POPULATION FLUCTUATIONS AND AGONISTIC INTERACTION OF PEREGRINE AND PRAIRIE FALCONS IN CENTRAL ALBERTA, 1960–2006

Abstract

ABSTRACT The Peregrine Falcon (Falco peregrinus) became extirpated during the 1960s, while the Prairie Falcon (Falco mexicanus) increased on a sympatric nesting range along a section of river in Alberta, Canada. Following the release of 233 captive-reared juvenile peregrines in 1992–96, some peregrines returned to the river to breed, reaching a high of seven pairs in 2000, but declining again to two pairs in 2006. In an inverse relationship, the number of Prairie Falcon nests dropped from five to two and then went back up to four. We hypothesize that the peregrine's failure to reach former densities in the study area was due to habitat deterioration which resulted in a reduced prey base compared to pre-1960s conditions. Interspecific strife with Prairie Falcons was common at contested nest sites. The Peregrine Falcons were the dominant aggressors, replacing pairs of Prairie Falcons at some, but not all, historical cliff eyries.