Prelaying exodus of Cory's shearwaters (Calonectris diomedea borealis) on Selvagem Grande

We stayed on the Portugese island of Selvagem Grande, 30 degrees N 16 degre es W, 300 km south of Madeira and 200 km north of Tenerife, from 9 May to 3 June 1991 in order to study the prelaying exodus in Gory's Shearwaters (Ca lonectris diomedea borealis) and its connection with the colony attendance cycles described earlier. Laying took place on 1.5 June +/- 3.4 days. Durin g the preceding weeks males and females of Gory's Shearwater show very diff erent attendance patterns. Females are absent for 19 +/- 4.8 days at minimu m. Absences of males are much shorter and also much more variable. They las t 7.9 +/- 6.0 days (coefficient of variation 76%, as against 12% in females ). The first incubation stint of the males is badly synchronised with the l aying: the delay between laying and the start of the males' first stint may amount up to one week, without compromising the breeding success. Two well marked attendance peaks have been observed, separated by an interval of 18 days, twice the length of that observed in June 1988. We suggest that this anomaly in the daily attendance graph is correlated with the prelaying exo dus. The function of the prelaying exodus and its relation with the energy cost of producing the egg and the necessity of storing lipid reserves are d iscussed.