Rostrhamus sociabilis (Snail Kite) nesting success and productivity were studied at the recently colonized Lake Kissimmee and historically occupied Lake Okeechobee in Florida during 1987 to 1993. Mean (± SD) clutch size for all lake-years was 2.79 ± 0.49, with 3-egg clutches (75.2%) being most frequent. No significant difference was detected in the overall distribution of 2-, 3-, and 4-egg clutches between lakes. The mean number of fledglings for all lake-years was 0.79 ± 0.99 fledgling/nest. Only 44.1% of all nests were successful in fledging birds. Fledging success did not differ significantly among years at Lake Kissimmee. However, significant differences occurred among years at Lake Okeechobee due to the high frequency of failed nests in 1990 (72.2%) and 1991 (74.3%) during low lake levels. Productivity at Lake Kissimmee (0.88 ± 1.00 fledgling/nest) was significantly greater than at Lake Okeechobee (0.74 ± 0.98 fledgling/nest).
Estimated nest survivorship (Sday: nest age in days) combined for all years and both lakes was as follows: S28 (hatching) = 0.67 ± 0.14; S42 (2 weeks) = 0.58± 0.14; S56 (4 weeks) = 0.46 ± 0.20, and S70 (fledging) = 0.45 ± 0.03 (n = 641 nests). There was a higher probability of nest survivorship to hatching and fledging at Lake Kissimmee compared with Lake Okeechobee (S28 = 0.74 ± 0.11 and S70 = 0.53 ± 0.03 versus S28 = 0.63 ± 0.16 and S70 = 0.40 ± 0.02, respectively). Lake level and breeding date during early incubation had a significant effect on nest survivorship at both Lake Kissimmee and Lake Okeechobee.