Relationships between Temperate Forest Bird Communities and Vegetation Structure

Abstract

General patterns of density, species richness, and relative abundance of breeding birds are examined in a wide variety of North American forests. Taking data from censuses published in American Birds, we use rarefaction to ordinate the species richness of communities in terms of samples of equal numbers of individuals. By this criterion young forests in secondary succession and mature deciduous forests can be equally rich in bird species; coniferous forests and dense successional stands having only one or two species of trees have the fewest species of birds. For our data set the density of birds is higher in mature deciduous forests than in successional stands. The species/area relationship (s/a) is a function of both this species/individuals relationship (s/n) and the individuals/area relationship (n/a). According to the Jonckheere—Terpstra statistic there is a statistically significant pattern to the order in which bird communities fall with regard to s/n and n/a. To see these relationships in terms of the structure of the vegetation, we present the positions of 56 stands in a graphic space determined by principal components. Tree species richness and canopy cover dominate the first axis, and variation in canopy height the second. Tree density has a somewhat independent pattern of variation and is expressed by the third axis. In the bivariate space of principal components one and three, a SYMAP contour diagram of the number of bird species expected in 10 ha (s/a) shows maximal values in mature deciduous forests, but not in those that have the highest trees species richness, canopy height, or tree density. The number of individual birds/10 ha (n/a) shows a similar pattern except that maximal density occurs at maximal values of tree species richness and canopy height. By this criterion both bird species richness and density are minimal in coniferous forests characterized by high tree density, low canopy, and few species of trees. These patterns are not discernible by the classic bird species diversity/foliage height diversity method proposed by Mac Arthur and Mac Arthur in 1961. They are masked by correlation coefficients, partly because the coefficients are insensitive to nonlinear relationships. We recommend rarefaction, principal components analysis, and contour diagrams to display relationships among communities.