PREDATION AND KLEPTOPARASITISM BY MIGRATING PARASITIC JAEGERS

Parasitic Jaegers (Stercorarius parasiticus) are thought to rely exclu sively on kleptoparasitism of seabirds to obtain their food while on m igration. We investigated this dependence of fall migrating birds at a stopover located on the shore of the St. Lawrence River, Quebec. We f ound that in addition to kleptoparasitism, jaegers also preyed upon in vertebrates (Gammarus spp.), ducks, and shorebirds. Small Calidris san dpipers were chased more often than expected according to their availa bility whereas Black-bellied Plovers (Pluvialis squatarola) were chase d less often. The capture rate was independent of the type of shorebir d chased. Capture rate was not influenced by the number of jaegers (on e to five) participating in a chase. The number of jaegers, however, s ignificantly affected the per capita hunting yield through prey defens e against gulls; pair members obtained the highest yield. Predation of shorebirds was mainly performed by territorial jaegers which restrain ed other jaegers from the main hunting area. Kleptoparasitic interacti ons were mostly aimed at Common Terns (Sterna hirundo), which were cha sed more often than expected considering their availability, whereas B lack-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 fi sh increased with jaegers' group size (one to seven), whereas the per capita feeding yield decreased. No such trend was observed when jaeger s chased kittiwakes. Occurrence of predation and kleptoparasitic event s was influenced by tide and time of day.