Population Energetics of the American White Ibis

Kushlan, James

The White Ibis population nesting in the fluctuating water ecosystem of southern Florida numbered up to 62,000 birds and used several nesting habitats including islands in large lakes, the vast freshwater marshes of the Everglades, and coastal estuaries. Periodic reproduction at large inland colonies, accounting for 93-98% of the nesting birds, sustains the population at its current level, but smaller, more consistantly successful coastal colonies provide recruitment during years of inland nesting failure. Clutch size, brood reduction pattern, and energy requirements showed intraregional variation. Clutch size differences may be related to habitat, with lowered clutch sizes along the coast. Despite clutch size differences, the number of young fledged was similar in different colonies. Intraregional variation in timing of brood reduction resulted in differences in the energy required for nesting. The daily energy expenditure of White Ibis was estimated from the time-activity budget and multiples of measured metabolic rates. Although existence metabolism was similar to that predicted bv Kendeigh's (1970) regression, daily energy expenditure was considerably below that predicted by extrapolating King's (1974) regression, suggesting a slower increase of DEE at higher body weights. The White Ibis consumed about 21% of its body weight daily, similar to that found in other studies of ciconiiformes. The energy required to raise a brood of young was 9.2-9.9 x io' kcal. The nesting population expended 930 and 640 x 106 kcal during 1972 and 1973 respectively. Over 70% of this energy was derived from the Everglades and over 90% from inland habitats. Contrasted with the Wood Stork, the smaller size and shorter nesting cycle enables nesting White Ibis to support 11 times the population on 1. 1 times the energy. The ability of smaller birds to maintain higher populations on a given resource base may supply a competitive advantage during times of intense resource-based selection. Together the south Florida populations of White Ibis and Wood Storks require 1.9 billion kilocalories per nesting season and are important determinants of energy flow in the wetland ecosystem.-U.S. National Park Service, Everglades National Park, Homestead, Florida 33030. Accepted 17 May 1976. (This paper was subsidized by the United States Department of the Interior, National Park Service.)

Population Energetics of the American White Ibis

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