Although Puffins fed their young on those species of small fish that were most abundant in mid water, there was some evidence of selectivity. Sandeels and Sprats were by far the most important prey species. Rockling and Whiting were also taken but they were less suitable because of their small size and low calorific value respectively. Many of the abundant gadoid and flatfish species were not preyed on to any extent. Some years Puffins on St Kilda fed their young on Whiting but in these years chicks fledged at lower weights than in years when young were fed on Sprats. It is suggested that Puffins on St Kilda were short of the best prey species compared to those on the Isle of May and were forced to feed their young on sub-optimal food.