Biogeographic aspects of the distribution of bird species breeding in Québec's peatlands

Abstract

Aim: The state of peatlands in eastern Canada is of growing concern. This habitat is in decline due to urban sprawl, agriculture, forestry, and peat mining. Moreover, reduction and fragmentation have led to increasing isolation of remaining peatlands. We determined how bird species distribution in peatlands conforms to expectations drawn from island dynamics. We also determined the factors influencing the occurrence of 10 common peatland bird species, two of which rely mainly on peatlands for nesting in the study region. Location: We sampled sixty-three peatlands in southern Québec, Canada, within a landscape characterized by a mosaic of forest stands and farmland. Methods: We sampled nesting bird populations within peatlands from 4 June to 14 July 1995, using both transect lines and fixed-radius point counts. Each sampled peatland was characterized by area, vegetation structure (microhabitats), and isolation. We used multiple regression to test the relationship between bird species richness, peatland area, heterogeneity, microhabitat richness, and relative isolation, after correction for sampling effort. Relationships between bird species abundances and the variables the environmental variables were investigated with Canonical Correspondence Analyses. We calculated probabilities of occurrence of individual species in peatlands by logistic regression, with the same explanatory variables as mentioned previously. Results: Bird species richness was mainly explained by microhabitat richness, and to a lesser extent, by sampling effort. By contrast, the occurrence of more than half of the species was mainly explained by peatland microhabitat heterogeneity. Palm warbler Dendroica palmarum (Gmelin) and upland sandpipers Bartramia longicauda (Bechstein) were the only species less frequent in small and isolated peatlands than in other peatlands. Main conclusions: The results for species richness support both the habitat diversity, and passive sampling hypotheses for patchy distribution of birds. By contrast, results from individual species emphasized the difference between factors affecting total species richness and individual species distribution. The distribution of palm warbler, the only species restricted to peatlands regionally, was consistent with expectations from island dynamics.