Status of Nesting Ospreys in Coastal Baja California, Sonora and Sinaloa, Mexico, 1977 and 1992-1993.
Henny, Charles J.
Anderson, Daniel W.
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We used a double-sampling technique (air + ground surveys), with partial double coverage and an additional adjustment for lack of nesting synchrony in southern latitudes, to estimate the size of the osprey (Pandion haliaetus) nesting population in the study area in 1992-1993. The osprey population was previously surveyed in 1977 by the same authors, although their published findings were not adjusted for early or late nests missed. An estimated 810 ± 55 (95% C.I.) pairs were nesting at the time of the aerial survey in 1977, but the population increased 68% to an estimated 1,362 ± 278 pairs at the time of the aerial survey in 1992- 1993 (Baja California surveyed in 1992, Sonora and Sinaloa in 1993). The new adjustment for nesting chronology to estimate the total nesting population (in- cluding early and late nests missed) adds 19% to the time of the aerial survey population estimates for both 1977 and 1992-1993. The surveyed area was di- vided into seven regions for summary purposes in 1977; the same as in 1992— 1993. The distribution of nesting pairs was similar during both time periods, except two range expansions to the north, which we attributed to the presence of artificial structures in flat terrain with no suitable cacti. The estimated number of nesting pairs on the Pacific side of Baja California (focused on Scammon's and San Ignacio Lagoons) more than doubled, while the population did not increase on the Gulf of California side of Baja California. Osprey pairs nesting on the Midriff Islands in the Gulf of California increased 64% — those nesting on islands nearer Baja California remained generally the same, and those on islands nearer Sonora showed the most increase. The nesting population in Sonora and Sinaloa also more than doubled with a higher rate of increase in Sinaloa. The use of human-made structures for nest sites is still small (only 6%). This population still nests primarily on cliffs (40%), cacti (37%), and the ground (16%).