DIVING BEHAVIOR IN SURF SCOTERS AND BARROW GOLDENEYES

I investigated diving behavior in foraging flocks of wintering Surf Scoters (Melanitta perspicillata) and Barrow's Goldeneyes (Bucephala islandica). Individuals in flocks of scoters and goldeneyes tended to dive and surface in a highly synchronous fashion. Diving-party size explained a large amount of variation in observed levels of diving synchrony. Synchrony increased with flock size as individuals in large groups followed one another more quickly during foraging dives than birds in smaller parties. Synchrony may be advantageous in large groups to maintain cohesion during foraging trips. Scoters and goldeneyes appeared not to adjust diving behavior in the presence of kleptoparasitic Glaucous-winged Gulls (Larus glaucescens). The length of the pause between consecutive dives was positively correlated with the duration of the preceding dive in the two species. Based on these relationships, the deviation from the estimated surface time was obtained for each series of dives. In both species, pause duration in sequences of many dives tended to oscillate around predicted values. This supports the idea that divers foraging on sessile prey complete more of the recovery after each dive than birds foraging on mobile prey who occasionally delay repayment of the physiological debt.