Ages of Storm Petrels Hydrobates pelagicus prospecting potential breeding colonies

Each year, ringers using sound lures mark and recapture immature Storm Petrels Hydrobates pelagicus prospecting potential breeding colonies. Attempts have been made to estimate the size of this population, but a full demographic model has not been possible since the age structure of this population has not been determined. To address this, between 1990 and 2004, 799 Storm Petrel chicks were ringed at the breeding colony on the Island of Mousa, Shetland, to establish a sample of marked known-age birds. There were 39 subsequent recaptures of these individuals, mainly by using sound lures at sites away from breeding colonies. Only one recapture related to a first-year bird: the largest cohort was of second-year birds and the relative frequencies of third-year and fourth-year cohorts progressively diminished as birds entered the breeding population and ceased responding to sound lures. These data provide demographic information to facilitate the construction of population models. Given the difficulty in determining the size of breeding populations of Storm Petrels and in long-term annual monitoring of breeding productivity, there may be considerable conservation benefit from ringing and recapture of Storm Petrels each year to monitor the size of the pre-breeding population.

J. David Okill and Mark Bolton, Ringing & Migration (2005) 22, 205-208