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-3- situations. There is, for example, the peregrine's preference for open ledges and the Prairie Falcon's acceptance of a wide range of nesting situations, illustrated by its use of potholes in the face of the cliff, open ledges, and old stick nests of other raptorial species. The pothole eyries probably enhance the survival prospects of young reared on west-facing cliffs and pro- bably offer greater protection from predation than do the open ledges; (5) a variation between the two species in the size of the nesting area and in the height of the cliffs and eyrie sites; (6) the presence, in abundance, of the prey species preferred by the peregrine in areas of Utah where the peregrine was most common, with partial partitioning by the two species of their food niches; (7) the peregrine's fidelity to the cliff. The competitive roles of the peregrine and the Prairie Falcon apparently change according to locality, based on the availability of food and nesting sites, and on the behavior and ecology of the raptorial species with which they must compete. For example, in the Arctic the Peregrine Falcon is a generalist, both in terms of its selection of nesting sites and in regards to trophic rela- tionships with its competitors (White and Cade, 1971); there, this species utilizes a wide variety of food, which is not a restrictive aspect of its economy in the Arctic, and a broad selection of n~st- ing sites. Its chief competitors in the Arctic are the Common Raven, the Gyrfalcon, and the Rough-legged Hawk ( uteo layopus). The Gyrfalcon, the peregrine's most closely related competitor, on the other hand, is a specialist in terms of nest site and food selection. The raven and Gyrfalcon have similar nesting require- ments, and since both are early nesters they have an earlier choice of eyrie sites. Thus, when the later-nesting peregrines and rough-legs arrive, the late arrivals are more or less limited to the remaining sites. Consequently, the peregrine utilizes "marginal" sites where it may have to compete with the rough-legs. Apparently, however, the peregrine is capable of usurping the rough-legs's nest. In addition, the two species may jointly occu- py the same cliffs, thus lessening the competition between them. White and Cade (1971) believe that since ravens and peregrines do not get along well together, the earlier nesting raven may force the peregrine into "marginal" and "submarginal" sites on those occasions when peregrines try to nest too close to the ravens. They believe that the same thing applies when peregrines attempt to settle too close to Gyrfalcons on the same cliff. The peregrine, nevertheless, does use "optimal" sites in the Arctic when they are available. In the desert, however, the pereyrine's role is the reverse of its role in tfie Arctic. Elere the peregrine is forced into the role of a specialist because the harsh arid environment produces few of the prey species preferred by the peregrine and because the Prairie Falcon competes more successfully for both the former's marginal food niche and its marginal nesting niche. The speciali- zation in the peregrine's food requirements is apparent only when