A Capture-Recapture Analysis of a Shearwater Population: With a Statistical Appendix

Abstract

The breeding population of Manx shearwaters, Procellaria puffinus, was analysed by the capture-recapture method. The recoveries of birds ringed as nestlings on Skokholm Island, Pembrokeshire, showed that most of them do not return to the island until they are 3 or 4 years old. The loss of rings causes an apparent high mortality in later years. Preliminary estimates of the survival-rate of shearwaters ringed as adults were between 50 and 80% but a consideration of the probable reproductive success of the species and the recoveries of birds used in homing experiments indicated that the survival rate must be above 90% per year. Further testing showed that the sampling was highly non-random, the nature of the bias suggesting that most birds visited the island at least 1 year before breeding, at which time they were more easily caught than later, when they spent less time on the surface of the ground. This view explains the high loss the 1st year after ringing, after which the survival rates rose to 88, 93 and 97% in the 2d or 3d years. A method of testing whether the ringed birds were being sampled at random is described.