THE SEARCHING SPEEDS OF FORAGING SHOREBIRDS - REDSHANK (TRINGA-TOTANUS) AND OYSTERCATCHER (HAEMATOPUS-OSTRALEGUS)

Foraging models predict terrestrial animals should maximize search spe ed whenever it is profitable to forage. Observations of foraging anima ls, however, suggest they seldom walk at their maximum speeds. Animals may walk slower than predicted because factors not accounted for in t he models influence the relationship between net energy gain and searc h speed in a manner that makes it more profitable to forage at speeds below the maximum. Alternatively there may be a limit on the speed at which animals can search because they cannot accelerate instantaneousl y from a standing start to their maximum speed. A theoretical model is developed that predicts the expected searching speeds of terrestrial predators as a function of prey encounter density (number of prey enco untered per unit distance searched), when they cannot accelerate insta ntaneously. This model predicts an inverse nonlinear relationship betw een speed and prey encounter density such that in most circumstances p redators will search at less than their maximum possible searching spe eds. Quantitative predictions were generated from the model for two ch aradriiform birds (redshank, Tringa totanus; and oystercatcher, Haemat opus ostralegus). Observations of the walking speeds of free-living re dshanks and oystercatchers had a form similar to those predicted by th e model, but they deviated by being lower, particularly at low prey en counter densities. Redshank may be acceleration limited in their prey- searching behavior at high prey encounter densities, but at low prey e ncounter densities and for oystercatchers other factors also appear to be important.