Previous studies documented that Black Skimmers, Rynchops niger; tend to forage at night but did not examine which environmental variables explain nocturnal foraging. Foraging frequency and environmental variables were studied at a large breeding colony of skimmers on an urbanized barrier island on the west coast of Florida, USA. To determine which variables influenced foraging, the number of skimmers departing to forage was counted for 120, one-hour periods through the day and night from 30 May–29 August, 2007. Using generalized linear models, factors were most associated with skimmers departing to forage were identified. These models included: light level, temperature, wind, wind direction, relative humidity, tide height, tide stage (incoming, outgoing, or slack) and status of the colony (mostly eggs, chicks, or fledglings). Light level was the only factor to significantly influence the amount of foraging (Wald = 9.40, d.f. = 1, p = 0.002). The relationship was negative, as light levels decreased, the number of birds departing to forage increased. The average proportion of the colony foraging per hour at night was 38.2% (SE = 4.6) and during the day the average proportion was 13.6% (SE = 4.0). The likely cause is that small, planktivorous fish come closer to the surface and shoreline at low light levels, allowing the skimmer to employ its unusual foraging technique of “skimming”.
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Vol. 33 • No. 4