A lot of the bird seed that is used to feed wild and domestic birds is contaminated with weed seeds, and is one way that invasive plants can move around. Whether the bird seed is imported or a Canadian product, it could contain seeds of invasive plants. The weed seeds may start to grow in your garden under or near the bird feeder, or they may be blown by wind or carried by birds into natural areas or agricultural fields. This can be the start of a new plant invasion.
If you notice unusual plants starting to grow near your bird feeder, they should be pulled out before they can reproduce, and you may want to consider changing the bird seed you are putting out.