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Some of us can never have enough field guides. It’s a kind of addiction. I’ve acquired guides to the flora and fauna of places I’ll most likely never visit, but, hey, you never know. How could I pass up Butterflies of the West Indies for just five bucks? Or Trees of New Zealand? And they’re handy for identifying things on the internet. There’s a whole Flickr pool called “ID Please!” 

In that spirit, then, I welcome The Stokes Field Guide to the Birds of North America to the crowded shelf (a gift from Ron.) It’s the best, in my judgment, of the current generation of all-photo bird guides, contains the latest American Ornithologists’ Union taxonomy (hello, Pacific wren!), and has good coverage of subspecies—for which Donald and Lillian Stokes, unlike David Sibley, provide the actual Latin names. For each species, a generous spread of photographs shows regional, seasonal, sex- and age-related plumage variations. 

One thing in the new Stokes that I don’t recall seeing in any previous bird guide is a list of known hybrids for each species that has been caught in the act. This was compiled from authoritative sources, including a web database (www.bird-hybrids.com), and includes only instances of cross-breeding known to have occurred under natural conditions. 

Odd things happen in aviaries. Falconers, parrot fanciers, and waterfowl keepers have intentionally crossbred their birds. I’ve seen a photograph of a peregrine-kestrel hybrid, a product of artificial insemination. I believe it was at the San Francisco Zoo that an American avocet paired up with a black-necked stilt, producing hybrid avostilt offspring. That’s understandable: the birds were cooped up in there, and the partner would at least have been a long-legged shorebird. But there’s at least one case of an avocet-stilt cross in the wild. 

Looking only at spontaneous crosses among wild birds, it’s interesting to compare patterns of hybridization among different avian families and orders. Based on the Stokes book, natural hybrids seem to be uncommon among wading birds, plovers, hawks, owls, corvids, wrens, thrushes, and sparrows. There are exceptions, of course. An injured raptor brought to the Lindsay Museum in Walnut Creek a few years ago was determined to be the offspring of a red-tailed hawk and a Swainson’s hawk. Most hybrids in such groups are between very close relatives, like the three species of bluebird. 

A couple of groups, on the other hand, seem particularly prone to hybridize. Cross-species liaisons have been documented in some 30 species of North American ducks. Mallards alone have interbred with wood ducks, gadwalls, American and Eurasian wigeons, American black ducks, cinnamon and green-winged teal, shovelers, pintails, canvasbacks, and common eiders. This is impressive. Wood ducks, canvasbacks, and eiders aren’t even in the same genus as mallards. We’ve seen a few wild hybrid ducks over the years, including the hooded merganser-Barrow’s goldeneye cross that winters at Lake Merritt. 

Grouse and pheasants have similarly profligate tendencies. Ring-necked pheasants have bred with, among others, ruffed grouse, sooty and dusky grouse (formerly known as blue grouse), and prairie-chickens. Likewise hummingbirds: our resident Anna’s hummers have hybridized with black-chinned, Costa’s, calliope and Allen’s hummingbirds. 

The common thread here could be the birds’ mating systems. Grouse and hummingbirds don’t form lasting pair bonds, even for a single season. Males display (in the case of many grouse and some tropical hummers, at shared arenas called leks) to attract females. After mating, she’s on her own with the nesting and brood care. Females of species with this kind of life history might be less discriminating than those that will be entering a working partnership. 

Ducks are a special case. Although they do form pair bonds, males are sexually aggressive and undiscriminating. A mallard drake will attempt to mate with anything vaguely duck-shaped. (There are some fascinating recent studies on the sexual arms race between male and female ducks that I won’t go into here. Google “duck genitalia” if you’re curious.) 

So where would you expect not to find a high incidence of hybridization? Maybe among large, long-lived birds that pair more or less for life, like geese? But no. Snow geese, for one, are known to have hybridized not only with the closely related Ross’s goose but with Canada, cackling, and greater white-fronted geese as well. 

The larger gulls, also with long-term pair bonds, are notorious hybridizers. Where the ranges of western and glaucous-winged gulls overlap on the Pacific Coast, hybrid birds abound, sometimes outnumbering the pure parent types. There may be another factor here: most of these species are very similar genetically, suggesting a recent evolutionary divergence. The mechanisms for recognizing a mate of the proper species may not be as well developed as in other groups. 

So much for grand theories of hybridization. But it’s the particularities that keep things interesting.