Birders are obsessed with trying to tell species apart, but sometimes the birds themselves aren’t so picky. Occasionally, individuals from two different species will come together and pop out offspring with characteristics of each parent. These unique, field-guide-defying birds are known as hybrids.
About 10 percent of the world’s avian species are known to have hybridized with another species. But why? There isn’t a clear answer. The romantic in me likes to think of it like Romeo and Juliet, where young lovers defy what’s expected of them and follow their hearts. Unfortunately, that’s probably not correct.
Given that the majority of hybrids are pairings between two closely related species, the explanation might simply be mistaken identity. In other instances, a bird may stray across species boundaries when it can't connect with a mate of its own kind. For example, Northern Parulas and Yellow-throated Warblers are common in the same habitats over much of the South, but they're apparently more likely to interbreed in the upper Midwest, where both are uncommon and potential mates are harder to find. Other cases could be the result of interspecific parasitism, in which a bird lays some of its eggs in the nest of another species. The resulting hatchlings then imprint onto their hosts and may grow up preferring them as partners. A complicated plot even Shakespeare would appreciate.
Whatever the reason, hybrids are out there and birders have to be ready. Encountering these misfits in the wild can be a real challenge: They sort of look like two species in your field guide, but don’t fit either description. Maybe it’s a hybrid . . . or maybe it’s just funky-looking. How do you decide?
Sadly, the bird can’t show you its family photos. So, you’re going to have to use context clues. Here are a few to help you connect the dots.
Look for a Familial Resemblance
Hybrids are most often recognized by physical features passed down from each of their parents. With that in mind, pick out field marks that remind you of a similar-looking bird. For example, if you come across a duck with a partial green head but a dark brown body, it’s probably the offspring of a Mallard and an American Black Duck.
Listen for an Odd Mashup Song
While it’s possible to identify hybrid birds by voice, it typically takes a very experienced ear and a string of spectrograms (visual maps made by recording sound waves). Sometimes they make it easy, though. A Dark-eyed Junco and White-throated Sparrow mix, for instance, might cap off a junco trill and a few sparrow pips. The exact repertoire will vary by individual.
Know Which Hybrids Live Nearby
Studying is always the best way to be prepared. If you’re in the Eastern half of the United States, Blue-winged and Golden-winged Warbler hybrids are common. If you’re in the Pacific Northwest, a cross between Western and Glaucous-winged Gulls is the norm (but, like most gulls, they’re still no picnic to ID). Many field guides will show the most frequently encountered hybrids, and online resources such as birdhybrids.blogspot.co.uk illustrate hundreds of known combinations.
Sometimes the features that identify a bird as a hybrid can be too subtle to see in the field, or require the subject to sit still for a long time—a tough proposition for anything with wings. Getting images of these tricksters can be useful for more detailed study and for soliciting the opinions of other birders online.