Tidal rhythm in a seabird

Abstract

GUILLEMOTS (Uria aalge) breed on cliff ledges in densely packed colonies often numbering many thousands of pairs. About 25% of the British population of these birds nests in Orkney¹, where their proximity to North Sea oil developments makes it important that the numbers breeding are continuously monitored to check for any adverse effects. One of the difficulties in estimating the breeding population is that the numbers present on cliff ledges show considerable variation, particularly before egg-laying². The data presented here were collected on the island of Copinsay, Orkney, between April 20 and May 13, 1976 with a view to determining the reasons for these fluctuations. They demonstrate the clear effect of a tidal rhythm, a phenomenon previously described for birds which feed close inshore^3,4, but not apparently for any species which feeds in deeper water, though Tuck⁵ makes passing reference to the possibility of such an effect at some of the guillemot colonies which he studied in Canada.