If you look at this bird and know immediately what it is, you should be writing this column, not me. If I had encountered this bird in the wild, I would have just stared at it while drooling down my shirt in bewilderment. He is a hybrid — the offspring of a hooded merganser and a common goldeneye. Nobody knows where he goes in the summer, but this is his seventh winter swimming around the docks in Southwest Harbor.

A hybrid is the result of two different species mating. It certainly is not unheard of among mammals. A cross between a horse and a donkey is a mule. A cross between a lion and a tiger is a liger. (A cross between a human and an ape is a rushlimbaugh.)

Our understanding of evolution is evolving. The determination of exactly what constitutes a species remains a matter of debate. In general, we think of a species as being a group of critters so similar that they breed amongst themselves. But it is a fuzzy definition and breeding outside of one’s species is possible. When the parent species have a different number of chromosomes, the offspring are usually infertile. For instance, horses and donkeys have a different number of chromosomes. They may be able to produce a mule together, but the mule is left with enough genetic confusion that it is sterile.

However, the wolf, coyote, jackal, dingo and domestic dog all have 78 chromosomes and can hybridize rather readily. Based solely on appearances, it’s hard to view a coyote and a German shepherd as two different species, while a Great Dane and a Pekinese are the same species. Theoretically, a St. Bernard and a Chihuahua can mate, though one will need a ladder and a good sense of balance.

Similar species are grouped taxonomically into a genus. Hybridization among waterfowl of the same genus is relatively common. Most dabbling ducks belong to the genus Anas. Mallards are, shall we say, enthusiastic about mating and will try to breed with just about anything that moves. They have been cross-breeding so often with American black ducks that scientists feared the latter would melt into the gene pool and disappear as a species. Only differences in range and nesting preference have kept them around.

Diving ducks in the Aythya genus hybridize with some consistency. Crossbreeds of ring-necked ducks and greater scaups occur, especially in the Midwest. The European tufted duck wanders to this continent occasionally and will mate with ring-necked ducks.

What makes today’s featured hybrid interesting is that hooded mergansers and common goldeneyes aren’t even in the same genus. There are vast differences between the two species. For instance, goldeneyes have typical duck bills and enjoy a varied diet of invertebrates, small fish and vegetation. Mergansers have serrated saw-tooth bills designed for holding slippery fish, and these bills can handle crayfish and mollusks. The genes of a hybrid throw together some kind of a compromise between the dissimilar traits, and it’s hard to imagine how such a compromise can work well enough to allow such birds to survive. Yet they do.

Genetics are not the only determinant for classifying an organism into a species. Range and behavior are taken into account. When a population of one species gets geographically isolated from the rest of its kind, it can evolve into another species over time as it adapts to the local characteristics of its home.

Biologists are debating whether there is just one species of red crossbill or many. Local populations have evolved bill sizes appropriate to the size of the cones they eat. A red crossbill dining on spruce cones in the East has a much smaller bill than those that eat the huge cones of lodgepole pines in the West. There are noticeable differences between the vocalizations of these crossbill populations.

The ranges of similar species also can converge as one expands its range into the other’s. When that happens, hybridization happens more frequently. Some hybridization is so common that the offspring get their own names. A cross between a blue-winged and golden-winged warbler is a Brewster’s warbler. Oddly, when the Brewster’s warbler goes on to mate, it produces a hybrid with its own characteristics called a Lawrence’s warbler. On the rare occasions when a yellow-throated warbler and northern parula get it on, the result is an unusual bird called a Sutton’s warbler.

What causes two very dissimilar individuals to hook up? Who knows? At least humans can blame beer.

Bob Duchesne serves as a Maine Audubon trustee and vice president of its Penobscot Valley Chapter. Bob developed the Maine Birding Trail, with information at www.mainebirdingtrail.com. Reach Bob at duchesne@midmaine.com.