brood parasites.

This might give the impression that brown-headed cowbirds lay eggs in any available nest and then abandon their eggs, but new research shows that they are much more sophisticated than that.

While some species of birds are able to distinguish between their eggs and cowbird eggs, the unlucky ones end up raising a cowbird. So, how does a brown-headed cowbird, learn to…well, be a brown-headed cowbird and not its host species? In fact, female brown-headed cowbirds choose their host nests wisely. They pay attention to how offspring are being raised by their foster parent(s). If their host nest has a positive reproductive outcome, they return to lay more eggs. If the brood fails, then the cowbirds don’t return. This means that the female brown-headed cowbirds are choosing the highest quality host nests for their eggs.

Researchers from the previous study found that brown-headed cowbird nestlings sneak out at night. They spend entire nights alone. They leave their host nest at sunset and return just before dawn.

Brown-headed cowbird. Image Credit: Orietta Estrada

According to researchers, they do this to avoid imprinting on their host species. They create independence from their host so that when they are sexually mature, they don’t attempt to breed with wrong species.

Brown-headed cowbirds grow faster than their nest mates and therefore require more food resources from their host parents.

If brown-headed cowbirds were to attempt to breed with their host species, their brood parasitism would be ineffective. It would be a reproductive dead-end for the species because two bird of different species cannot produce offspring. Other species that might be unwittingly raised together, such as tree swallows and Eastern bluebirds, do imprint on their host species and attempt to breed — unsuccessfully of course.

Female Cowbird
Female brown-headed cowbird. Image Credit: Orietta Estrada

Brown-headed cowbirds grow faster than their nest mates and therefore require more food resources from their host parents. Nestlings are have been known to roll eggs out of their host nest, push other nestlings out or smother them at the bottom of the nest, according to All About Birds.

These mechanisms lead to the fascinating success of brown-headed cowbird brood parasitism. While brood parasitism might not be something to celebrate, it is certainly something to marvel.

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