. The Canadian field-naturalist. 20 The Canadian Field-Naturalist Vol. 112 20 15 â active [a SUCCESSFUL. n 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 Figure 3. Variation in tiie numbers of active and successful Northern Gosiiawk nests monitored each spring during â 1971-1992 in the northeastern Wisconsin study area. « male on his return with prey. The adults were held in appropriate holding tubes while the nest tree was climbed and the young banded. Adults were aged, sexed, weighed, measured, and marked with bands before release. A third visit was made to a t
. The Canadian field-naturalist. 20 The Canadian Field-Naturalist Vol. 112 20 15 â active [a SUCCESSFUL. n 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 Figure 3. Variation in tiie numbers of active and successful Northern Gosiiawk nests monitored each spring during â 1971-1992 in the northeastern Wisconsin study area. « male on his return with prey. The adults were held in appropriate holding tubes while the nest tree was climbed and the young banded. Adults were aged, sexed, weighed, measured, and marked with bands before release. A third visit was made to a territory post-fledgling to assess chick survival. Snowshoe Hare, Ruffed Grouse and Fisher Population Estimates Because both the quantity of prey available and presence of significant predators affect Goshawk population trends, we used data made available from WDNR's Bureau of Wildlife Research on the rela- tive size of Snowshoe Hare, Lepus americanus. Ruffed Grouse, Bonasa umbellus, and Fisher, Martes pennanti populations through the years. For prey species we utilized the mean number of drums/ transect from the spring drumming counts taken in northern Wisconsin as an index of breeding densities and population size for Ruffed Grouse, and the annu- al hunter harvest of Snowshoe Hare as an index of hare abundance (Figure 2). Collection of prey remains and pellets from plucking perches and nest sites has shown the two main prey items to be Snowshoe Hare and Ruffed Grouse, although many other species are also taken (unpublished data). To evaluate the relationship between Goshawk repro- ductive success and prey densities, an index of prey availability was constructed from the WDNR data, with Prey Index = (Snowshoe Hare index^* Ruffed Grouse index)''^. The index was weighted towards Snowshoe Hares because hares weigh approximately times more than Ruffed Grouse and represent a greater return on foraging time investment. The cube root was used to return the index value to the r
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