FIRST-YEAR SURVIVAL OF GREAT HORNED OWLS DURING A PEAK AND DECLINE OFTHE SNOWSHOE HARE CYCLE

Most bird species have low survival rates in their first year of life, and the highest losses occur when juveniles become independent and di sperse. Young great horned owls (Bubo virginianus), monitored by telem etry in the southwestern Yukon, Canada, survived well during the peak of the population cycle of snowshoe hares (Lepus americanus). Subseque ntly, juvenile survival collapsed parallel to the decline in hare dens ities. The proportion of starving owls did not increase, but there was a significant increase in mortalities involving parasitism and predat ion, probably as an interaction with food shortage. The mortality rate s of juvenile great horned owls peaked before, not during, dispersal. We propose that extended parental care makes the postfledging stage sa fe during optimal conditions, but that the relatively slow development during this stage incurs the cost of increased susceptibility to dise ase and other mortality factors under environmental stress.