Bird Community Dynamics and Energetics in a Northern Hardwoods Ecosystem

Abstract

One of the central goals of ecology is to understand the organization and functional relationships among natural assemblages of organisms and their complex interaction within ecological systems. Although the structures and productivities of terrestrial plant communities and their role as primary producers have been frequently studied (Whittaker 1970), there is little comprehensive information on whole communities of consumer organisms, especially those such as birds living above ground and often high in forest canopies. This is due in large part to the methodological difficulties of obtaining quantitative data simultaneously on the seasonal and annual fluxes in numbers and energy expenditures of a large number of populations. Recent work has demonstrated, however, that with intensive field investigations, bird community structures can be determined quantitatively (Emlen 1971, 1972; this paper) and that the energy expenditure of freeliving birds can be realistically estimated, using information on avian bioenergetics that has recently become available (Holmes & Sturges 1973). Several studies provide information on the structure and energy flow of individual bird populations, e.g. old field sparrows in winter (Odum, Connell & Davenport 1962), breeding red-winged blackbirds, AgelaEus phoeniceus* (Brenner 1968), and marsh wrens, Telmatodytes palustris (Kale 19653, and tree sparrows Passer montanus (Pinowski 1968) on an annual basis. At the community level, Uramoto (1961) and Karr (1968, 1971) have estimated the populations sizes and energy expenditure of birds breeding in deciduous forest in Japan and in Illinois and Panama, respectively, but their results were not discussed in an ecosystem context. Varley (19703 and Phillipson (19733 have modelled energy pathways through a temperate oak woodland, with emphasis on consumer organisms, although their data on the avian community were very incomplete. The most thorough and comprehensive analysis of avian community structure and energetic role is that of Wiens (1973) for grassland bird communities in summer. Inthis paper, we report some results from 51 continuous months of intensive and quantitative studies of bird community dynamics and energetics in the northern hardwoods ecosystems of the Hubbard Brook Experimental Forest in West Thornton, New Hampshire, U.S.A. The major objectives of this paper are to examine the seasonal and annual Ruxes in bird community composition, absolute population densities, biomass, productivities, and energy expenditures, and to consider the place of birds in the energy flow pathways of northern hardwood forests. A report with procedures for estimating