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Journal of Raptor Research






The Raptor Research Foundation

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We examined the reproduction of Ferruginous Hawks (Buteo regalis) in Utah's West Desert from 1997–99. We found 100 occupied territories during the study; 80 of them contained an active nest (i.e., evidence of eggs laid). Most active nests (91%) were successful in producing at least one hatchling, and 67% of nests with hatchlings produced at least one fledgling. We followed the fate of 202 Ferruginous Hawk hatchlings; 58% survived to fledging. We radio-tagged 46 of these fledglings; 72% survived the fledgling period and dispersed from their natal territories. Most juveniles that died were killed during the late nestling (58%) and fledgling (24%) period; mortality was lowest early in the nestling period (18%). Across all years, 42% of hatchlings did not survive long enough to disperse from their natal territory. Lagomorph abundance increased each year of our study and during 1999 was over 100 times higher than during 1997. Concomitantly, there was a significant difference among years in the proportion of nests that produced a hatchling and in the survival rate of hatchlings and fledglings. For all of these dependent variables, reproduction was lowest during 1997 when lagomorph densities were low and highest during 1999 when lagomorph densities were high. Yet, most juvenile mortalities were from depredation and not starvation. Most depredated juveniles were apparently killed by avian predators. We also found no relationship between the probability of juvenile depredation and either an index of parental nest attendance or an index of intensity of nest defense.

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