Many people think these sunny little birds leave for the winter and migrate to southern warmer climates. However they stay here in the north all year long. They simply change color in the winter months. You have likely seen them at birdfeeders all winter long and not recognized them. These amazing bright yellow birds simply turn brownish yellow in the winter. This color change is a perfect camouflage to let these little fellows make it through the winter.
American Goldfinches are unique in that they nest later than most other birds in the spring. They use thistle down to build their soft little nests in shrubs. They raise their young later in the summer than other birds and rely primarily on thistle seeds although they will eat many other types of seed, including sunflowers. Birds in the north generally all raise their young early in spring, relying heavily on proteins sources such as insects to raise their young. Migrating birds are in a hurry to hatch their chicks and get them growing so they can make their travels back south in the fall. Native birds are eager to raise their young on the heavy spring insect hatches so that they can grow and be prepared for a diet of seed in the winter. American Goldfinches eat only seeds so they need to wait to hatch and raise their young when there are seeds to eat.
You've certainly seen American Goldfinches in the summer, their bright yellow and black coloring makes them impossible to miss. The trick is to notice them in the winter.