Are wintering Common Redpolls Carduelis flammea more exposed to avian predators than other small passerines?

Sammendrag

The Common Redpoll Carduelis flammea is a highly social flocking bird which forms cohesive groups outside the breeding season. Being small birds (12-15g), they need to forage almost continuously during a few hours of daylight in winter to meet their energy demands. Although predation risk is reduced as a result of improved surveillance of many eyes, increased flock size may also increase the agonistic interactions among individuals within the flock and as a consequence lower the probability of detection of a predator’s approach. To examine whether Common Redpolls, living in relatively large winter flocks, are more exposed to avian predators than small passerines living in groups of only a few individuals, I recorded the responses of Common Redpolls, Willow Tits Poecile montanus and Great Tits Parus major to a life-like stuffed specimen of two different predators, the Siberian Jay Perisoreus infaustus (body length 30 cm) and the Hooded Crow Corvus cornix (50 cm). After the presentation of a predator model, Redpolls returned to the seed sites on the average earlier than the tits, and significantly so versus both tit species after exposure to the Siberian Jay, and for Great Tits after exposure to the Hooded Crow. Whereas Willow and Great Tits seemed to pick up sunflower seeds in the snow without much competition, Common Redpolls displayed a more conspicuous, aggressive, intraspecific behaviour. Thus, the individual vigilance of Common Redpolls was most likely reduced and exposure time to predation increased. The seed-eating Common Redpolls may be more food-stressed than the year-round resident Willow Tits that have stored food within their territories. If so, Common Redpolls may be forced to take greater predation risks because of a higher hunger level. If the differences in return times after exposure to a predator model reflected an adaptation to perceived predation risk, the Common Redpolls apparently evaluated the Siberian Jay as less dangerous than did the tits.