Foraging responses of stilts (Himantopusspp.: Aves) to changes in behaviour and abundance of their riverbed prey

Abstract

Riverbed foraging by pied stilts (Himantopus h. leucocephalus) and the endangered black stilt (H. novaezealandiae) was studied in the Cass River Valley, South Island, New Zealand. The staple riverbed diet of both species was aquatic larvae of Deleatidium spp. (Ephemeroptera), usually captured by pecking. When the larvae were inactive and hidden from view, particularly when the water temperature was below 5°C, black stilts used the non‐visual methods of probing or raking in the gravel to find food. Pied stilts did not use non‐visual methods and so were forced to feed at a low rate or move to other temporarily more profitable habitats. Black stilts foraged mainly at the riffles where prey was most abundant, whereas pied stilts foraged mainly at pools where prey was less common but more easily seen. During the emergence drift of Deleatidium, both species of stilts switched from feeding on larvae to feeding on drifting subimagines.