Breeding ecology and habitat of Northern Goshawks (Accipiter Gentilis Laingi) on Vancouver Island: a hierarchical approach




Ethier, Thomas Joseph

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Northern Goshawk breeding habitat was conceptualized and analyzed as a system composed of different sized components organized within a hierarchy. The organization of the hierarchy was asymmetric with smaller components nested within and subordinate to larger components. Components considered in the breeding habitat hierarchy of goshawks were nest tree, nest site, nest territory, foraging area, and landscape. Choice at any level within the hierarchy was contrasted with the next level in the hierarchy. Thus it was found that goshawks chose nest trees with a largerdbh (79.0 ± 4.4 cm) than the nest site average (43 ± 2.7 cm) (p=0.000036); the averagedbh of the nest site (43.2± 3.1 cm) was significantly larger than the territory (33.2± 1.9cm) (p=0.018). The nest territory when contrasted with random unoccupied locations in the landscape demonstrated a composition of a central site of trees with largedbh and fewer stems when compared with the surrounding forest. I hypothesize that this pattern reduces thepredation risk at the nest site. I investigated the nest site relative to the assumed foraging area (radius of 3000m; approximately 3000ha). Goshawks selected nest sites which were significantly further from young clear cuts (>10 ha). The average distance from a nest site to a recentclearcut greater than 10 ha was 1,350 m of the nest while for unoccupied sites the average distance was 770 m (p = 0.011). Finally, at the landscape level, areas of approximately 10,000 ha, goshawks on Vancouver Island were more abundant, had more nest sites, and had higher productivity per active nest in old growth forests (>120 years) than in either second growth (majority less than 80 years) or fragmented landscapes. The majority of goshawk prey, as determined though pellet analysis was red squirrel (69%). Red squirrels were equally abundant in second growth and old growth yet less abundant in fragmented landscapes. Despite similar abundance of their main prey item goshawks were not as abundant in second growth forests as in old growth forests. I conclude that the structure of second growth and fragmented landscape reduces the availability of key prey species and therefor the suitability of these landscape fpr goshawks is lower.



red squirrels, nest tree, nest site, nest territory, foraging area, landscape