Apparent survival and dispersal in a recovered Osprey population: effects of age, sex and social status

Dispersal and survival are important determinants of population dynamics and should be monitored in both threatened and recovered populations. We used colour ringing with a capture–mark–recapture approach to estimate the age distribution, survival and dispersal in a recently recovered Baltic Osprey Pandion haliaetus population. The mean age of breeding Ospreys was 5.5 years (5.5 years in males and 5.4 years in females). The mean age of floaters was 3.1 years (3.3 years in males and 2.9 years in females). The re-sighting probability was best modelled as a time-independent function of five age classes separately for both sexes, and it increased with age. In females, the re-sighting probability increased from 0.03 in 1-year-old birds to 0.77 at 5 years and older; in males, it increased from 0.01 to 0.54. The best models for apparent survival included two age classes. Survival was lower for 1-year-old females (0.14) than males (0.27); the survival difference was retained at > 1 year of age (females 0.64 and males 0.74). Apparent survival (0.72) was similar for satellite-tracked adult Ospreys. Floaters were rare; however, their survival (0.70) was similar to that of breeders. Mean natal dispersal distances were significantly longer for females (164 km) than for males (47 km). Breeding dispersal was recorded in 5% of breeders (mean distance, 11.1 km). The relatively high (apparent) mortality in the Baltic population may be partly explained by competition in breeding grounds and threats during migration and wintering. However, permanent emigration may contribute to the low apparent survival.