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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