This is a disturbing thought, birds being killed by a plant. Common Burdock is found throughout most of the U.S. it is a non-native species from the old world. What is apparently happening, is that birds are getting caught in the velcro-like spines of the seedheads, and dying. Here is an article discussing the problem, as it relates specifically to Golden and Ruby-crowned Kinglets.
Here is the initial link, with a photograph of a kinglet trapped in the burrs, so that you can see what it entails. I'm putting it as a link instead of placing the photo here because although it's not graphic, it is very sad. This site also has a link to the article I linked to above.
Ruby-crowned Kinglet I saw at SJWS with Alex on Sunday