The effect of manual thinning and glyphosate application on breeding bird communities in southern British Columbia

• Main Author: Easton, Wendy Elizabeth
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/6437

Removal of deciduous vegetation in managed conifer forests is a major component of most silviculture programs. Despite controversy over non-target effects, research regarding the impact of vegetation management on birds is limited. I examined the effect of removing 90-96% of the deciduous tree volume on breeding bird communities in young conifer plantations during 4 years. Trees were removed by two treatments: manual thinning, and manual thinning plus application of the herbicide, glyphosate. The control and two treatments were each replicated three times. During the 3 post-treatment years, the herbicide-treated sites remained depauperate of deciduous vegetation while the manually thinned sites experienced deciduous regrowth. Variation in richness of bird species across all sites was solely due to the presence or absence of rare species. Richness declined, abundance increased, and common species dominated after herbicide application. Richness, abundance, and evenness increased after manual thinning. Turnover of bird species was highest in the herbicide-treated areas and lowest in the control areas. Residents, short-distance migrants, ground gleaners and conifer nesters increased significantly after herbicide application. Deciduous nesters and foliage gleaners declined in abundance (nonsignificantly) in areas treated with herbicide. The Warbling Vireo (Vireo gilvus), a deciduous specialist, declined in areas treated with herbicide. Although treated areas exhibited similar increases in total abundance, nesting success of open-cup nesting species was significantly lower in the herbicide-treated than manually thinned areas. Warbling Vireos, Swainson's Thrushes, and Dusky Flycatchers preferred to nest in untreated or recovered habitat within larger treated areas. Successful nests were more more likely to be present in areas with willow. Overall, the composition of bird communities became more homogeneous after herbicide application, and showed little change after manual thinning.