Foraging by Common Redshanks Tringa totanus on the polychaete worms Hediste diversicolor and Nephtys hombergii: A re-analysis of data first published in 1977.

Abstract

Goss-Custard (1977) constructed an empirical model to test the hypothesis that free-living Common Redshanks Tringa totanus selected the size classes of prey that maximised intake rate. Model parameters were estimated during the winters 1969–70, 1970–71 and 1971–72 in Redshanks foraging on 16 mudflats containing two species of polychaete worms, the Ragworm Hediste diversicolor and the catworm Nephtys hombergii. Model predictions agreed with the predictions of the hypothesis. Subsequent research has found that environmental factors, namely, season, state of the tide and ambient temperature, affect the behaviour of the worms and their availability to shorebirds, including Redshanks. Our initial intention here had been to test by multivariate analysis whether taking the effect of these potentially confounding variables into account would significantly change model parameter values and thus its predictions when testing the rate-maximising hypothesis. The analysis revealed that not only were parameter values substantially changed but that major implicit assumptions in the model had been false. Perhaps most fatal to the model was that the responses of Redshanks to variations in the abundance of the two species of polychaete worms are so different that the two species cannot be considered jointly to comprise the birds’ food supply, as had previously been assumed. To our great surprise, intake rate decreased as the numerical density of Nephtys increased, exactly opposite to the Type II functional response found in the well-known prey species of Redshanks, Hediste. Nephtys is now known to be an important predator of Hediste. We hypothesise that Hediste may avoid Nephtys by moving deeper into their burrows below a depth of 5 cm, an anti-predator response that would put them out of reach of the 4 cm long bill of Redshanks. This could reduce the numerical density of accessible Hediste when Nephtys are present and so the birds’ intake rate.