Wear of rings used on Guillemots Uria aalge: Caution in the estimation of survival rates

Abstract

When ashore, a Guillemot Uria aalge usually walks with the tarsus in contact with the ground and this results in severe wear of rings. Even the change from soft to hard metal rings in the early 1960s only increased the useful life of a ring placed on a Guillemot to around 10 years, after which the ring numbers still became illegible if the ring had turned or had been put on upside down. During the early 1980s the British Trust for Ornithology (BTO) developed a special ring for use on Guillemots in an attempt to reduce the adverse effects of wear on the longevity and legibility of rings. The ring is approximately triangular when viewed end-on with a flat base that should be in contact with the ground, the number stamped on each side and the return address placed above one of the numbers. This ring came into general use in 1983, since when over 200,000 have been used with 80% of these being on chicks. The ring has proved extremely successful in eliminating the loss of the ring number by abrasion. Recently, however, a new problem has come to light in that some rings placed on chicks have turned, with the result that the pointed apex of the ring is in contact with the ground. This concentrates the wear on a small area and exposes the return address to abrasion. Each year since 1983 about 300 Guillemot chicks on the Isle of May, southeast Scotland have been metalringed, individually colour-ringed and had their ages estimated from their wing lengths. In 2000-03 MPH used a x60 telescope to determine whether or not the BTO rings had turned on 389 birds that had been colourringed as chicks and had subsequently returned to prospect for breeding sites. Twenty-five birds with turned rings were caught and their rings replaced. Most (82%) of the 389 birds examined were between three and six years old, an age when birds prospecting for nest-sites are easily observed. Older birds are normally breeding which makes rings difficult to see and two-yearolds rarely attend colonies. On 168 (43%) of these colour-ringed birds, the BTO guillemot special ring had turned. These rings are an excellent fit on adult Guillemots and we have found it impossible to reverse a turned ring without removing it from the bird. None of the guillemot special rings placed on 748 full-grown Guillemots that we colour-ringed at the same time have turned, so it appears as though turning occurred soon after chicks were ringed, presumably before they become full-sized, some 2-3 months after fledging. We examined the effects of year and wing length at ringing on the chances of a ring turning using logistic regression with the response as a binary score (ring turned = 1, ring not turned = 0). There was a highly significant effect of year ( 2 9 = 38.62, P < 0.001) with the proportion of rings turning in years where there were reasonable samples varying from 28% to 70% (Table 1). The variation between years could not be accounted for by annual variation in wing length since there was also a highly significant effect of year after allowing for wing length ( 2 9 = 38.24, P < 0.001). We cannot explain these between-year differences. The chances of a ring turning were reduced with an increase in wing length at ringing (ignoring year effects: 2 1 = 5.06, P = 0.024; allowing for year effects: 2 1 =