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<br />-z- <br />and to test its applicability elsewhere in the arid West. <br />The Prairie Falcon was less selective than was the peregrine <br />in its choice of nesting sites, sometimes selecting sites which <br />were seemingly shunned by the peregrine. The former species, for <br />example, utilized sites that were located on smaller ledges with <br />a smaller total nesting area, as well as sites located on lower <br />cliffs nearer the base of the cliff or otherwise more easily acces- <br />sible to humans and to mammalian predators, than did the latter <br />species. Furthermore, ravens which are common in Utah, seemingly <br />alter the nesting habitat in a beneficial way for Prairie Falcons <br />by providing additional nests that are frequently used by the fal- <br />cons, whereas the peregrine apparently is little affected by the <br />presence of ravens. <br />The Prairie Falcon is a true desert species. It apparently <br />evolved in the grid environment of western North America, and as <br />expected, in its association with the peregrine it appears to be <br />the dominant competitor in the following ways: (1) it has a <br />greater reproductive potential than does the peregrine, based on <br />its larger clutch size; (2) it is less selective than is the pere- <br />grine in choice of nest sites and thereby has more nesting situa- <br />tions to choose from; (3) it lays earlier in the season than does <br />the peregrine; thus it may have first choice of cliffs and eyrie <br />sites; (4) it shows less selectivity in its choice of prey species <br />as food for its young; consequently it has a wider range of spe- <br />cies to choose from, including birds, mammals, and reptiles; and <br />(5) because of its selection of prey species other than birds, it <br />is not as obligate to open water for food, nor is it as obligate <br />to open water for bathing, and thus it may nest many miles from water. <br />The Prairie Falcon, then, would appear to have a marked <br />adaptive advantage over the peregrine, especially in marginal areas <br />where the peregrine's ecological tolerance is restricted and where <br />the peregrine's preferred food and/or nesting sites are in short <br />supply. The Prairie Falcon's adaptive advantage over the pere- <br />grine may contribute to the restriction of the peregrine to the <br />more optimal aquatic habitats near streams and marshes where food <br />and nesting sites are not in short supply, and where the peregrine <br />competes successfully with its congener. <br />Some of the reasons the peregrine in Utah is able to compete <br />successfully with the Prairie Falcon for food and quality nesting <br />sites may be: (1) the relative tolerance of the two species for <br />each other while nesting close together; (2) the utilization of <br />one another's alternate eyries, coupled with the inability of <br />either species to consistently gain a dominance over the other in <br />aerial combat, although recent observations by Ogden (1972) and <br />R. Fyfe (pers. comm.) suggest that the peregrine may occupy the <br />most propitious sites because it is capable of forcing the Prairie <br />Falcon from them; (3) the possible partitioning of the nesting <br />cliff with each species having distinct preferences for different <br />types of nesting sites of a wide variability in acceptable nesting <br />