Effects of manual, mechanical, and aerial herbicide conifer release on songbird numbers in regenerating spruce plantations in Northwestern Ontario

Abstract

This study examined the effects of conifer release with herbicides [Vision® (glyphosate) ], [Release® (triclopyr) ] , and proposed alternatives to herbicides with manual brush saws and mechanical brushsaws, on breeding songbird densities in regenerating spruce plantations in northwestern Ontario. Pre-treatment (1993) cind post-treatment (1994) densities of songbirds were determined by territory mapping during June. In July and August of both years, birds were captured in mist nets and colour banded to document reproductive success and to determine whether resident birds continued to use these areas after treatment. Transects were walked during August and September of both years to identify species using the plantations. Post-treatment data revealed no changes between years in breeding bird species richness in the plantations. Overall mean songbird density decreased non-significantly from approximately 69 pairs per 10 ha in the pre-treatment year to approximately 63 pairs per 10 ha in the first post-treatment year. Analysis of variance revealed significant decreases in mean density of Chestnut-sided Warblers (Dendroica pensylvanica) between the brushsaw and Silvana treatments and the controls in the post-treatment year. Paired t-tests showed that, for most of the common species, there were significant year to year increases or decreases in mean densities in most areas where treatments were applied. Sparrow densities increased and the densities of foliage gleaning insectivores decreased. Ten percent of the 826 birds banded in 1993 were recaptured in 1994, 80% of these were within 100m of the point of initial capture the previous year. Twenty individuals moved from treated areas into adjacent areas with more typical breeding habitat. More White-throated Sparrows Zonotrichia albicollis than warblers (Dendroica sp.) were observed utilizing the plantations during August and September in the post-treatment year compared to the pre-treatment year when there were no differences. Clumps of untreated vegetation (skips) remaining within the brushsaw and Silvana treatments provided the only available nesting sites for Alder Flycatchers Empidonax alnorum, Chestnut-sided Warblers, and Mourning Warblers Oporonis Philadelphia. Conifer release treatments had non-significant impacts on the breeding densities of most songbird species. Evidence suggests, however, that treatment effects were masked by other confounding factors such as the presence of skips, male site tenacity, and too few degrees of freedom in the MANOVA analyses.