Interspecific Territoriality

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.
