Bird behaviour on and entanglement in invasive burdock ( Arctium spp.) plants in Winnipeg, Manitoba

Burdocks ( Arctium spp.) are invasive plants native to Eurasia. They produce their seeds within large burrs that adhere to vertebrates for seed dispersal, i.e., epizoochory. Small animals may become entangled in these burrs and die. Through active searches of natural areas in Winnipeg, Manitoba, from 2000 to 2005, we documented 29 songbirds entangled in burdock burrs and recorded observations of bird behaviour on burdock plants. We analyzed trends among birds found entangled by taxon, sex, age, migratory status, and season, and we compared them to observations of bird activity on burdock. Thirteen species of birds were found entangled. Most birds found entangled (62%) were warblers (Parulidae), but there were also species from five other families (<15% per family). There was no significant difference in the distribution of birds found entangled by sex or age. A significant proportion of birds found entangled were migrants, and the average number of birds found entangled per search visit differed significantly by season, with most birds found entangled in fall. By comparison, we recorded 34 species of birds (n = 178 observations) active on burdock plants. Warblers (38%) and sparrows (Emberizidae) (24%) were the most active, followed by small numbers of birds from eight other families (<9% per family). Species that were observed actively foraging on burdocks more often became entangled than those species observed only perching on burdocks. Species found entangled had a significantly lower body mass than species that were not found entangled but were active on burdocks. Overall, we suggest that small-bodied fall migrants that forage on burdock plants are most at risk of becoming entangled.