Awareness of the spread of invasive plant species has grown, but quantitative measures of their impacts are lacking. This study analyses the impact of Himalayan blackberry (Rubus armeniacus) on breeding bird diversity finding a significant difference in bird diversity between “natural” and R. armeniacus-dominated understoreys. More bird species were noted in habitats with greater structural and compositional diversity. Simpson’s richness/evenness index was significantly different between habitat types for Stanley Park and Maplewood Flats (P<0.05) but not Jericho Park (P>0.05), likely due to lower overall bird diversity at Jericho Park and lack of overstorey trees at R. armeniacus thickets. When R. armeniacus is the dominant understorey shrub in a forested setting it has the greatest negative impact on breeding bird diversity.