Aims To document the changes in return dates over a 44-year period and to identify the factor associated with these changes.
Methods We compared changes in return date at Shetland colonies with those for the Isle of May, south-east Scotland, and with the available information on population size, the abundance of fish species eaten by Common Guillemots and large-scale changes in the oceanography and climate of the eastern Atlantic as reflected by the winter index of the North Atlantic Oscillation (NAO).
Results Common Guillemots normally return to colonies in Shetland in late winter. However, during the 1960s return dates became gradually earlier with birds present from early October. Autumn return remained the norm for about ten years after which return dates gradually reverted back to late winter. In contrast, Common Guillemots on the Isle of May 400km south of Shetland, showed no marked shift, returning in October each year. There was strong negative correlation between date of return Shetland birds and population size, whereas on the Isle of May birds came back earlier when there was a large positive winter NAO index. There was no convincing evidence that changes in wintering areas or fish abundance influenced when birds returned to the colonies, although the fish data may not have been collected on the correct spatial scale.
Conclusion Competition for high-quality nest-sites is the most likely reason for Common Guillemots returning to the colonies during the autumn and winter.