Relationships between Tidal Oscillations and Ruddy Turnstone Flocking, Foraging, and Vigilance Behavior

Abstract

Ruddy Turnstone (Arenaria interpres) flocking behavior, aggression, vigilance, prey choice, foraging rate, and foraging success were examined in relation to tidal fluctuations and food distribution on a Pacific beach in Costa Rica. Turnstones foraged only when the tidal flat was exposed; they rested in flocks during high tide. Flock size, peck rate, and success rate decreased during the ebb and increased with the flood. Foraging velocity increased until low tide and then decreased. Vigilance was related only to flock size and flock size did not affect foraging rates or success. Vigilance apparently did not directly reduce foraging efficiency or deter flock breakup. Increased aggression, however, decreased foraging efficiency, and may have partly caused reduction in flock size. Turnstones fed on invertebrates close inshore and on crabs farther out on the flat, where the latter were more abundant. Rates of predation on crabs were correlated with crab density, decreased foraging rate, and success. The switch in the birds' diet and the substrate zonation appeared to be largely responsible for the tidally and spatially correlated patterns in foraging rate. Possibly the nature and distribution of the prey influenced the turnstones' levels of aggression and hence, determined the optimal sizes of their flocks.