Efficacy of Migration Counts for Monitoring Continental Populations of Raptors : An Example using the Osprey (Pandion haliaetus)

Abstract

Recent research has confirmed the efficacy of migration monitoring to estimate trends in the populations of raptors sampled at traditional watch-sites. We used autumn satellite tracks of 57 adult Ospreys (Pandion haliaetus) captured on the breeding range in North America between 1995 and 2000 to assess the extent to which migration monitoring sampled their populations. We used (1) 3-km-wide and 6-km-wide linear trajectories (migration paths) that connected locations with straight lines and (2) utilization distributions derived from Brownian bridge movement models to estimate the proportions of Ospreys likely to have been detected by watch-sites and watch-sites likely to have detected tagged birds, and to describe the geography of southward migration between North America and South America. The migration path method estimated continental detection rates of 12–23%, with regional maxima of 21–36% in eastern North America. This analysis indicated that 8–20% of all watch-sites could have detected ≥1 of the satellite-tracked Ospreys. The Brownian bridge method estimated that 95% of the utilization distributions of migrating Ospreys in North America intersected ≥1 watch-site and that 89% of all watch-sites intersected ≥1 utilization distribution. Using this method, regional probabilities of detection (mean ± SD) for individuals were estimated to be 33.8 ± 28.8% in eastern, 5.8 ± 6.6% in midwestern, and 4.7 ± 4.9% in northwestern (Pacific coast) North America. Migrating Ospreys appear to concentrate along well-defined, narrow fronts and to use land bridges where available, rather than travel along broad fronts and engage in large water crossings during autumn migration.