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) hav e 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 contri bute to a higher fledging success, hence greater recruitment rate to t he Ring-billed Gull population. To provide insight into this hypothesi s, we conducted a comparative study of the two species at one location in one year using standardized data collection procedures. While ther e was no difference in hatching success between the two species, egg f ailure in Herring Gull clutches was evenly distributed among A-,, B- a nd C-eggs, whereas in Ring-billed Gull clutches, C-eggs had the highes t failure rate. Chick failure in Ring-billed Gull broods was mostly du e to known death or disappearance, principally among C-chicks, and mos tly before seven days of age. Chick failure in Herring Gull broods was almost entirely due to their disappearance and A-, B- and C-chicks we re lost with equal frequency. As egg and chick loss were independent o f lay/hatch order in Herring Gulls, we suspect a higher incidence of c onspecific 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 pro portions. Ring-billed Gull parents fed chicks in 2-chick and 3-chick b roods at a significantly higher rate than did Herring Gull parents, pa rticularly during the first 16 days of chick age. Breeding success (ch icks 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-p air 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-bi lled Gulls relative to Herring Gulls noted throughout the Great Lakes.