A male chestnut-sided warbler like this one served as the male mate that produced an interesting hybrid in Pennsylvania this year. Credit: Bob Duchesne

Something extremely rare happened this past summer, and it knocked scientists for a loop. We humans like to put the natural world into a logical order. Sometimes nature doesn’t cooperate.

A bird is a thing with feathers, and only birds have them. Scientists logically lump all birds into one class of animals, then further subdivide them into orders and families, and lastly into a genus and species. For birders who keep a list of their lifetime sightings, this is their scorecard — a life list.

If only it were that simple. Taxonomists are constantly tinkering with the classifications, especially with the advent of DNA analysis. Heretofore, birds that looked and acted alike were considered to be the same species. But there are regional variations among species, and many birds in the east don’t look like their western counterparts. Yet where their ranges overlap, they routinely interbreed. For a long time, a propensity to interbreed was a key factor in deciding if two dissimilar birds might be the same species.

But interbreeding among species is not rare. Horses and donkeys produce mules and hinnies. Even more exotic pairings can produce offspring, such as horses and zebras, and lions and tigers. As long as enough of the genes line up between the two parents, a hybrid baby is possible. Usually, hybrid offspring are sterile.

In the bird world, interspecies coupling happens more often than most people would suspect. Up to 10 percent of the world’s 10,000 species have been known to do it. Mallards will mate with just about anything that moves. Some pairings happen so often that the offspring get their own names. Among these, the most famous is the matchup between blue-winged and golden-winged warblers.

Blue-winged warblers are a rare breeder in southern Maine. Golden-winged warblers do not nest here, though they share a range over much of the eastern United States. When the two species mate, they produce a youngster with plumage characteristics from both parents. This hybrid is called Brewster’s warbler. If this youngster goes on to mate with either of its parents’ species, the result is a color variation called a Lawrence’s warbler.

The blue-winged warbler is getting the better of this match-up. It is generally outcompeting its golden-winged cousin, and the dominance of its genes may eventually hybridize the golden-winged warbler out of existence. Golden-wings used to range down the length of the Appalachians, but 98 percent of those birds are now gone. Nearly half of the world’s remaining golden-winged warbler population is now confined to Minnesota.

This is where it gets more complicated. Most hybrids are the offspring of birds within the same genus; that is, they are birds that have been classified as a group sharing very similar characteristics. But some interbreeding happens between birds that are not within the same genus. These dissimilar birds produce some pretty weird-looking progeny, and because of the genetic differences, most are sterile.

In May, a Pennsylvania birder noticed an odd warbler in his backyard. It looked like a Brewster’s warbler, but with some characteristics that resembled a chestnut-sided warbler. He kept track of it, and when it matured enough to start singing, it sang the chestnut-sided song. Astonished, he reported the bird to the Cornell Lab of Ornithology, which immediately investigated.

Sure enough, it was the offspring of a hybrid Brewster’s female and a chestnut-sided warbler male. It was the genetic product of three different species. It was also the first time that a Brewster hybrid had been known to pair up with a chestnut-sided warbler, which belongs to a different genus.

While the new hybrid is scientifically exciting, it’s also worrisome. It may mean that some uncommon warblers have declined so far that they can no longer find members of their own species to mate with.

Or maybe evolution is smarter than we think. Most critters evolve away from each other, becoming so genetically distinct that they can’t hybridize. Birds seem to have retained enough compatibility that many can pair up with another species in a pinch. If you can’t be with the one you love, love the one you’re with. This encourages diversity and adaptability among birds during a time of major changes in climate and habitat.

But if crossbreeding is so possible, why doesn’t it happen more often? Most birds have well-defined breeding rituals that encourage females to mate with their own kind. The singing ability of males is typically more important than appearance in mate selection. That would also explain how Mick Jagger fathered eight kids.

Bob Duchesne serves as vice president of Maine Audubon’s Penobscot Valley Chapter. He developed the Maine Birding Trail, with information at mainebirdingtrail.com. He can be reached at duchesne@midmaine.com.

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