From 1992 to 1997, I monitored population size and productivity of Ospreys (Pandion haliaetus) nesting along 80 km of coast on the eastern side of the Gulf of California, Mexico. During the study period, the number of nesting pairs increased by an estimated 5–14%. The percentage of successful pairs varied significantly among years, ranging from 5–50%. In three of the six years, the productivity of breeding pairs exceeded the estimated level necessary to maintain Osprey population size in at least parts of the species' range. However, three of the last four years were characterized by low reproductive success and by a higher proportion of late clutches. In 1997, the most unproductive of the six years, 38 of 40 (95%) nesting pairs failed to produce any fledgling. That same year, however, the observed productivity of 10 pairs on two nearby islands was 15 times higher than on the mainland. In 1994, 1995, and 1996, successful pairs were significantly clumped, indicating that in the study area, nesting success was strongly influenced by local factors. The results of this study warrant further research to identify the source(s) of annual and spatial differences in nesting success.
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Vol. 71 • No. 2