The attendance cycles of the Cory's Shearwater Calonectris diomedea borealis on Selvagem Grande.

In the Cory's Shearwater Calonectris diomedea borealis of Selvagem Grande (30 degrees 09'N, 15 degrees 52'W), the visits the birds pay to their colonies are cyclic throughout the breeding period. The number of visitors varies regularly between a minimum and a maximum following a sinusoid, the half-period of which is 4.8 +/- 1.5 d (2-8 d). No variation exists according to the status of the birds (breeders or non-breeders) or to the stage of the breeding cycle (pre-egg, incubation or chick rearing) and moon phases have no influence. Among Procellariids, such attendance cycles are related to the length of the pre-egg stage. They are absent when it is short (less than 50 d), present when it is long (more than 60 d), and indifferently present or absent when it is intermediate (50-60 d). Considering the comparative scarcity of the visits of the birds to the colonies during the pre-egg stage, these cycles are likely to play a part essentially in the synchronisation of individuals in colonies and of mates in pairs.