Teasel and seeds of other invasive plants sometimes are accidentally (or maybe even purposely, but unwisely?) fed to birds. There are excellent sources of information available today to learn more about invasive plants, how to avoid spreading them, and how they have negative effects on bird populations.
Start with SEWISC (The Southeastern Wisconsin Invasive Species Consortium, Inc. (SEWISC): http://sewisc.org/
US Forest Service page on Invasives: http://www.fs.fed.us/pnw/invasives/index.shtml
Effects on bird communities: http://userwww.sfsu.edu/~efc/classes/pa/Chelsea.pdf
Effect on nesting success: http://www.inhs.uiuc.edu/inhsreports/autumn-01/nesting.html