COMPARATIVE BREEDING SUCCESS AND DIET OF RING-BILLED AND HERRING-GULLS ON SOUTH-LIMESTONE-ISLAND, GEORGIAN BAY

In recent years, numbers of Ring-billed Gulls (Larus delavarensis) have increased throughout the Great Lakes while numbers of Herring Gulls (L. argentatus) have remained relatively stable. We hypothesized that greater diet flexibility and fewer conspecific predation events contribute to a higher fledging success, hence greater recruitment rate to the Ring-billed Gull population. To provide insight into this hypothesis, we conducted a comparative study of the two species at one location in one year using standardized data collection procedures. While there was no difference in hatching success between the two species, egg failure in Herring Gull clutches was evenly distributed among A-,, B- and C-eggs, whereas in Ring-billed Gull clutches, C-eggs had the highest failure rate. Chick failure in Ring-billed Gull broods was mostly due to known death or disappearance, principally among C-chicks, and mostly before seven days of age. Chick failure in Herring Gull broods was almost entirely due to their disappearance and A-, B- and C-chicks were lost with equal frequency. As egg and chick loss were independent of lay/hatch order in Herring Gulls, we suspect a higher incidence of conspecific predation than in Ring-billed Gulls where mostly C-eggs and chicks were lost. The diet of Herring Gull chicks was primarily fish whereas Ring-billed Gull chicks received fish and insects in equal proportions. Ring-billed Gull parents fed chicks in 2-chick and 3-chick broods at a significantly higher rate than did Herring Gull parents, particularly during the first 16 days of chick age. Breeding success (chicks fledged per pair) was significantly higher for Ring-billed Gulls. We conclude that the higher feeding rate and greater diet flexibility of Ring-billed Gulls compared to Herring Gulls yielded a higher-per-pair productivity. If the patterns identified in this one-year, single-island study can be generalized, differences in feeding and (suspected) predation may have contributed to the numerical increases of Ring-billed Gulls relative to Herring Gulls noted throughout the Great Lakes.