Rafting behaviour and predator disturbance to Steller's EidersPolysticta stelleri in northern Norway

Abstract

Steller's Eiders responded to overflying Great Black-backed GullsLarus marinus by “spooking” — flushing from feeding and aggregating on open water. Birds did not react at high tide when aggregated into dense roosting flocks. The significant reduction in inter-bird distance associated with this escape behaviour suggests that rafting in this way (and at rest) serves an anti-predator function. In 32% of spooks, up to 56% of the flocks were displaced from feeding sites; even when all birds remained, they took an average of 3.5 minutes for half of the flock to resume feeding, representing a minimum total of 17% loss of feeding time throughout the 24 hour period. At observed rates, the cost of flight behaviour in response to gulls was estimated to equate to a minimum of 7.8% of daily existence energy. Hence such predator responses can cost Steller's Eiders loss of access to favoured feeding area, loss of feeding time (already restricted by tidal exposure) and additional energy costs.