Black Skimmer Breeding Ecology and Behavior

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Abstract
Black skimmers [Rynchops niger] arrive in their breeding colonies at approximately the same time as most of the terns, usually in late April or early May. They are commonly found in dense colonies on open sandy beaches in association with other seabirds, most frequently common terns [Sterna hirundo] in Virginia [USA]. The usual clutch size is large for a seabird (.hivin.X = 3.6), with 4-egg clutches being most common. Incubation begins with the 1st egg laid. Hatching success was high (nearly 80%) in the colonies studied, but fledgling success was low in both 1973 (0.40 young/pair) and 1974 (0.37 young/pair). Probably food acts to limit production as 10 of the 11 surviving fledglings were the first hatched in the brood. Survival of 2nd, 3rd and 4th hatchlings drops off precipitously. Adult skimmers are sexually dimorphic, the male being approximately 25% larger than the female. Both parents incubate and share parental duties until fledging. The growth characteristics of the black skimmer appear to follow the logistic model developed by Ricklefs. Only after the midpoint of the fledging period (about 11 days) do male chicks begin to grow more rapidly than females. Male chicks fledged at an average of 295.2 g while females fledged at 264.4 g. Adults captured a fish approximately every 5 min. Most foraging was done at low tide in marsh habitats. No pulse of high-fishing activity at dawn and dusk was noted as has been reported in the literature. Birds foraged alone in most cases, except when the immatures began fishing. Apparently learning occurred at this time. Each young was fed on the average of 0.43 fish/h during the day; the low rate may largely explain the low fledging success in 1973-1974. The extent of nocturnal feeding requires investigation. The diet consisted of 100% fish, with silversides [Menidia spp.] and killifishes [Fundulus spp.] predominating.