Migration-Based Monitoring

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Science  03 Dec 2010:
Vol. 330, Issue 6009, pp. 1289
DOI: 10.1126/science.330.6009.1289-a

Counts of birds passing traditional hawk-watching sites provide a means for estimating population trends in migrant raptors. To gauge the effectiveness of such efforts, Farmer et al. explore the degree to which the existing network of sites could sample the autumn tracks of 57 adult ospreys (Pandion haliaetus). Birds from four breeding areas across the United States were fitted with satellite transmitters. An average of 1.5 relocations per day were observed per bird. To analyze flight trajectories, the authors identified migration paths that connected locations with 3-km- or 6-km-wide corridors. Across North America, 12% of the narrow and 23% of the broader pathways intersected one or more active watch sites; these values rose to 21 and 36% in the eastern region (through which the majority of the continent's population probably passes). Models that assume a conditional random walk between consecutive positional fixes estimate that 95% of the birds' utilization distributions cross at least one observation area. Disparities in mean regional detection probabilities for individuals (east, 34%; midwest, 5.8%; northwest, 4.7%) reflect the relative distribution of hawk watches. The satellite tracks also show southbound ospreys concentrating along narrow fronts and avoiding large water crossings. These results suggest that migration counts do offer a practical means of monitoring populations of the North American osprey and perhaps many other hawks.

Auk 127, 863 (2010).

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