Feeding methods and diving rates of migrating larids off Deer Island, New Brunswick

A study of three species of migrating larids was undertaken off Deer Island, New Brunswick, to compare feeding methods and diving rates of birds exploiting various prey types. Bonaparte's Gulls (Larus Philadelphia) fed mainly by diving (dipping and plunging) and surface seizing, whereas Common and Arctic Terns (Sterna hirundo and S. paradisaea) acquired food by diving only. Common and Arctic Terns foraged from greater heights than Bonaparte's Gulls, and both the gulls and the terns maintained a higher foraging height while feeding on small fish than while feeding on euphausiids or, in the case of the gulls, insects. Common and Arctic Terns had a steeper dive angle than Bonaparte's Gulls, and both the gulls and the terns dove more steeply when feeding on small fish than on euphausiids. The highest diving rate occurred when Bonaparte's Gulls were dipping for insects trapped in the water surface layer. There was no significant difference in mean dipping or plunging rates between the gulls and the terns feeding on euphausiids or small fish. Bonaparte's Gulls had a lower mean rate of incomplete dives than did terns, probably because of the gulls' lesser aerial manoeuverability reducing the likelihood of altering course after dive initiation. The mean dipping rate was higher than the mean plunging rate for both the gulls and the terns since most food was taken at the surface, and plunges were necessary only for deeper prey (usually small fish). Intervals between hovering and diving were similar for both the gulls and the terns.