Observations on colony size, breeding success, recruitment and inter-colony dispersal in a Tasmanian colony of Short-tailed Shearwaters Puffinus tenuirostris over a 30-year peroid

Abstract

A programme to mark a small colony of Short-tailed Shearwaters, by means of monel leg-bands, was begun on Fisher Island during 1947-50 as part of a comprehensive study. Banding of all adults and their young has colitinued every year since (to 1980).

The numbsrf breeding burrows occupied each year declined steadily during the first 25 years of the study, at the end of which numbers had been reduced by 71%. More recently, numbers have begun to increase again. No change has occurred in the relative sizes of three sub-colonies existing within the island. Annual rates of breeding success were usually lower than those reported from other less-studied colconies and have tended to be either relatilvily high (> 55%) or else poor (< 40%). Some chicks were raised every year; on average, the smallest sub-colony was as successful as the largest.

Nearly all breeding failures occurred before the chick-rearing stage and few successfully hatched chicks failed to fledge. No banded young were recaptured at the colony before age two and most not until three years and upwards. Age at first breeding for both sexes ranged from five to at least ten years. Forty-one per cent of chicks banded over 20 consecutive seasons were subsequently recovered alive on their natal island but only 35% of these were recruited as breeders. Losses of immatures and young adults through emigration were offset by an annual recruitment of unbanded immigrants, most of which could not have been bred on the island. After 30 years, the proportion of Fisherbred breeders in the colony was stable at 41-46%. Band recoveries confirmed inter-colony dispersal by young adults. Fisher-bred birds found in nearby colonies were encountered mainly in the nearest breeding area on the closest neighbouring island.

Repeated disturbance of the colony during the course of the study is thought to have contributed to the decline of the colony, due to increased burrow desertion, decreased breeding success and decreased recruitment of young adults.