One of These Chicks Does Not Belong in This Nest

To identify the imposter, start with the adult.

It's a sultry summer day, and all is quiet in the Eastern woods. Suddenly, there's a flicker of wings: an adult songbird, foraging for grubs in the high leaves. You track it with your binoculars as it flies through the canopy . . . all the way back to its nest and hungry chicks.

The view is heartwarming, but as a birder, you're not satisfied until you ID this family. Start with the adult. With that slightly hooked beak and yellowish flush, it must be a vireo. There are three similar-looking and common species in your area: Warbling, Philadelphia, and Red-eyed Vireo. But which one is it? On closer glance, the thick "eyebrow," ashy cap, and bright crimson eyes make it clear: You've got a Red-eyed Vireo.

Of course, the chicks must be of the same ilk, right? The baby in the foreground certainly fits the Red-eyed bill: yellow gapes, with white feathers growing in on its chest, belly, and flanks. The nestling in the back is a bit of an anomaly, though. Its plumes are just starting to sprout, but you can already tell that it has a different palette than the rest of the family; the beak is much heftier, too. Also, WHY IS IT SO BIG?

Aha! This must be a young parasitic cowbird—a common scene to stumble upon during songbird-breeding season. Brown-headed Cowbirds frequently infiltrate vireo nests because the cup-shaped structures are usually out in the open and are easy to drop an egg into. It's probably safe to say that every North American vireo species has been a target to cowbirds, Audubon field editor Kenn Kaufman says. Studies show that in some areas, up to 72 percent of Red-eyed nests host brood-parasite eggs. The probability is even higher at sites along fragmented forest edges. 

While some species have ways of defending against the cowbird's ingenious childcare strategy, Red-eyed Vireos haven't shown much resistance. And though this imposter's presence doesn't bode well for the little vireo chick (its parents will spend way more time feeding the Hulk in their nest), there are few situations where cowbirds have actually had population-wide impacts on a species. One beleaguered bird is the Black-capped Vireo in Texas. But even in that case, Kaufman notes that humans are what propel their spread. If cowbirds are a problem, it's our fault.