Monitoring results of the northern goshawk nesting areas in the greater Yellowstone ecosystem: Is decline in occupancy related to habitat change?

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I monitored a subset of Northern Goshawk (Accipiter gentilis) nesting areas on the Targhee portion of the Caribou-Targhee National Forest in eastern Idaho and western Wyoming from 1998— 2002 (recent period) to provide occupancy and productivity data for U.S. Forest Service monitoring requirements. A total of 16 randomly selected nesting areas, half in undisturbed and half in timber-sale project areas, were surveyed each year. Occupancy in 1998—2002 averaged 34%, which was significantly lower than the 61 % measured at these nesting areas from 1992—95 (baseline period) using similar survey methods and effort. The productivity of successful nests was similar in the two periods. I used the dawn vocalization survey method in 2001—02, in addition to standard broadcast survey methods, to determine if low occupancy reflected a poor detection rate of pairs that occupied sites but failed to reproduce. The detection rate of goshawks during the courtship period in these 2 yr averaged less than 50%, indicating that number of pairs reoccupying known nesting areas surveyed was low. I found no relation between weather factors and lower occupancy. Occupancy at nesting areas located in past timber-harvest areas in the recent period was significantly lower compared to those in less disturbed habitat (22% occupancy versus 45%, respectively) suggesting that occupancy may be influenced by the long-term effects of timber-management practices. Whether the observed decline during the recent period reflects spatial shifts of nesting pairs, short-term demographic responses to variation in weather or prey, or longer-term responses to changes in forest structure and age resulting from timber-management activities, cannot be determined using the current monitoring program. Long-term monitoring of study areas in the western United States, based on statistically valid study designs and adequate sample size, is needed to understand if the apparent decline in goshawk occupancy reported here and in other recent studies has serious implications for the conservation of this species.

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Shelf Note: Electronic Files - Zoology: BIRDS

Included in the Shelf Note is a supplemental article published by NRCC News, No. 19(1), Spring 2006 discussing Susan Patla's northern goshawk research in the greater Yellowstone ecosystem.