Guillemots on the Isle of May fed their young on Sandeels and Sprats, the proportion of the latter being highest late in the day and the season. There was a peak of feeding after dawn when many large Sandeels were brought. Weather had no effect on feeding rate but this did increase with the chicks age. Food appeared to be abundant. A large Sprat was the most efficient return for effort and Guillemots may select the most energy-rich prey. Display fish (mainly small Sandeels) were brought throughout the season, mostly by females which often ate them later. The significance of the fish-carrying display is obscure.