vogelpsittacines

Who will win?

The Brown-headed Cowbirds are brood-parasites, which means the females lay eggs in the nests of other birds instead of building their own nest and raising their young themselves. Cowbirds have a shorter incubation period than most other birds, so they are usually the first in the nest to hatch. They are also larger than the hatchlings of many other species and grow quicker too. Therefore they often thrive at the expense of the host's own babies and can threaten the livelihood of some species.

They are known to lay eggs in the nests of hundreds of other songbirds. Most of those hosts go ahead and raise the foster chicks without seeming to notice the difference. 

However.

Their opponent, the Grey Catbird, are schmarter than they look. Though they are frequent targets (cowbirds may lay eggs in ~50% of catbird nests), the females learn the look of the first egg in their nests, their own, and reject/toss out any that look different.

Hah! Take that, Cow(bird). I guess the Cat(bird) takes this round. All that meowing must've paid off.

Fun fact: According to Dr. Stephen Rothstein, because the catbird’s recognition of her own eggs is learned, not innate, it can lead to strange results. It might happen if a cowbird lays an egg in a nest and removes the catbird’s first egg, so that when the female catbird returns, she memorises the cowbird egg pattern. Thereafter, if she continues to lay eggs, she'll identify them as not matching her mental image of the prototype. She’ll toss her own eggs out of the nest after she lays them, while continuing to incubate the cowbird egg. Clearly this is bad news for the catbird, but it’s such a rare occurrence that it shouldn’t take away from the Gray Catbird’s status as a stellar nest defender.