Modifications des communautes d'oiseaux de la sapiniere suite au ravage d'une epidemie d'insectes | Changes in balsam forest bird communities following insect infestation

This study addresses the relationships between changes in the structure of bird communities and changes in the composition and structure of forest vegetation resulting from an insect infestation. The study compares two balsam fir-white birch forests located in similar physiographic settings. One of these was protected against spruce budworm infestation through insecticide spraying over a ten-year period. Following the infestation, the unprotected, defoliated forest underwent significant changes in structure, particularly the mortality of the coniferous tree strata and the consecutive expansion of the deciduous shrub strata. Yet, despite the infestation, the defoliated forest had just as many bird species and individual birds as the healthy stand protected through the use of insecticides. In addition, half of the avian species occurred with the same frequency in both types of forest. Aerial feeders (flycatchers) and those found in the underbrush (including four species of Turdinae) best withstood this type of infestation, whereas all species that fed chiefly at the tops of conifers decreased in number. This decrease in population could be the result of changes in stand physiognomy (for species such as Boreal Chickadee, Pine Grosbeak and Blackpoll Warbler) or predator-prey relationships affecting birds that feed mostly on spruce budworm larvae (Yellow-rumped, Baybreasted and Tennessee Warblers). As the defoliated forest became mixed with hardwood stands, some birds increased in number (Black-throated Green Warbler and Solitary Vireo). However, two other species, Winter Wren and White-throated Sparrow, both ground species, were more common in the defoliated stand, probably because of gaps in the forest and the accumulation of dead wood on the ground, where the infestation left openings in the previously dense stands of mature balsam fir.