Bautista, L.M., J.C. Alonso and J.A. Alonso. 1998. Foraging
site displacement in common crane flocks. Anim. Behav. 56:1237-1246.
ABSTRACT. Optimal foraging theory predicts that an individual
should resort to intraspecific kleptoparasitism when this foraging strategy
contributes to maximize its intake rate. Thus aggressor and victim should be
foraging at lower and higher rates, respectively, than the flock average
(intake rate maximization). Independent of the maximization principle,
moreover, an aggressor should attack when its intake rate falls below a
threshold critical for survival, and select a victim foraging at an intake rate
enough to ensure survival (starvation risk minimization). We tested both
hypotheses using 324 aggressive displacements from feeding sites observed in
flocks of common cranes, Grus grus, foraging on cereal fields. Aggressors
attacked cranes feeding at higher rates than average birds. The immediate
consequences of a successful attack were an increase in intake rate for the
aggressor and a decrease for the victim. The intake rate of the aggressor prior
to the attack was lower than both the mean intake rate of the flock and the
minimum intake rate necessary to cover basic metabolism needs. After displacing
its victim, the intake rate of the aggressor was higher than before the attack
and also higher than the average intake rate of the flock. The intake rate of
the aggressor after the attack was not higher than the mean intake rate of the
flock, however, when the time spent on the attack was included. We conclude
that cranes used a kleptoparasitic strategy to recover from temporary
reductions in feeding rate. This was particularly the case below the threshold
of intake necessary to survival.
This material has been published in Animal Behaviour (1998) 56:
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