Status of Nesting Ospreys in Coastal Baja California, Sonora and Sinaloa, Mexico, 1977 and 1992-1993.

Abstract

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