. The Canadian field-naturalist. 1978 Adamcik et al.: Great Horned Owls and Snowshoe Hares 159 5 O 20. 1970 1971 1972 1973 1974 1975 Figure 1. Relationship between Great Horned Owl nesting and estimated densities of Snowshoe Hares on a 162-km- study area (130 km- in 1966), 1 April, near Rochester, Alberta. Figures in parentheses are known numbers of territorial pairs of owls on the area. while young fledged elsewhere may or may not move into the area. If the number leaving equalled the number arriving, net ingress and egress would both be zero. Mean annual mortality rates of Great Horned Owls
. The Canadian field-naturalist. 1978 Adamcik et al.: Great Horned Owls and Snowshoe Hares 159 5 O 20. 1970 1971 1972 1973 1974 1975 Figure 1. Relationship between Great Horned Owl nesting and estimated densities of Snowshoe Hares on a 162-km- study area (130 km- in 1966), 1 April, near Rochester, Alberta. Figures in parentheses are known numbers of territorial pairs of owls on the area. while young fledged elsewhere may or may not move into the area. If the number leaving equalled the number arriving, net ingress and egress would both be zero. Mean annual mortality rates of Great Horned Owls banded as nestlings in forest and parkland regions of Alberta, Saskatchewan, and Mani- toba were 55% in the first year of life, 39% in the second, and 22% thereafter (Adamcik and Keith 1978). There was no detectable difference in post-fledging mortality as owl populations increased and decreased in relation to the hare's cyclic fluctuations; hence, we applied the same mean rates of mortality each year when calculating ingress and egress at Rochester (Table 3). Ingress contributed importantly to the in- crease of horned owls at Rochester during 1966-1971 (Table 3). If, as is generally beheved, territoriality and nesting are not evident among yearlings (birds approaching 1 year of age), then ingress accounted for between 31 and 58% of the territorial owls present during 3 years of population growth. If all yearlings had entered the territorial cohort, ingress would still have exceeded 30% in 3 of 5 years from 1967 to 1971. Conversely, egress played a major role in the ensuing population decline, being equivalent to 62% of the remaining territory holders in 1974 (if these were adults only), and 35% in 1973 (if both yearlings and adults were territorial). Overall, increases in numbers were always associated with ingress and decreases were associated with egress. Food Habits Snowshoe Hare biomass in the spring diet of horned owls exhibited an approximately four-. Please note that th
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