. The Canadian field-naturalist. 88 The Canadian Field-Naturalist Vol. 118. FiuuRt 2. Ruby-ihroated Hummingbird entangled in burdock found 14 September 2002, Delta Marsh (photograph by T. J. Underwood). The large number of hummingbird entanglements in burdock over a short period in August 1985 seems unusual. Burdock apparently has increased in abun- dance on the ridge forest (Kenkel and Graham 1994) since our smdies began at Delta in 1973. This suggests that the frequency of entanglement should have in- creased or at least remained constant. However, search effort has varied widely since 1985
. The Canadian field-naturalist. 88 The Canadian Field-Naturalist Vol. 118. FiuuRt 2. Ruby-ihroated Hummingbird entangled in burdock found 14 September 2002, Delta Marsh (photograph by T. J. Underwood). The large number of hummingbird entanglements in burdock over a short period in August 1985 seems unusual. Burdock apparently has increased in abun- dance on the ridge forest (Kenkel and Graham 1994) since our smdies began at Delta in 1973. This suggests that the frequency of entanglement should have in- creased or at least remained constant. However, search effort has varied widely since 1985 and migration rates and weather patterns affecting bird movements through the ridge forest vary from year to year. Hence, it is difficult to assess from occasional records the overall effect of burdock at Delta Marsh on migrating birds. Nevertheless, the growing number of reports of burdock-related deaths in birds (, McNicholl 1994; Raloff 1998; Underwood and Underwood 2001) sug- gests that this type of mortality may be more important than originally beUeved, particularly at places like Delta Marsh, and King's Park in Winnipeg (Underwood and Underwood 2001), where burdock and large numbers of migrating birds are concentrated. Further study of the interaction between birds and burdock and the pos- sibly fatal consequences for birds should result in a better understanding of the effects of this exotic plant species on bird populations. Acknowledgments We thank Janis Klapecki for cataloguing bird speci- mens in The Manitoba Museum and Carla Dove, Smithsonian Institution, for attempting to identify feathers. Some specimens were collected by G. C. Biermann, J. V. Briskie, H. E. den Hann, K. A. Hobson, E. Kiviat, A. Mcllraith, and R. M. Underwood. H. E. den Haan provided details on the banding of the Yel- low-rumped Warbler from the files of the Delta Marsh Bird Observatory and assisted with age/sex identifica- tion of specimens. We thank the reviews, A. J. Erskine and M. K. McNi
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