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. SEE: Great
Plains Hybrids;
Avian
Invaders;
Species
and Speciation;
Superspecies;
Feeding
Birds;
Habitat
Selection;
Bird
Communities and Competition;
Interspecific
Territoriality. Copyright
® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl
Wheye.