We addressed the following question: Do conifers within aspen stands (conifer invasion) increase bird species diversity in western landscapes? We tested the hypotheses that bird species diversity, measured as species richness or with the Shannon-Weiner diversity index, responds to aspen-conifer ratios (from 0% to 100% conifer) in a quadratic manner with a maximum occurring at an intermediate ratio of aspen and conifer. Extra sum-of-squares F tests comparing quadratic with linear models suggested that migratory bird diversity was inversely linearly related to the extent of conifer invasion. These linear responses were moderate (species richness: R² ≥0.34, P < 0.01; Shannon-Weiner diversity index: R² ≥0.34, P < 0.01). Resident species diversity appeared quadratically related to conifer invasion. However, variation was poorly described for species richness (R² ≤0.13, P ≥0.09) and was marginal for the Shan-non-Weiner diversity index (R² ≤0.27, P ≤0.01). We concluded that mixed aspen-conifer stands do not have higher bird species diversity than pure aspen stands and that management activities should focus on heavily conifer-invaded stands to increase bird diversity in western landscapes and help reverse the decline of aspen habitat due to conifer invasion.