Predation and Kleptoparasitism by Migrating Parasitic Jaegers

Abstract

Parasitic Jaegers (Stercorarius parasiticus) are thought to rely exclusively on kleptoparasitism of seabirds to obtain their food while on migration. We investigated this dependence of fall migrating birds at a stopover located on the shore of the St. Lawrence River, Quebec. We found that in addition to kleptoparasitism, jaegers also preyed upon invertebrates (Gammarus spp.), ducks, and shorebirds. Small Calidris sandpipers were chased more often than expected according to their availability whereas Black-bellied Plovers (Pluvialis squatarola) were chased less often. The capture rate was independent of the type of shorebird chased. Capture rate was not influenced by the number of jaegers (one to five) participating in a chase. The number of jaegers, however, significantly affected the per capita hunting yield through prey defense against gulls; pair members obtained the highest yield. Predation of shorebirds was mainly performed by territorial jaegers which restrained other jaegers from the main hunting area. Kleptoparasitic interactions were mostly aimed at Common Terns (Sterna hirundo), which were chased more often than expected considering their availability, whereas Black-legged Kittiwakes (Rissa tridactyla) and Ring-billed Gulls (Larus delawarensis) were chased less often. Terns were more likely to drop food than kittiwakes. The success rate at inducing a tern to drop a fish increased with jaegers' group size (one to seven), whereas the per capita feeding yield decreased. No such trend was observed when jaegers chased kittiwakes. Occurrence of predation and kleptoparasitic events was influenced by tide and time of day.