On predicting the presence of birds in Eucalyptus forest types

Abstract

We examine the relationship between the number of bird species and environment within 500 000 ha of eucalypt forests in south-eastern New South Wales, Australia. Birds were surveyed at 39 sites within 31 eucalypt communities, which were, in turn, scored by altitude, temperature, rainfall, basal areas of trees and levels of the nutrients nitrogen, phosphorus, potassium and magnesium in the eucalypt foliage. Ninety bird species were recorded. Numbers per site, averaged over the whole sample period, ranged between 20 and 38 over all surveys, 24 and 42 in ‘summer’ (October to March) when migrant species were present, and 13 and 38 in ‘winter’ (April to September). A forward stepwise Poisson regression model was used to fit bird species richness to the environmental variables. Over all surveys and in ‘summer’, foliar magnesium and tree basal area (including the basal area of dead trees) were significantly (positive) correlated with the number of bird species. In ‘winter’ the correlations were with altitude and temperature (both negative), presumably the result of emigration of species that avoid cold weather, and tree basal area (positive). The positive association of number of species over all surveys and in ‘summer’ with tree basal area, including dead trees, and foliar magnesium may index the level of forest maturity and productivity. Forest management directed solely to timber production reduces the basal area and the number of dead trees and may thereby reduce bird species richness permanently. A methodology is offered, based on the relationship between the physical environment and the more productive eucalypt communities, for determining forest areas containing the most species of birds.