A detailed investigation of the effects of weather conditions on the feeding behaviour of a pursuit diving seabird, the Common Guillemot Uria aalge, was carried out during the chick-rearing period at a colony in southeast Scotland in 1997. There was no evidence that the rate at which adults brought back fish for their chicks was related to weather conditions but the type of prey delivered was affected with birds bringing in smaller sandeels Ammodytes marinus in stormy weather. This resulted in a significant decrease in the mean energy value of a prey load and in the overall rate of energy intake of chicks. The attendance behaviour of adults also varied in response to the prevailing weather conditions. The proportion of sites with chicks which had both adults present was significantly lower in stormy conditions indicating that, although the frequency at which Common Guillemots provisioned their chicks was not affected, birds were spending longer away foraging. Furthermore, information on diving behaviour collected using radio telemetry suggested that birds were working harder when conditions were bad with the number of dives in a bout, and hence the amount of time spent underwater, increasing significantly and the interval between consecutive diving bouts decreasing significantly. Analysis of long-term data on chick provisioning and adult attendance for the study colony indicated that the observed reductions in the mean energy value of loads and the proportion of chicks attended by both adults in stormy weather, which were evident in the 1997 data, were also apparent across years. If the current climate change prediction of an increase in the frequency and/or intensity of storms at mid to high latitudes proves correct, such trends could have marked energetic consequences for Common Guillemots in terms of increased foraging costs of adults and reduced energy intake of chicks.