Comparative breeding biology of Guillemots Uria spp. and Razorbills Alca torda at a colony in the Northwest Atlantic

We compared various aspects of the breeding biology of Razorbills, Common Guillemots, and Brünnich’s Guillemots (the “intermediate” auks) at the Gannet Islands, Labrador. Canada, in 1997. In all three species, laying followed a strongly right-skewed pattern, with median laying dates falling within a narrow window between 27-29 June. In comparison to previous years, laying was late, and relatively synchronous among species. Incubation periods were 2 days longer in Razorbills (median 35 days) than in either guillemot species (33 days), whereas Common Guillemots had longer nestling periods (mean 24 days) than Razorbills (19 days) or Brünnich’s Guillemots (20 days). In all species, there was a tendency for late-laying pairs to contract their breeding periods, mainly by reducing the duration of the nestling period. Breeding success was high in Razorbills (73%) and Common Guillemots (85%), but low in Brünnich’s Guillemots (51%), largely due to low hatching success. Late-breeding Brünnich’s Guillemots were more likely to fail than were early pairs, but there was little indication of this in the other two species. Seasonal patterns of colony attendance suggested that there were many young, pre-breeding Brünnich’s Guillemots and Razorbills present; populations of these species appear to be faring well at this colony. Despite a major shift in chick diets since the early 1980’s, caused by a decline in the availability of capelin in Labrador, chicks of all three species grew relatively quickly and departed the colony at normal masses.