CHANGES IN THE BIRD COMMUNITIES OF A BALS AM FIR WHITE BIRCH FOREST FOLLOWING AN INSECT PEST EPIDEMIC

This study addresses the relationships between changes in the structur e of bird communities and changes in the composition and structure of forest vegetation resulting from an insect infestation. The study comp ares two balsam fir-white birch forests located in similar physiograph ic settings. One of these was protected against spruce budworm infesta tion through insecticide spraying over a ten-year period. Following th e infestation, the unprotected, defoliated forest underwent significan t changes in structure, particularly the mortality of the coniferous t ree 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 thro ugh the use of insecticides. In addition, half of the avian species oc curred with the same frequency in both types of forest. Aerial feeders (flycatchers) and those found in the underbrush (including four speci es of Turdinae) best withstood this type of infestation, whereas all s pecies that fed chiefly at the tops of conifers decreased in number. T his decrease in population could be the result of changes in stand phy siognomy (for species such as Boreal Chickadee, Pine Grosbeak and Blac kpoll Warbler) or predator-prey relationships affecting birds that fee d mostly on spruce budworm larvae (Yellow-rumped, Bay-breasted and Ten nessee Warblers). As the defoliated forest became mixed with hardwood stands, some birds increased in number (Black-throated Green Warbler a nd Solitary Vireo). However, two other species, Winter Wren and White- throated Sparrow, both ground species, were more common in the defolia ted stand, probably because of gaps in the forest and the accumulation of dead wood on the ground, where the infestation left openings in th e previously dense stands of mature balsam fir.