Diving Behavior in Surf Scoters and Barrow's Goldeneyes

Abstract

-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. Divingparty 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. Received 13 September 1991, accepted 22 February 1992. IN A NUMBER of bird species, search for food occurs underwater. A typical dive cycle in these birds includes underwater time, during which individuals travel to the foraging area, search for food, and return to the surface, and surface time between successive foraging episodes. Pauses between dives may be used by the birds to recover from the physiological effects of a prolonged period of time spent underwater. A great deal of research has focused on the physiology of diving (e.g. Butler and Jones 1982), but comparatively little attention has been paid to the behavioral ecology of diving in birds. Two aspects of diving behavior in birds have, however, received increased attention recently: synchronicity; and the relationship between dive and pause duration. In flocks of diving birds, submersion schedules are often temporally clustered. Individuals in groups of birds thus tend to dive and surface in synchrony (McKinney 1965, Stewart 1967, Schenkeveld and Ydenberg 1985, Wilson et al. 1986). In the particular case of birds foraging on sessile prey, synchrony may allow individuals to follow one another to extremely localized food patches (McKinney 1965). In addition, I Present address: Department of Biology, Concordia University, 1455 Ouest Boulevard de Maisonneuve, Montreal, Quebec H3G 1M8, Canada. Schenkeveld and Ydenberg (1985) suggested the possibility that individuals in these flocks gain protection while surfacing with a prey by swamping potential kleptoparasites. In a quantitative study of diving by Surf Scoter (Melanitta perspicillata) flocks, Schenkeveld and Ydenberg (1985) provided evidence that diving and surfacing are highly synchronous in this species. Surfacing synchrony also tended to increase in the presence of Glaucous-winged Gulls (Larus glaucescens) that commonly rob mussels from scoters. Little is known about other factors that lead to large variation in the degree of synchrony exhibited by different flocks. Diving birds also show flexibility in the control of dive duration. Data from a variety of species indicate that pause duration is generally positively related to the amount of time spent underwater in the preceding dive (see review in Ydenberg 1988). This supports the assumption that pause duration is linked to recovery from the physiological effects of diving. However, recent evidence also suggests that surface time in a series of dives may be altered to suit local foraging conditions. For instance, Western Grebes (Aechmophorus occidentalis) can delay repayment of the accumulated physiological debt until after an encountered school of fish escapes (Ydenberg and Forbes 1988). Shorter pause duration than would be needed for full recovery may allow a forager to relocate and exploit more