THE EFFECT OF VEGETATION MANAGEMENT ON BREEDING BIRD COMMUNITIES IN BRITISH COLUMBIA

Abstract

Removal of deciduous vegetation in managed conifer forests is a major component of most silviculture programs in North America. We examined the effect of removing 90-96% of the volume of deciduous trees on breeding bird communities in young conifer plantations during four years. Trees were removed by two treatments: manual thin- ning (manual treatment), and manual thinning plus application of a herbicide, glyphosate (herbicide treatment). The control and two treatments were each replicated three times. During the three post-treatment years, the herbicide-treated sites remained depauperate of deciduous vegetation while the manually thinned sites experienced regrowth of deciduous trees. Number of species declined, total number of individuals increased, and common species dominated after herbicide treatment. Number of species, total number of individuals, and evenness increased after manual treatment. Turnover of bird species was highest in the herbicide-treated areas and lowest in control areas. Residents, short-distance migrants, ground gleaners, and conifer nesters increased significantly after herbicide treatment. De- ciduous nesters and foliage gleaners increased in abundance (nonsignificantly) in control and manually thinned areas. Warbling Vireos (Vireo gilvus), which are deciduous specialists, declined in areas treated with herbicide and may be particularly susceptible to glyphosate application. Although treated areas exhibited similar increases in the total number of birds, nesting success of open-cup nesting species was significantly lower in the herbicide-treated than in manually thinned areas. We suggest that habitat variability may be a mechanism for producing nested subset structure of bird community composition. Overall, the com- position of bird communities became more homogeneous after herbicide treatment, and it showed little change after manual thinning.