Hybridization occurs when individuals from genetically different groups breed. In most cases, those distinctive groups are separate species.

Hybridization occurs widely in the natural world and birds are no exception. Some authors claim that at least 10 percent of the world’s 11,000 species form hybrids with other closely related species.

In Maine, mallards frequently mate with American black ducks. In fact, wildlife biologists worry that the frequency of hybrids may eliminate the American black duck as we know it. Mallards are much more common than American black ducks. Hybrid pairings represent only a small fraction of all mallard matings but a more significant fraction for the less common American black duck. This process, called genetic introgression, means that American black ducks are accumulating mallard genetic material as time goes on.

Mallard-black duck hybrids are hardly the only duck hybrids. Ducks are notorious for hybridization, sometimes producing baffling individuals whose provenance is unclear. Hybrids I have had the pleasure to see in the wild include Barrow’s goldeneye/common goldeneye, greater scaup/tufted duck, American wigeon/Eurasian wigeon and mallard/northern pintail.

Some bird hybrids occur often enough that they have been given names. For some of these named hybrids, early ornithologists thought they were separate species and described them as such. Later work indicated they were actually hybrids of related species. Thus we have Brewster’s warbler, a hybrid between a blue-winged and golden-winged warbler, along with the rarer Lawrence’s warbler (one parent is usually a Brewster’s warbler!).

Nelson’s gull was described in 1884 as a new species. We now know that it is actually a hybrid between a glaucous gull and a herring gull.

Viable hybrids represent flies in the ointment as we try to identify distinct species. Many biologists (and most members of the American Ornithologists Union Check-list Committee, the arbiter of the North American species list) subscribe to the biological species concept, first developed by Dr. Ernst Mayr in 1940. Mayr defined a species as a group of interbreeding individuals that are reproductively isolated from all other groups. In birds, reproductive isolation can occur by differences in plumage, differences in vocalization or differences in courtship behavior.

If mallard and American black ducks can interbreed to produce fertile offspring, should they not be considered a single species? It’s a thorny issue, but the upshot is that occasional hybridization is tolerated before two hybridizing species are considered one.

A recent paper by Jennifer Walsh and colleagues in the Auk explores an intriguing hybrid zone in salt marshes in Maine. Field guides from 20 years ago or older had an entry for the sharp-tailed sparrow with a breeding range along the Eastern Seaboard from Virginia to Nova Scotia. In 1995, this species was split into two species: the more southerly salt marsh sparrow and the more northerly Nelson’s sparrow. The two species can be distinguished morphologically. The salt marsh sparrow has more distinct dark streaking on the breast and flanks and a bright orange cheek patch. The salt marsh sparrow is slightly bigger with a longer, thinner bill.

One of the birding attractions of Scarborough Marsh is that both of these sparrow species nest together. We also know that hybrids of the two species occur here.

Walsh and colleagues sampled sparrows in 34 marshes from Washington County to the North Shore of Massachusetts. At each site, they captured sparrows, took various measurements and drew blood samples for DNA analysis.

Most of the Massachusetts marshes had only salt marsh sparrows, and the Penobscot Bay to Washington County sites had only Nelson’s sparrows. In between, however, hybrids occurred. DNA comparisons showed that 52 percent of individuals in the hybrid zone had mixed ancestry. However, because of the complicated crossing and backcrossing of individuals, pure and hybrid birds cannot be distinguished on the basis of morphology. Hybrids are rampant but can only be discerned by genetic analysis.

Herb Wilson teaches ornithology and other biology courses at Colby College. He welcomes comments and questions at:

whwilson@colby.edu