Molt of Cory's Shearwater during the Breeding Season

Abstract

We studied the molt of adult Cory's Shearwater Calonectris diomedea borealis breeding at Vila Islet, Azores. Breeders appear to undergo a complete annual molt that overlaps considerably with the breeding season. Molt is minimal during most of the prelaying and early incubation periods (March-June). The cycle starts at middle incubation (main period July), with ventral body feather replacement at a locus on the upper breast that progresses downwards and laterally to the belly and flanks. Dorsal body molt apparently follows similar topographical and temporal patterns to those of ventral body molt. Primary renewal seems to be simple and descendent and it is estimated to take a mean of 207 days for completion, between 28 August and 22 March. Rectrix renewal apparently starts on the wintering grounds since it is advanced but not completed in March-May; then it is apparently suspended until August, when the renewal of the few remaining old rectrices is concluded in most birds by September. Such an overlap of wing molt and breeding has not been previously described in a pelagic and highly migratory seabird as the Cory's Shearwater. We suggest that the timing of molt in the species is dictated by intrinsic rhythms acquired by evolution rather than imposed by nutritional or energetic constraints.