“TWO-EGG CLUTCHES” IN CORY’S SHEARWATER (CALONECTRIS DIOMEDEA)

Abstract

— In the Cory’s Shearwater (Calonectris diomedea) of Selvagem Grande, “two-egg clutches” represented 0.52% of the total number of clutches. “Two-egg clutches” were found principally in attractive nests occupied by inexperienced breeders. The first egg seemed to be laid mostly by lonely females taking advantage of the pre-laying exodus to enter the temporarily empty nest of a young inexperienced pair. These females deserted their egg after a few days; then the legitimate tenants layed the second egg. The first egg was deserted on average after three days and replaced four days later. The breeding success of “two-egg clutches” was significantly lower than that of one-egg clutches, because of the failure of the first egg. Among other Procellariiformes, as in Cory’s Shearwaters, “two-egg clutches” are always rare and show little success.