Geography Reference
In-Depth Information
subdivisions. All but three—black-headed grosbeak, chestnut-backed chicka-
dee, and yellow-rumped warbler—were more abundant in established devel-
opments than in forest reserves. About half of the adapters colonized or peaked
in abundance midway through the development sequence, before avoiders
were eliminated. They used environmental features created by clearing and
construction such as the ponds and gravel pads defended by red-winged black-
birds and killdeer, dead tree promontories commandeered by olive-sided l y-
catchers, and weedy hillsides ruled by American goldi nches, Bewick's wrens,
savannah sparrows, white-crowned sparrows, and orange-crowned warblers.
Some of these species continue to persist in older neighborhoods, but only at
very low densities.
Adapters included a mix of neotropical and shorter distance, regional mi-
grants as well as residents. They nested on the ground, in shrubs, and in dead
tree cavities; forest canopy species, as noted earlier, avoided development.
Adapters ate what subdivisions of ered—insects, seeds, fruits, and nectar. They
searched for their meals on lawns, in forest openings, along marsh edges, and
on bare, rocky ground. They were an extremely diverse lot of open-country
birds plus a few birdseed addicts such as the black-capped and chestnut-
backed chickadee.
The ephemeral nature of the habitats adapters used is rel ected in their
regional population trends. Most are declining along annual survey routes scat-
tered throughout the western United States, some substantially so. Pine siskins,
willow l ycatchers, rufous hummingbirds, killdeer, and American goldi nches
have declined annually since the late 1960s by an average of 2 percent or more
on western Breeding Bird Survey routes. (The Breeding Bird Survey is a long-
standing national ef ort to track bird population changes along hundreds of
twenty-i ve-mile-long routes using standardized techniques. The routes are
i xed in location, and most are outside of the city, which likely accounts for the
discrepancy with our observations.)
