Attributes of a breeding population of Peregrine Falcons associated with reservoirs on the Colorado River

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We describe results from a comprehensive effort to survey and monitor Peregrine Falcons (Falco peregrinus) breeding in Lake Mead National Recreation Area (LMNRA) from 2006–2010. We identified 37 breeding territories, and the annual occupancy rate averaged 94%. Pooled over time, breeding success was 72%, and average reproductive output was 1.8 young per nesting attempt. The closest distance between eyries from neighboring territories was 1.2 km; the lowest annual mean nearest-neighbor distance (NND) was 6.3 km. No relationship was apparent between NND and breeding success or reproductive output. Nesting attempts occurred twice as often in eyries with a north-facing (68%) aspect than in eyries with a south-facing (32%) aspect. Pairs using south-facing eyries began incubating 5 d earlier than those using north-facing eyries, although the difference was not statistically significant. On a finer scale, pairs most commonly selected northwest-facing eyries (45% of nesting attempts), despite experiencing a trend of lower mean breeding success (64%) than in eyries with aspects facing all other quadrants (83%). Within territories, peregrines used alternate eyries following 58% of nesting attempts; however, switching eyries between years did not influence breeding success. Peregrines appear to be largely year-round residents at LMNRA, based on monthly surveys at five territories during a nonbreeding season (August 2008 through January 2009). We also detected peregrines at 10 of 24 territories in September and October 2009 using 10-min call-broadcast surveys at eyrie cliffs. Our results contribute to knowledge of increasing populations of peregrines following the DDT era in the southwestern U.S., and provide insight about how reservoirs may influence local breeding populations.