THE BREEDING BIOLOGY OF LEACH'S PETREL, OCEANODROMA LEUCORHOA HENRY M. WILBUR THE Procellariiformes are the most pelagicof all seabirds.A delayed reproductive maturity, a reduced clutchsize,a prolonged incubation and nestling period,and low mortality ratesare consequences of their pelagic ecology. Lack (1954, 1966), Ashmole (1963), Serventy (1967), and others have discussed the evolution of this breeding strategy. Theseau- thors conclude that nesting on predator-freeislands limits seabirdsto small clutches by the need to searchfor widely dispersed or fluctuating food organisms. The age of first breeding is delayedbecause young, inex- perienced birds cannot providefor the additional demands of reproduction and nestling care. The prolongation of the incubation and nestling periods is thoughtto be an adaptationto an irregular feedingschedule.The re- duced reproduction rate, coupledwith a density-dependent mechanism, necessitates the Iongevities characteristic of the group. This paper adds to the life history of Leach's Petrel as described by Gross (1935), Ainslieand Atkinson(1937), Gross (1947), and Hunting- ton (1962, 1963) and relatesthe renesting attemptsof failed breeders to Lack's theory of the reproductive rates of birds. The observations reported in this paper were madeduring a study of the movements of Leach's Petrel, Oceanodro•na leucorhoa, at the Bowdoin ScientificStation on Kent Island, Grand Manan, New Brunswick, Canada. Kent Island supports the south- ernmos.t largecolony of Leach's Petrel in the Atlantic; an estimated 15,000 pairs breed on the island (Wilbur, MS). I spent 20 weekson the island, from 3 June to 14 August1965 and from 6 June to 17 August 1966. METHODS The study area was a section of the colony that had not been used previously for biological studies. Lattices of twigs were placed over all burrows in this section and were checked daily for at least a week after they were erected. When a lattice was found disturbed, the burrow was examined for the presence of nest material, an egg, or adult birds. Adults were removed from the burrows only during the day in an effort to minimize the number of egg desertions. In 1965 the study area contained about 140 burrows of which 86 containedeggsor chicks. In 64 of these burrows both adults were banded, in 22 burrows only one adult was banded. In 1966 the study area was enlarged to about 1.4 acres. We found 274 burrows of which 154 contained eggs m' chicks. In 113 of these burrows both adults were banded, in 32 burrows only one adult was banded, and in the remaining 9 burrows neither adult was identified. These data do not necessarily reflect a change in the breeding population of the colony. From 8 to 13 August 1966, 444 previcusly undislurbed petrel burrows adjacent to the s[udy area were examined. The total breeding population of the colony was estimated by dividing the island into 43 communities disti, nguished by the changes in the color and lexture of their 433 The Auk, 86: 433-442. July, 1969