American goldfinch prefer can be found in a variety of habitat but they prefer to live around weedy fields. This is handy because they are almost exclusively vegetarian. Thistle seeds are one of there favorite foods. It is a lot of fun to watch them tear into the plant pulling out the hairs, called pappus, to get to the seeds at the bottom.
Besides being a consciences vegetarian the American goldfinch is also a good environmentalist. Instead of throwing away all of the pappus from the thistle plant they will take some back and use them to build their nest. This may be part of the reason why they nest much later then other birds, in June and July.