Birds have all sorts of crazy antics and nesting behaviors—from multiple female anis who all lay all their eggs in the same nest and raise the chicks communally, to swallows who use nothing but mud and spit to build their nests, and more, but in my opinion, one of the most fascinating behaviors of all time belongs to a group of species called cowbirds. There are three species of cowbirds in the United States– the brown-headed cowbird (ubiquitous across all of the US), the shiny cowbird (Florida) and the bronzed cowbird (mainly in the Southwest). Cowbirds evolved following herds of buffalo across the country. Buffalo were nomadic, so the cowbirds needed to become nomadic as well, in order to follow their food supply—bugs gleaned from the ground which the buffalo were kicking up. This proved to be problematic when nesting season came around, but the birds found a creative solution.
Cowbirds began laying their own eggs in other species’ nests and continuing on following the buffalo herd, leaving the other species to incubate their eggs and care for their young. The unknowing surrogate mothers incubate the cowbird egg or eggs, along with the rest of their own eggs, and because of the short incubation period, typically the cowbird eggs are the ones to hatch first. Cowbird chicks are often more robust and louder than the female’s own offspring. Additionally, since they don’t have to expend the energy to build a nest, cowbirds can focus solely on producing eggs, and can lay up to three dozen eggs in a single summer.
After anxiously awaiting the hatching and growth of chicks in our Quinta Mazatlan just-above-eye-level Hooded Oriole nest in the parking lot, we discovered a cowbird chick…
Screaming chicks are the equivalent to a huge sign reading “Eat me, I’m right here!” for any nearby predators, so a bird parent’s instinct is to quiet them as soon as possible by shoving their mouths full of food. Being larger and more aggressive, the cowbird chick typically is fed more, and will sometimes push the other species out of the nest. Cowbirds are blamed for part of the decline of the endangered Black-capped Vireo population in central Texas, Kirtland’s Warblers in Michigan, and here in the Rio Grande Valley, cowbirds are particularly a problem for Hooded, Altamira, and Audubon’s Orioles. One study showed more than 50% nest parasitism on Altamira Orioles!
This male yellow warbler was feeding an angry brown-headed cowbird chick several times his size at Tinicum NWR in Pennsylvania.
Cowbirds eggs have been documented in at least 220 species of nests in the United States—but were only successful at parasitizing 144. You can imagine that Blue-winged Teal and Ruby-throated Hummingbirds did not make good foster parents for cowbirds. A few species, including Yellow Warblers, actually have the ability to recognize the eggs, and will sometimes begin building a new nest on top of their current one, to prevent incubation of the cowbird egg (though of course they will have to lay a new batch of their own eggs as well).
Before you pass judgement on cowbirds too quickly though, think about how cool it is that these species evolved such a specialty breeding behavior. Next time you look at a flock of cowbirds, you can marvel and wonder at how many different species must have been responsible for raising them.
Keep a look out for those mismatched parent/child pairings in the field, and if you have an interesting cowbird story, please share it with me in the comments below!