Mortality of Prairie Falcons during the Fledging-Dependence Period

We monitored the survival of 152 radio-tagged Prairie Falcon (Falco mexicanus) nestlings from 37 broods through the fledging-dependence period in southwestern Idaho during 1992 and 1993. Mortality rates were high but consistent between years, 28% in 1992 and 34% in 1993, despite a significant decline in the primary prey of falcons in 1993. Predation by Great Horned Owls (Bubo virginianus) was the primary cause of mortality. To a lesser extent, mortality was influenced by the presence of hematophagous ectoparasites (Haematosiphon inodorus) in aeries. We were unable to find or show an effect of parental behavior (territory attendance and prey delivery rates) during the late brood-rearing period, hatching date, brood size or fledging mass on mortality. The frequency of predation in our study may have negated or diminished the influences of these factors. Survivorship was lower during the fledging-dependence period than during the incubation or nestling periods.