Colony attendance of Cape petrels on Nelson Island, South Shetland Islands, Antarctica, fluctuated in a non-random pattern. In the pre-breeding period a relatively weak cycle of about 6 days was observed. Low attendance was associated with brief periods of either SE wind or strong wind, the effects of which lasted 2 days. During breeding, regular attendance cycles were approximately twice as long (8 days) as the average incubation shift (3.8 days), which may have resulted from synchronization of unemployed birds not engaged in incubation. Breeding attendance was lowered on 2 days after strong wind and on days with SE wind, but only wind speed showed a significant effect when both variables were considered. Post-breeding colony attendance showed a continuation of the rhythm established during breeding, but with the majority of birds participating. Correlations found between wind factors and colony attendance suggest that some weather characteristics, which themselves were not truly cyclic, could act to synchronize the endogenous rhythms of individual birds. Seasonal changes of wind effects on colony attendance could be interpreted in terms of foraging behaviour, i.e. as a consequence of the feeding grounds in summer being not restricted by sea ice to one particular direction from the colony.
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