Last summer, NRC avian biologists conducted surveys for the Northern Goshawk (Accipiter gentilis) in southern Wyoming as part of the National Environmental Policy Act process for an electric transmission project. During our background research and field excursion, we came upon some interesting factors affecting habitat suitability for the species in southern Wyoming – those findings are presented below.
Throughout much of the species’ range, suitable nesting habitat consists of large, old conifers in stands that are typically denser than the surrounding forest. In southern Wyoming, however, where the forested landscape is dominated by lodgepole pine (Pinus contorta), the birds seem to be selecting sites that are, on average, less dense than the surrounding forest. At first glance, it appears that Goshawks in Wyoming are doing something very different from Goshawks across the rest of the species’ range. On closer inspection, however, we came to realize that this had more to do with the dominant forest type than the birds themselves.
Lodgepole pines grow in thick “doghair” stands in sub-alpine forests. These stand in stark contrast to the other forest types used by the Northern Goshawk, which are typically thinly stemmed forest with open understories (e.g. ponderosa pine [Pinus ponderosa] forests). Throughout most of the species range, nesting stands must be denser than the surrounding forest in order to achieve the preferred density. In southern Wyoming, however, where the forests can have densities of over 1,000 trees per hectare (compared to ponderosa pine stands with typical densities in the low hundreds), the preferred density for nest sites is lower than the surrounding forest. Goshawks appear to select for a specific range of densities, rather than a relative density, based on the surrounding forest.
While this caveat to traditional thinking about Northern Goshawk nesting habitat does not represent a scientific revolution, it does serve as a good reminder to carefully and critically evaluate technical information when designing or conducting surveys for any species. Most survey protocols, academic literature and technical reports state broadly that suitable Goshawk nesting habitat is characterized by greater than average stem density. Had we not critically evaluated the applicability of these conclusions to our specific project, we would have spent a lot of time surveying habitat that had very little chance of being home to a nesting Goshawk. Instead, we focused our surveys on less dense stands with larger diameter trees. In doing so, we were able to significantly reduce survey time and cost while still evaluating all potentially suitable habitat.
To cap it all off, two Northern Goshawks were detected during the surveys (including one nest). And, as if to prove the point, both detections occurred in the two least dense stands we observed throughout the study area.
More About Northern Goshawks:
Check out this video of one of the female Goshawks we found defending her nest:
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