Flock Size and Vigilance in Parrots

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Abstract
We observed the effect of several flock and extrinsic variables on the vigilance and foraging behaviour of red-rumped parrots, Psephotus haematonotus (Gould), and galahs, Cacatua roseicapilla (Vieillot), during the non-breeding and early breeding season in the Canberra region. Birds devoted less time to vigilance and more to foraging as flock size increased. However, log-linear modelling revealed that this relation was affected by a complex set of variables, inevitably in the direction of higher vigilance as predation risk increased. Although both species adjust scan rates, red-rumped parrots were more likely to adjust the length of scans than galahs, possibly because the opportunity to increase scan rates is restricted when scan rate is already high. Changes to food resources and the physiological demands of winter cause adjustments to the diurnal pattern of foraging, but not to the total time spent foraging, or to the pattern of flocking. We conclude that the risk of predation is the dominant influence on flocking by parrots.