Dandelions and Canada thistle are also a couple non-native plants with seeds that American Goldfinches devour. The birds don’t read memos about native and non-native, they just know yummy, high fat food when they taste it. Nyjer is a thin, black seed with a high oil content, an excellent source of nutrition that comes from African daisies, not thistles. It is a favorite of redpolls, juncos, doves, finches, buntings and siskins. People like to offer nyjer because it is heat treated and won't sprout below the feeder.
More than 55 million adults in the United States feed birds around their home, which makes bird-feeding the second most popular hobby in the country after gardening. And the seed we use in the bird feeding industry is dependent largely on food grown for human consumption.
The most common birdseed ingredients, sunflowers, peanuts, safflower, corn, millet, and nyjer are all crushed for oil, grown as staple crops or grains for animal feed. And most of these plants have been hybridized to produce high yields with the highest nutrition.
Finches do forage for native seeds from plants like asters, coneflowers, grasses, native milkweed and trees such as alder, birch, western red cedar, and elm. They like to hang out in weedy fields, open floodplains, and other overgrown areas with some shrubs and trees for nesting. To encourage goldfinches into your yard, you can plant native seed producing plants and hang sunflower and nyjer bird feeders. I find their song especially warm and sunny during the winters here in Michigan.
Different Goldfinches in America http://goo.gl/aI9msF
A Closer look at Dandelions http://goo.gl/YfhxHY
Round fluffy white seeds floating in wind http://goo.gl/xmKiCN
Sunflowers Up-close: The Strange Journey of an American Plant http://goo.gl/eitZa1
Nyjer (thistle) isn't related to Canada Thistle http://goo.gl/ZZBcgM