Corvids colonize an extraordinarily wide range of habitats. The extremes serve to illustrate the breadth and diversity— alpine choughs have been observed feeding in the Himalayas at 2,700 ft (822 m); the "desert" choughs live in the steppe deserts of central Asia.

Corvids evolved as arboreal species, and some genera have remained wholly dependent on forest habitat. Jays of the Cyanolyca genus of Central and South America carry out nearly all of their foraging in the canopy, rarely, if ever, descending to the ground. Genera that have evolved specialized feeding techniques are largely restricted to particular types of forest and, exceptionally, even particular types of tree. Some forms of spotted nutcracker (Nucifraga caryocatactes) have unusually slender bills for feeding on the seeds of the Arolla pine (Pi-nus cemra); the two species may have evolved in symbiosis. Other genera are omnivorous, but show a marked preference for certain foods. The Garrulus jays favor oak acorns and oak-living invertebrates: their range usually coincides with that of oaks (Quercus spp.).

Those genera which rely on trees only for nesting and roosting occupy a much wider variety of habitats. Magpies of the Pica genus and some Corvus species, such as the Eurasian rook (Corvus frugilegus), are able to feed diurnally in a range of open habitats, returning at night to their tree roosts. Some species, most notably the American, carrion, and house crow, have become well adapted to living in areas of dense human settlement. Studies in New York have shown that although urban American crows have smaller territories and nest at

A gray-breasted jay (Aphelocoma ultramarina) in flight over Arizona. (Photo by Joe McDonald. Bruce Coleman Inc. Reproduced by permission.)

higher densities than rural crows, nesting success is broadly similar.

An adaptation to nesting on cliffs and on the ground or low bushes has enabled a number of species to colonize open country. The five species of ground-jay are all terrestrial feeders, and the Pyrrhocorax choughs feed on invertebrates on grazed pasture. Most raven species nest both in trees and on cliff ledges.

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