Geography Reference
In-Depth Information
crows and squirrels prey on robin and thrush nests, leading to an increased
risk of nest failure far from hawk nests. Failure very near hawk nests is likely
due to actual predation by the hawks on the nestling songbirds. As with most
things in life, moderation with respect to nesting near a Cooper's hawk nest
is the best strategy.
Despite their ef ects on the predators and prey that nest near them,
Cooper's hawks didn't overly inl uence the organization of Seattle's suburban
bird communities. Their presence was unrelated to overall diversity, though
it did provide some check on the dominance of the robins and Swainson's
thrushes.
When species form new communities, as birds do in response to the creation
of cities and suburbs, they may i nd themselves in the company of ancestral
foes. Through evolutionary time, similar species that require the same, rare
resources compete. This interspecii c competition is thought to eliminate in-
ferior species and also foster divergence. Because of competition, similar spe-
cies may come to dif er in critical aspects of their structure, such as the size or
shape of their beaks, as happened with the famous Galapagos i nches and
splendid Hawaiian honeycreepers. The habitats where they live and their
behavior, such as the tactics they use to catch a meal, can also be shaped by
competition. This “character displacement” may happen gradually or rapidly
during brief and infrequent ecological crunches, for instance, during years
when critical seed resources are especially rare. Because interspecii c compe-
tition works to diversify close competitors over evolutionary time scales—
typically thousands to millions of years—species that have evolved together in
the past often show little present-day competition. In a sense, battles long ago
have already settled the score. Species that coexist today can be thought of as
