Most
birds defend their territories only against members of the
same species; some, however, defend against individuals of
other species as well. Generally such interspecific
territoriality occurs between species that are very similar
-- as might be expected if territoriality is a way of
guarding resources or mates. Closely related species are
most likely to have similar resource requirements, and are
also most likely to attempt to copulate with the territory
owner's mate. For instance, interspecific territoriality is
found in two extremely similar Empidonax flycatchers, the
Gray and Dusky Flycatchers. These birds are so much alike
that they can be told apart with assurance only by their
songs or by careful measurements of specimens. The Gray
Flycatcher breeds in small trees and sagebrush and tends to
forage in the open; the Dusky Flycatcher lives in forest and
chaparral. Logging operations in eastern California in the
mid-1800s brought these two flycatchers together by opening
clearings in the forest, where the species retain their
habitat separation but defend their territories
interspecifically. The birds' appearances and challenge
calls are so similar that each respects the other's domains.
Even with the high degree of similarity, there is no sign
that the species hybridize. Similarly, on a Scottish
island, Great Tits (relatives of chickadees) and Chaffinches
(relatives of goldfinches) defend their territories against
one another, even though the birds belong to entirely
different taxonomic families. On the mainland the two
species do not exclude each other. Great Tits and
Chaffinches have similar feeding habits and areas, and
presumably the simpler island environment provides less
opportunity for each to use different resources. Ecologists
generally expect to find interspecific territoriality when
the habitat is relatively simple, restricting the variety of
resources (usually kinds of food) available, and when the
birds involved are specialists in their use of resources (so
that it is not easy for one or both species to change its
resource use in the face of competition from the other).
When breeding, North American hummingbirds usually live in
separate habitats, but during migration, more than one
species often occur together and all use the same nectar
resources -- and then interspecific territories are
defended. Some groups of birds defend
interspecific territories communally. Acorn Woodpecker
groups attempt to exclude Acorn Woodpeckers belonging to
other groups, Lewis' Woodpeckers, jays, and squirrels from
the territories that they establish around their large
caches of acorns. They also defend against European
Starlings, which may appropriate the woodpeckers' nest
holes. Finally, birds may defend
their territories against insects; some tropical
hummingbirds chase bees and butterflies away from nectar
sources. This behavior has not been reported in North
American hummers, but you should watch for it. SEE: Territoriality;
Sibling
Species;
Hoarding
Food;
Hybridization;
Species
and Speciation. Copyright
® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl
Wheye.