Back in 2004, a study done by Oregon State University researchers revealed that wild bird seed can be a haven for noxious weeds. Weed scientists examined seed from ten different brands of seed found more than fifty different weed seed contaminants, including the very nasty puncturevine (Tribulus terrestris) and nuisance species like bindweed (Convolvulus arvensis) and bull and Canada thistle (Cirsium vulgare and C. arvense).
Now the authors of the study have gone on to make a series of recommendations for how bird lovers around the world can keep noxious weeds out of their bird seed and out of their yards. For example, something simple, like checking below bird feeders for odd new plants. But given the propensity of birds to grab seeds and flee the scene, the advice to seek heat-treated seeds may make more sense. A full list of suggestions can be had in this press release from the Weed Science Society of America, along with a list of ten of the thirty seed contaminants found in the study.
The ISW first posted about the issue of bird seed contaminants in Oregon back in early 2007.