Abstract
There was a steady increase with latitude in the size of Puffins and Kittiwakes breeding in Norway. Data which supports earlier suggestions to disregard the subdivision of F. arctica arctica and F. a. grabae are presented. Males of both species were consistently heavier and larger than females in measurements of wing length, head length, bill length and bill depth. Discriminant analyses showed that head length, sometimes combined with bill depth, distinguished the sexes most precisely. Both sexes of both species steadily lost weight from the start of incubation to the peak chick feeding period. It is argued that this weight loss is voluntary and adaptive.
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