The Role of Winter Bird Distribution in Conservation Planning
While some large mammals are capable of seasonal migrations of some distance, most amphibians, reptiles, and mammals do not migrate. Many of these species avoid harsh weather by becoming inactive for considerable periods. Many birds, on the other hand, migrate considerable distances from breeding areas to warmer regions where they spend the winter. During breeding season, birds have a limited home range around their nest. Many species are also associated with specific habitat types during their breeding season, and these frequently correspond to vegetation cover types that can be mapped using remote sensing technology. As a result, GAP has emphasized mapping the breeding distribution of birds. As M. D. F. Udvardy notes in his book Dynamic Zoogeography (1969:154-155), "In many respects the most important area is that on which successful propagation of the species regularly occurs. The distributional area of the animal therefore usually coincides with the breeding area (breeding range, reproduction area)."
It is undeniable, however, that in situ conservation of a species must deal with maintenance of its habitat throughout the year. There is, therefore, a sound argument for mapping winter distribution of a species as well as its breeding distribution. However, winter distribution of birds is less predictable than their breeding distribution. Birds tend to move from one area to another during the winter, often in response to changes in the weather. Some species that ordinarily migrate south may remain on their breeding grounds during mild winters.
Although Christmas Bird Counts, sponsored by the National Audubon Society, provide the best information on winter distribution of birds, they take place over a span of weeks in late fall and early winter, which introduces variability into these data. National maps based on these records have been compiled by Root (1988). Birds also tend to use a wider variety of habitat types during the nonbreeding season, making habitat-based mapping more difficult. For example, many passerine species with specific breeding habitats occur opportunistically throughout agricultural areas during winter. Finally, the individuals of a species that breed in an area may migrate south, only to be replaced by individuals of the same species that breed farther north. This has been documented for species with recognizable subspecies, such as the Fox Sparrow (Passerella iliaca).
As a result of the many factors which influence winter distribution of a species, it is inadvisable to combine the breeding and winter distributions of migratory species into a single data layer. Those state programs that elect to include winter distribution of birds in their data sets should maintain them separately from breeding bird distribution data layers. This will preserve the information content of breeding bird maps while addressing the desire to incorporate important bird wintering grounds in biodiversity planning. In Oregon, for example, many bird species have a chance of being seen at virtually any low elevation site during the course of a winter (although they may not remain in most areas throughout the winter). A map of this type of winter distribution has a low information content. Mapping winter distribution of birds may fail to highlight important migratory stopovers, such as major wetlands. Such areas important to many migratory species should also be maintained as a separate data layer.
Root, T. 1988. Atlas of wintering North American birds: An analysis of Christmas Bird Count data. University of Chicago Press, Chicago.
Udvardy, M.D.F. 1969. Dynamic Zoogeography. Van Nostrand Reinhold Company, New York.
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