Timber harvest and tree size near nests explains variation in nest site occupancy but not productivity in northern goshawks (Accipiter gentilis)

Conservation concern for the northern goshawk (Accipiter gentilis) reflects evidence that goshawks may abandon nest sites or suffer from reduced nesting success in response to some forms of timber harvest. However, this evidence is mixed and has yet to be reviewed systemically and quantitatively. Therefore, we conducted a meta-analysis to assess the extent to which timber harvest and tree size explain variation in goshawk productivity and site occupancy. Goshawk productivity was not significantly explained by the presence of nearby timber harvest nor by the average size of nearby trees either in North America or in Eurasia or when averaged across all studies. Effect sizes differed dramatically among studies and the average effect size was close to zero (Zr = 0.04). However, timber harvest and tree size together more strongly explained goshawk occupancy of nest sites or territories. Within studies, goshawk nest sites or territories with less timber harvest nearby or relatively larger trees were, in most cases, more likely to be occupied. When we estimated average effect sizes separately for the two continents, the averages were moderate, consistent (Zr = 0.23-0.27), and significantly > 0. When we combined studies from North America and Eurasia, average effect sizes for timber harvest (Zr = 0.24) and tree size (Zr = 0.25) were similar in strength and both significantly > 0. Thus taken together, our results suggest that although both timber harvest and a lack of large trees are associated with lower occupancy by nesting goshawks, pairs that nest near timber harvest or in small trees have indistinguishable nesting success from pairs nesting in large trees or farther from timber harvest. We found substantial heterogeneity in results among studies, especially within North America, which is not surprising given that studies differed greatly in research methods, forest type, and forest management. In conclusion, our results suggest goshawk nest sites in populations of conservation concern, such as A. g. laingi, may need more protection from timber harvest than they are currently receiving. Equally important, to better understand effects of forest management on goshawks, we recommend additional studies designed to: (1) better identify the spatial and temporal extent of the effect of timber harvest on goshawk site occupancy; and (2) determine what goshawks do and where they go after a timber harvest. (c) 2016 Elsevier B.V. All rights reserved.