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.. Appendix The selection of eyrie sites by peregrines in Utah appears to be associated with the availability of suitable nesting sites adjacent to a marsh or stream where prey species are available in adequate numbers. The utilization of aquatic prey species as food for nestling peregrines in Utah undoP.btedly is a major factor in the species's proclivity for nesting sites near water. The Golden Eagle and Prairie Falcon are the two species most likely to compete with the peregrine for food and nesting sites. Direct competition between the peregrine and Golden Eagle for food probably is minimal, since ma~alian prey species contribute greatly to the eagle's diet, and no direct evidence of competition between the two species for nesting sites has been observed in Utah. The Prairie Falcon, on the other hand, is a probable competi- tor, which is expected because both species belong to the same genus, both are basically equivalent in ecological function, and both are approximately equal in size and strength. Although the habitat of the two species overlaps, habitat separations are pre- sent. The Prairie Falcon, for example, may nest in the desert many miles from water. In the zone of contact between the two spe- cies, its selectivity in habitat and food encompasses and exceeds that of the peregrine. The zone of contact between the two species appears to be restricted only by the paucity of suitable nesting habitat adjacent to an adequate supply of food for the peregrine. We have no data regarding the food niches of the Prairie Falcon in the zones of contact between the two species at river sites in the desert. However, we would expect a greater overlap in the avian prey of the food niches of the two species in these areas due to the elimination of a great portion of aquatic birds from the diet of the peregrine (see Gabrielson and Jewett, 1940). Our data, however, suggest that the extensive utilization of ro- dents, particularly ground squirrels (Citellus sp.), by the Prairie Falcon in the desert areas would tend to lessen the impact of the competition between the two congeners for avian prey in those regions. In regions of Utah where the populations of the peregrine -- were greatest, pairs of Prairie Falcons and peregrines nested much closer together than did pairs of peregrines or pairs of prairies. The two species sometimes even used one another's alternate nest- ing sites. When they nested close together, their nesting sites were not known to be visible from one another. Although they were observed in aerial combat, neither species appeared to be able to consistently dislodge its congener from its nesting site. Our data suggest that both species select eyrie sites on the basis of availability, but when given a choice they seemingly se- lect them on the basis of directional exposure to the sun. The peregrine shows a preference for north- and east-facing cliffs, and the Prairie Falcon shows a preference for south- and west- facing cliffs. This relationship between the two species needs additional investigation to further test its validity in Utah