Body composition of the european Barn Owl during the nonbreeding period

Abstract

The body composition of Barn Owls (Tyto alba) killed on motorways in the northeastern part of France during autumn and winter was studied in relation to their age, sex, body size, and season. Body composition differed greatly among individuals. In mature and immature birds of both sexes, body mass and body lipid were highest in early winter, whereas body protein did not show seasonal changes. Thus, body mass increase was due to accumulation of body lipid. Maximum (14%) and average (10%) adiposity of males and females were very similar, but body protein was higher in females than in males. This difference is explained by the larger skeletal size of females, and it disappears after size-adjustment. Relative to other data on captive fed or fasted Barn Owls, and based on body lipid, 54, 40 and 6% of the present birds were considered as nondepleted, partly-depleted and emaciated, respectively. The adiposity of nondepleted males and females and of mature and immature individuals were similar and averaged 12.5%, intermediate compared to other bird species. In contrast to previous suggestions, we conclude that Barn Owls accumulate lipid reserves as frequently as do other bird species, in anticipation of cold weather and food scarcity. During mild winters, most of the road-killed Barn Owls were in good nutritional condition. We found no evidence for a sex difference in the energy strategy of Barn Owls during the nonbreeding period.