Reproductive activity in the Osprey (Pandion haliaetus) on Kangaroo Island, South Australia

Abstract

This study reports on aspects of the breeding biology of Ospreys (Pandion haliaetus) on Kangaroo Island from data collected over 18 breeding seasons between 1985 and 2004. Over this period an average of nine pairs were located each year, and a total of 145 occupied territory years were monitored for breeding activity and outcomes. Of these, active breeding occurred in 103 (71%), with 60% of these successfully fledging young. Productivity was found to average 0.66 young fledged per year per occupied territory, and 0.92 young fledged per year per active nest. This level of productivity, while similar to that of other studies in Australia, is below the minimum recruitment levels needed to maintain migratory Osprey populations in the northern hemisphere. However, such high rates may not be needed in the non-migratory population of Australia. Although some early dispersal was recorded among marked Osprey young, strong philopatric recruitment was also evident, with 22% of survivors either remaining on the island, or returning at maturity to join the breeding population. Through the re-identification of individuals, Osprey pairs were found to remain together over many seasons and to use the same primary nesting site. The breeding season began later than reported elsewhere in Australia, extending from August to February, with most laying occurring in September. Undetected nest predation and human disturbance was suspected at accessible nests as contributing to the high level of nest failures recorded. The apparent elongated nestling development period found is comparable to that determined in other studies where fluctuating prey availability directly influenced nestling growth and survival. These factors, plus geographical isolation, suggest the Osprey may be precariously balanced ecologically at the southern extent of its breeding range in Australia.