At about this time every year, there are reports from coastal birders about sightings of large flocks of gulls (i.e. Bonaparte’s and Ring-billed Gulls) feeding on plankton (barnacle larvae).

     Among the observations noted from these large feeding flocks has been the act of dip-feeding from the surface and picking plankton while in flight. Birders have also mention the sudden disappearance of these feeding flocks from an area, where hundreds, if not thousands of gulls had just been moments before.

     It was my belief over the years that these gulls were more interested in the bait fish eating the plankton, rather than the plankton themselves. Why would a gull with a bill not adapted to feeding on microscopic prey bother with plankton, when it could eat the much larger bait fish gorging on the plankton?

     Does the sudden disappearance of hundreds of gulls coincide with sudden disappearance of floating plankton, or does it make more sense that the bait fish moved on?

I can't give you a definitive answer, but it is certainly the case that gull predation (including by Bonapartes gull) on planktonic crustaceans is documented in the lieratutre (see e.g. abstract below). Most gulls are opportunistic and will take whatever food they can get! While fish do comprise a large part of their diet I would expect that a high concentration of large planktonic species near the surface actually provides an easier meal to catch. Fish are faster moving and, being generalist predators and scavengers means that gulls lack the particular adaptations for fish predation by diving seen in some seabirds (e.g. auks, gannets).





J. Plankton Res. (1987) 9 (3): 483-501. doi: 10.1093/plankt/9.3.483

The relationship between plankton-feeding Bonaparte's and Mew Gulls and tidal upwelling at Active Pass, British Columbia

Kees Vermeer, Ildy Szabo and Paul Greisman



Abstract

The food habits of Bonaparte's (Larus Philadelphia) and Mew Gulls (L. canus) were studied at Active Pass, British Columbia, in relation to upwelling of zooplankton. Bonaparte's Gulls fed mostly on planktonic crustaceans during September-November and again during April-May, while Mew Gulls foraged there chiefly in February and March. Both species ate predominantly the euphausiid, Thysanoessa raschii, in spring, while Bonaparte's Gulls fed mainly on the amphipods, Parathemisto pacifica and Calliopius laeviusculus, in fall. Year-round sampling of zooplankton and collection of temperature and salinity data showed gull numbers to correlate with times of maximum upwelling and abundance of zooplankton prey in surface waters. Outside the upwelling zone in Active Pass, Bonaparte's and Mew Gulls fed mostly on fishes and intertidal organisms, but also on zooplankton along tidelines.