Brown-Headed Cowbird

A new bird showed up at the feeder a few weeks ago that I didn’t recognize. Despite photos and books and apps it took me a while to figure out what it is. I’ve finally decided that my new visitor is a brown-headed cowbird.

I’ve learned that what this bird lacks in attractiveness, it makes up for in audacity. Notably, these birds do not build nests. Nope. Not at all. Instead, they lay their eggs in active nests of other birds. Brown-headed cowbirds will use nests of all shapes in sizes in a variety of locations including marshes, on the forest floor, in shrubs, at the tops of trees, and even in tree cavities. And they’re not overly picky about who they leave their eggs with. Studies have also revealed that cowbirds will leave their eggs in the nests of over 220 different foster species, anything from tiny kinglets to much larger meadowlarks. Before laying their eggs, cowbirds watch other birds building nests. Then, when the time is right, they flush the parents from a nest and lay the cowbird eggs among the others. Apparently they even choose nests where the size of the eggs is smaller than their own. How to do they know this???

Female cowbirds can lay an egg a day for several weeks in a row. She may lay 40 or more in a single season. And once she lays her eggs, she done. As in, she does not rear her young at all. She leaves it to the foster family. This is called brood parasitism (new term!) – as Merriam says, “characterized by a bird of one species laying its eggs in the nest of a bird of another species and giving no parental care to the eggs.” Unfortunately for the foster family, young cowbirds will sometimes push the other eggs out of the nest. Some birds do recognize the cowbirds eggs as not their own. Larger species will simply crack or remove the cowbird eggs; smaller bird that cannot physically manage this may build a new nest over the old, like the yellow warbler does. Overall, though, most birds do not recognize the cowbird eggs as different, and they end up rearing another bird’s young.

It gets even more interesting – the cowbird eggs hatch often faster than their foster siblings. This gives them an advantage in getting more food from the parents. In addition, they usually grow faster than the others and will sometimes toss the smaller birds out of the nest or smother them. Yikes. Makes me wonder where my neighborhood cowbird has laid its eggs and hope that they are getting along with their foster siblings.