We assessed potential factors contributing to variation in counts of staging Bonaparte’s Gulls (Larus philadelphia) on the Niagara River in southern Ontario. Much of the Mississippi Flyway population of this species stages each autumn at this site, en route to the Gulf coast and Mexico. Between 1986-96, weekly or bi-weekly counts were made of gulls flying to their nocturnal roost at the mouth of the Niagara River entering Lake Ontario, to assess the seasonal and temporal patterns of movement. Counts began each year in early October, corresponding to the ‘second wave’ of gulls. Generally, this second wave of autumn movements began in early October to late November (median 1 November) and continued until late January or mid-February (median 5 February). Model selection based on a repeated measures analysis (Generalized Estimating Equations) demonstrated a quadratic relationship between counts and time of year and a linear relationship with daily count duration. Less evidence existed for the role of mean daily temperature in explaining variation (positive relationship) in counts; more data, or inclusion of different weather variables (e.g., wind, cloud cover), may demonstrate that it is more important than our models suggest. Maximum counts were from early November to late January (median 6 December). Peak daily totals per season varied between 6,300 (1993/94) and 40,000 (1990/91). Migration counts may be useful to help monitor populations of this species, provided that variables affecting seasonality in counts are accounted for and the turnover rate of individual gulls staging in the area is determined (e.g., through mark-recapture or radio-telemetry studies).
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Vol. 31 • No. 2