by Whit Gibbons

April 30, 2006

Someone wrote to Marilyn vos Savant asking whether crossing a blue jay with a cardinal was genetically feasible. The inquirer was interested in producing a new national bird, though whether he wanted a purple bird or one with blue wings and a red body, I don't know. But whatever color a blue jay/cardinal mix might be, Marilyn's answer was that the birds "belong to different species, so they won't crossbreed." She is right about blue jays and cardinals--no crossbred specimens are known. But she is wrong about species not crossbreeding simply because they are different species. The phenomenon is called hybridization, and literally thousands of documented records exist for birds.

The Handbook of Avian Hybrids of the World (2006; Oxford University Press) by Eugene M. McCarthy is my source for asserting that Marilyn's statement that different species cannot crossbreed is just plain wrong. In fact, "crossbreeding" between two species is included among the definitions for the term "hybridizing." Almost everyone is familiar with one of the most common animal hybrids, mules, the result of matings between horses and donkeys, two distinct species. A goal of the Handbook is an attempt to list all the crosses between different species of birds ever reported, whether in the scientific literature or on the Internet. The crosses include ones occurring naturally in the wild as well as the numerous ones between captive species.

The author states that one of his primary interests in hybridization is its "role in evolutionary processes." Evolution is of course occurring constantly, as genetic combinations change from one generation to the next. But can hybridization be responsible for the formation of offspring that are sufficiently different from either parent that they constitute a viable new species? Mules are sterile and cannot propagate more mules. Only horses and donkeys can do that. But research within the last three years on sunflowers has provided scientific proof that hybrids can result in offspring that become their own species, not only genetically and functionally different from either parent but also able to reproduce.

In discussing hybrid birds, McCarthy identifies three primary causes of hybridization. One is imprinting, a common trait among birds in which newborn and young juveniles fixate on the first object they see, normally a parent, and ultimately are programmed to mate with that species (which under normal circumstances would be their own). But their own species is not always the first one a young bird sees, particularly in captive situations where one species of bird might serve as the foster parent of another. Because of mixed signals during mating, a bird that has imprinted on another species might attempt to mate with one of that species rather than one of its own. If the two species are genetically similar enough to successfully produce offspring, but genetically dissimilar enough to be distinct species, the young will be hybrids.

A second situation is one in which the urge to mate overrides the restriction of selecting one's own species. Captive birds in which only one individual of a species is present will sometimes mate with other species, producing hybrids. Crossbreeding because of the lack of same-species mates can even occur in natural situations, such as when storm-blown birds end up as the only one of their kind in an isolated area.

A third commonly proposed cause of hybridization among birds and other animals is that human disturbance of natural habitats places individuals of different species in contact with each other when they would not be so naturally. However, McCarthy does not consider habitat disruption to be a valid cause of hybridization among birds. Instead, he finds that hybridization in birds occurs much more commonly in natural settings with no human disruption than previously believed. Therefore, invoking human alteration of the environment is not necessary for explaining hybridization.

Bird crosses that produce hybrids are generally between closely related species; for example, wood ducks have successfully interbred with more than three dozen species of waterfowl, including mergansers and geese. But the phenomenon of hybridization among birds is worthy of consideration by biologists as a natural phenomenon that can affect ecology and evolution. Whether we will ever end up with a new national bird as a result of crossbreeding is doubtful, though not impossible.

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