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

-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. Many shorebirds forage in intertidal habitats where tides cause predictable changes in the foraging substrate. Exposed areas fluctuate and the timing of tides varies regularly. Food items may be clumped in tidal pools and their availability may vary according to when (Green and Hobson 1970) or how long different areas have been exposed to sun and air (Vader 1964, Recher 1966). The effects of tidal fluctuations on shorebird activities have had little study. Several researchers have documented changes in shorebird abundance, habitat choice, or dispersion in relation to the tides (Recher 1966, Thomas and Dartnall 1971, Burger et al. 1977, Hartwick 1978, Harris 1979, Kelly and Cogswell 1979, Connors et al. 1981). Others have attempted to relate levels of feeding or foraging activity to tidal changes (Ehlert 1964, Harris 1979, Hartwick and Blaylock 1979, Morrell et al. 1979). In order to avoid such tide-related variations, Baker (1974) recorded data only during the low tide. As the ebb tide exposes the foraging area, shorebirds may occupy the space, flocks may disperse and different prey may be taken. As individuals spread out and new prey types are sought, foraging success or techniques may change. In addition, the breakup of flocks could significantly alter relations among the birds and the susceptibility to predation. Increasing flock size often heightens aggression and competition (Recher and Recher 1969, Silliman et al. 1977, Burger et al. 1979), and lowers individual vigilance (Abramson 1979, Barnard 1979, Caraco 1979, etc.) and the likelihood of predation (Page and Whitacre 1975). Increasing f ock size also usually improves foraging success (Krebs 1974, Abramson 1979, Barnard 1979, Caraco 1979; but see Smith 1977), but if this is balanced against the costs of competition and vigilance, flocking may yield no foraging benefits (Krebs and Barnard 1980). I studied the foraging activities of Ruddy Turnstones (Arenaria interpres) on the Pacific coast of Costa Rica, and attempted to determine some of the relationships among tidal oscillations, food distributions, and various aspects of turnstone behavior. STUDY AREA AND FLOCKS I made my observations on a clay tidal flat at Playa Sirena, Corcovado National Park, Puntarenas Province, Costa Rica, from 6 to 16 August 1979, and from 9 to 23 April 1980. Playa Sirena frames a crescent-shaped bay and stretches about 2 km between the mouths of the Rio Sirena and the Rio Claro. A sandy beach slopes downward about 20 m from a second-growth woodland to the edge of the extensive clay flat (hereafter called the "beach's edge"). Regardless of the tide, turnstones were never seen feeding on the sandy beach. The clay flat itself was fairly level and at low tide extended 150-250 m from the beach's edge. Extent of total exposure depended on location