photograph of a Hermit Thrush by Rohan Kamath
|The geographic areas occupied by bird species change through time. For example, various woodland and forest edge species moved into the Great Plains as farmyards, suburbs, and city parks provided nesting trees. The Inca Dove, having adapted to life in Mexican pueblos, first arrived at Laredo, Texas, in 1866 and then gradually spread north of the border as human settlements created suitable habitat. Inca Doves have been seen as far north as Kansas and Arkansas, and as far west as southern California. Gardens and hummingbird feeders may have been responsible for the eastward advance of Anna's Hummers, which appear to have colonized the Davis Mountains of west Texas, and for the northward expansion of ranges of Violet-crowned and Berylline Hummingbirds in Arizona. For instance, Beryllines attempted to nest near a feeder in the Chiricahua Mountains of southern Arizona in 1976, and a pair|
nested near feeders in the Huachuca Mountains in 1978 and
fledged two young.
Similarly, Great-tailed Grackles have followed irrigated farmland and lawns northward out of Mexico. In the middle of the last century they were rare visitors to the Rio Grande Valley; now they nest in central California, central Nevada, southeastern Colorado, and southwestern Nebraska. It was reported that when this species first came into contact with the very similar Boat-tailed Grackles in southeastern Texas and southwestern Louisiana, hybrids were formed. The species now occur together without interbreeding, suggesting that the hybrids were less successful than "pure" offspring and that the process of speciation is complete.
The consequences of range expansion of one species for closely related species in the communities that are invaded can be considerable. The interesting example of the Wood Thrush, which expanded its range northward in this century, has been described by ecologist Douglass Morse of Brown University. Sometime after 1950, this species started to breed in Maine localities previously occupied by two other woodland thrushes, the Hermit Thrush and the Veery. The latter two, presumably displaced by the Wood Thrushes, moved into different habitats, the Hermit Thrush into relatively dry situations, especially pine-oak woodland, and the Veeries into damp deciduous forests.
The habitat preference of the invading Wood Thrushes lies in between the other two, and its ecological distribution overlaps both. The Wood Thrushes are socially dominant over the Hermits and Veeries, and defend their territorial boundaries against them. Interestingly, these interspecific territories are set up over a period of about a week in the spring, after which there are few if any obvious encounters between the species. To avoid missing the action, birders must be in the woods at precisely the right time. While the amount of habitat available to Hermits and Veeries is reduced by the presence of Wood Thrushes, all three species should coexist in areas that have habitats ranging from dry to wet woodlands.
Thus range expansions are of interest because they often signal important changes in habitats, because they may bring together populations that have partially (or just) completed the process of speciation, and because they may have interesting consequences for the communities that are invaded. They are also one of the many areas where observations by the numerous amateur birders in North America have contributed to scientific knowledge of birds -- for without the help of the birding community, professional biologists would be hard pressed to maintain even a skeletal knowledge of avian distributions. Fortunately, ornithologists David DeSante and Peter Pyle have produced an excellent Distributional Checklist of North American Birds that will make it much easier to detect range changes.
Great Plains Hybrids;
Species and Speciation;
Bird Communities and Competition;
Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.