| Summary: | Increases in numbers of Herring (L. argentatus) and Great Black-backed (L. marinus) gulls, and concurrent decreases in numbers of other beach-nesting birds, led to the proposed use of avicides to control gulls on Monomoy NWR, Chatham, Massachusetts. The purpose of this study was to obtain biological information on Monomoy's gull populations, and to use this information to develop and evaluate gull control strategies. Landfill use differed between species. Herring Gulls used more, and traveled farther to, landfills than did Great Black-backed Gulls. Herring Gulls used fewer landfills during the non-breeding than breeding season. Numbers of Herring Gulls in landfills were significantly related to human populations served by those landfills (r$\sp2$ = 0.63, P = 0.0003), Great Black-backed Gull numbers were not. Although both species consumed refuse and natural foods, more Herring than Great Black-backed gull stomachs contained refuse $(X\sp2$ = 7.88, P $<$ 0.005 in 1988; $X\sp2$ = 13.93, P $<$ 0.0002 in 1989). Refuse was a minor dietary component for both species' chicks. The passage of large gulls over nest sites had little impact on Monomoy's Common Tern (Sterna hirundo) and Laughing Gull (L. atricilla) colonies. Few fly-overs (49, 0.04%) at altitudes of 20 m or less resulted in mobbings or disturbances. Terns mobbed large gulls more often than did Laughing Gulls (G = 16.61, P $<$ 0.005), but numbers of birds per mobbing did not differ between species (t = 0.95, P $>$ 0.05). Both species habituated to fly-overs and ignored most large gulls. We developed a deterministic simulation model to identify the effects of different types and levels of control on gull population dynamics. Effectiveness of gull control varied with technique, but no method completely eliminated Herring or Great Black-backed gulls. Model construction and interpretation of outputs are described.
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