When two populations of distinct but closely related birds come into contact, members of those populations may mate with each other and successfully reproduce. That process of "hybridization" creates problems for taxonomists, but is one sign of the continuous nature of the process of speciation -- the evolutionary formation of new kinds of organisms. If hybrids are formed between two populations that are barely differentiated, they may remain undetected, since their features may fall within the range of variability of one or both of the populations. One would expect, if those populations were to remain in contact, that they would blend together and lose their distinctness. On the other hand, two populations may each have diverged so far from their common ancestor that their individuals no longer recognize each other as potential mates. In that case, biologists are agreed that the two populations should be considered separate species. They will not fuse back into a single species.

It is between those two extremes of complete blending and total distinctness that hybridization can provide glimpses of the complex process of differentiation -- of evolution in action. Speciation normally occurs in geographic isolation, but the distributions of birds are complicated and ever changing. Populations once isolated often come into contact, and when they do, the amount, duration, and results of hybridization will vary from instance to instance.

Something on the order of 10 percent of North American birds that are considered specifically distinct hybridize with other species. One of the most thoroughly investigated examples involves two warblers. In the middle of the last century, the Blue-winged Warbler was restricted to the central Midwest (Missouri, southern Iowa, southern Illinois, southern Indiana, Kentucky, Tennessee, etc.), but it expanded its range into New England as tracts of farmland began to revert to brush and woodland. There it came into contact with the very closely related Golden-winged Warbler, which, like the Blue-winged, breeds in successional habitats -- woodland edges, brushy fields, etc.

When the ranges of the two warblers began to overlap, they began to hybridize. At first the offspring of their matings were considered to be separate species. Before their hybrid origin was uncovered, the most common hybrid type was called Brewster's Warbler, and a rarer one was known as Lawrence's Warbler. Hybridization has continued wherever the species live together. The hybrids are fertile, being able both to mate with each other and with the parental types. There is, however, no sign that the two species are fusing back into one. Instead, the Blue-winged appears to be replacing the Golden-winged. The mechanism of replacement is not clear, but the Blue-wings may simply be out-competing the Golden-wings, and simultaneously acquiring (through back-crossing) further ability to penetrate into the historical range of the Golden-wings.

Some studies suggest that the decline of the Golden-wings is not entirely due to the expansion of the Blue-wings, but that changes in habitats may also be involved. The Golden-wings, more than the Blue-wings, need to breed in those early stages of the succession that gradually changes abandoned farm fields into woodland. Thus increased reestablishment of forests and the destruction of bogs and fields by advancing suburbia may both be important factors in the Golden-wing's troubles. Whatever the exact mechanism, the overlap of these two hybridizing species may lead to the replacement of one by the other within about 50 years. If the trends of the middle half of this century continue, the Golden-winged Warbler could be threatened with extinction before 2025.

The relationship between these warblers is simple compared with those found between gulls of the Herring Gull group, where varying degrees of hybridization between species occur (and lead to endless debates on which should be considered "good species"!). The Herring Gull itself, for example, is involved in a "ring of races" with the Lesser Black-backed Gull. The Herring and Black-backed overlap widely with little hybridization in Europe, but they are connected by a circle of hybridizing populations that extends across Siberia, North America, and then the North Atlantic.

The situation is made more complex by the propensity of the Herring Gull to also hybridize with the Glaucous Gull (in Iceland), the Glaucous-winged Gull (in Alaska), and the Slaty-backed Gull (in Siberia). In turn, the Glaucous and Glaucous-winged gulls hybridize around the Bering Sea. Furthermore, the Iceland Gull hybridizes with Thayer's Gull (on Baffin Island). All of these gulls, together with the California and Western Gulls, and the Mexican Yellow-footed Gull, compose one of the most interesting groups of birds -- illustrating many degrees of genetic differentiation. So when you are in the field trying to sort out which of these gulls you have in your binoculars, take heart. The gulls themselves also have problems telling who is who.

SEE: Species and Speciation; Natural Selection; Range Expansion; Great Plains Hybrids.

Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.