Sexual size dimorphism (SSD) may favour physiological peculiarities in diet, behaviour and home-range size both across species and within species. Sex-specific differences in diet and behaviour have been reported in several bird species but there are fewer studies of foraging area size in sexually dimorphic bird species. Foraging area size should be greater in the bigger sex according to home-range size predictions based on body mass. We tested this prediction in a winter study of foraging area size in the Great Bustard Otis tarda, the most sexually size-dimorphic bird species, which forages in unisexual flocks. In this species the temporal pattern of a flock's feeding intensity; the proportion of birds actively feeding (FI) and the size of the morning foraging area (MFA) of each sex are unknown. We recorded the behaviour and movements of unisexual flocks of Great Bustards during winter mornings and sampled food availability to take into account its effect on FI and MFA. FI increased and then decreased through the morning in both sexes, and was lower in males than in females. This sexual difference was greater where legume availability was smaller. Legumes were the most preferred substrate type. Consequently, MFA sizes were smaller in sites with more legume availability. We did not find sexual differences either in the size of MFA or in the selection of the two preferred substrate types: legumes and stubble fields. MFA and FI were determined to a greater extent by ecological factors such as food availability than by metabolic requirements derived from body size differences. These results obtained from a short-term study do not preclude an effect of sexual size dimorphism on MFA size and FI of Great Bustards over longer periods but show that the body size effect on foraging behaviour may be smaller than predicted only by SSD.
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Vol. 64 • No. 2