Flock Size and Position Effects on Vigilance, Aggression, and Prey Capture in the European Starling

Abstract

We studied autumn foraging flocks of European Starlings (Sturnus vulgaris) foraging on grass lawns to determine how flock size (up to ca. 250 individuals) and position within the flock (edge vs. center) affect the birds' foraging behavior. The birds fed exclusively by probing their bills into the ground, gaping, and extracting beetle larvae or earthworms, which were directly observed by us. Neither flock size nor position affected the rate of prey capture. Edge birds, however, spent more time vigilant and probed less than center birds. Rates ofagonistic interactions did not differ between flock positions; therefore, the percentage of time actually spent foraging (i.e., walking, searching, and probing) was lower for edge birds due to their higher level of vigilance. The only behaviors affected by flock size were percentage of time spent vigilant and percentage of time spent foraging, but only for edge birds in flocks of 520 individuals. The rate of agonistic interactions did not increase with increasing flock size for edge or center birds. Probe rate was not correlated with prey capture rate, placing caution on indiscriminate use of probe (or peck) rate as an index of prey capture rate. The likely nature of the dispersion pattern of the starling's prey is discussed in relation to our results.