Data from: 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.

北苍鹰（Accipiter gentilis）的保护关切，源于有证据表明苍鹰在应对某些形式的木材采伐时，可能会放弃巢址，或是出现繁殖成功率下降的情况。不过，这类证据存在分歧，且尚未得到系统性、定量的综述。因此，我们开展了一项元分析，以评估木材采伐与树木尺寸在多大程度上可以解释苍鹰繁殖生产力与巢址占用率的差异。整体来看，无论是北美、欧亚大陆的数据集，还是合并所有研究的平均结果，周边木材采伐的存在与否，或是周边树木的平均尺寸，均未对苍鹰的繁殖生产力产生显著影响。不同研究的效应量差异显著，平均效应量接近零（Zr = 0.04）。不过，木材采伐与树木尺寸的联合作用，却能更强地解释苍鹰对巢址或领地的占用情况。在多数研究中，周边木材采伐程度更低、或是拥有相对更大树木的苍鹰巢址或领地，其被占用的概率更高。当我们分别估算两大洲的平均效应量时，结果均为中等程度且一致（Zr = 0.23–0.27），且显著大于0。将北美与欧亚大陆的研究合并后，木材采伐（Zr = 0.24）与树木尺寸（Zr = 0.25）的平均效应量强度相近，且均显著大于0。综上，我们的研究结果表明，尽管木材采伐与缺乏大型树木均与筑巢苍鹰的占用率降低相关，但在木材采伐附近筑巢、或是在小型林木中筑巢的苍鹰，其繁殖成功率与在大型林木中、或远离木材采伐区域筑巢的苍鹰并无显著差异。我们发现不同研究间的结果存在显著异质性，尤其是在北美地区——这并不令人意外，因为各研究在研究方法、森林类型与森林管理措施上均存在巨大差异。综上，我们的研究结果提示，像A. g. laingi这类处于保护关切中的苍鹰种群，其巢址可能需要比当前更强的木材采伐保护措施。同样重要的是，为了更深入地理解森林管理对苍鹰的影响，我们建议开展以下额外研究：（1）更精准地识别木材采伐对苍鹰巢址占用影响的时空范围；（2）探明木材采伐后苍鹰的行为活动与迁移去向。