The influence of sexual size dimorphism on the dietary shifts of Capercaillie Tetrao urogallus during spring

Abstract

In boreal forests during spring, the highly size-dimorphic Capercaillie Tetrao urogallus changes its diet from pine needles to more nutritious ground forage. At Varaldskogen in southeast Norway, we compared the dietary shift of males and females from March to end of May based on faecal analysis (n = 199). The proportions of ground plants were significantly higher among the female samples than among the male samples during all three months. Both sexes had significantly higher proportions of ground plants in samples from May than the preceding months. When taking into account actual quantities consumed and the lower digestibility of pine needles, males were estimated to ingest about the same amount of sprouting ground plants as females. Males require much larger quantities of food than females, and thus, a smaller fraction of males' total food requirement is obtained from sparsely distributed ground plants. Consequently, males experience a smaller net benefit from feeding on ground food and supplement their diets with a larger fraction of easily accessible pine needles of lower nutritional quality. Moreover, male lekking activities and mutual avoidance behaviour during daytime may restrict foraging time, and the availability of ground forage is probably lower within the restricted daytime ranges around leks than outside.