Productivity of Ospreys in the Gulf of California

The 1977 and 1978 breeding seasons of non-migratory Ospreys (Pandion haliaetus) in the Gulf of California extended from early January through mid-June with a high degree of asynchrony among pairs that was not caused by renesting. Thirty-two and 28 pairs produced eggs in 1977 and 1978, respectively. Mean clutch-sizes (±SE) were 2.7 ± 0.1 in 1977 (N = 28) and 2.9 ± 0.1 (N = 24) in 1978. Hatching success (0.6 nestlings per egg) did not differ in the 2 years although the number of young fledged per active nest decreased from 1.0-0.9. Twenty-one pairs in 1977 produced at least one young to fledging (x̄ = 1.5) and 14 pairs in 1978 averaged 1.9 fledglings. In 1977, pairs that initiated egg-laying during the first half of the laying period were more successful in producing fledglings than were pairs that laid later. Thirty-two fledglings were produced. In 1978, 26 fledglings were produced and there was no significant association of reproductive timing and success. Success in raising nestlings to fledging was similar in both years, indicating that factors influencing the differential production for the two breeding seasons occurred early in the breeding and/or environmental phenology. A comparison of patterns of reproductive loss indicated that the greatest difference between 1977 and 1978 breeding seasons in patterns of reproduction was an increased loss of reproductive potential due to non-breeding.