SUMMER AND WINTER BIRD POPULATIONS ASSOCIATED WITH 5 AGE-CLASSES OF ASPEN FOREST IN ALBERTA

Summer and winter bird use of aspen forests was studied in west central Alberta. Forest age-classes in the study included 1- and 2-year-old clear-cut areas and 14-, 30-, 60-, and 80-year-old stands. Habitat structure and composition were quantified by a combination of measurement and estimation of variables. Density of the breeding bird population was estimated by territorial mapping. Changes in habitat structure between different age-classes resulted in a successional replacement of bird species. Number of individuals was greatest in 14-year-old stands at 656 territories per 100 ha, probably owing to the presence of remnant snags and of large trees that had survived past fires. The 30-year-old stands supported 312 territories per 100 ha, 60-year-old stands 380 territories, and 80-year-old stands 231 territories, respectively. Recently clear-cut areas supported only 110 territories. Number of species followed the same pattern with 25 in 14-year-old stands, 22 in the 30-year-old stands, 21 in the 60-year-old stands, and 14 in the 80-year-old stands. Recent clearcuts supported 10 species. In winter 13 bird species were recorded in the combined clear-cut, 14-, 30-, and 60-year-old stands. The highest number of species (11) were recorded in the 60-year-old stand during winter but the greatest winter bird density (125 individuals/100 ha) occurred in the 30-year-old stand. Three principal components extracted from the habitat data explained 83.2% of the variance in bird use. The components corresponded to gradients in canopy cover, occurrence of conifers, and shrub density.