When I looked at the Natural Events Calendar on my office wall today, the event for August 7 was "Goldfinches begin nesting as thistles go to seed." It seemed late in the year for a bird to begin nesting, so I looked up some more information on the habits of the American Goldfinch, which I found interesting.
Goldfinches are one of the latest nesting bird species in Missouri, with their reproduction tied closely to the fruiting of thistles. The link between goldfinches and thistles is evident, even in the Latin name for the bird (Carduelis tristis), because Carduus is a genus of thistles. Goldfinches eat thistle seeds and use the downy fruits to line their nests, sometimes using caterpillar or spider silk to bind its edges. The nests are so tightly woven that they reportedly will hold water.
Goldfinches are almost exclusively seed eaters, apparently able to get all of their protein needs from their seed diet. Many of their food plants are in the sunflower family. The birds have a rudimentary crop that allows them to accumulate some undigested food. The chicks are fed a sticky mass of seed pulp that the adults regurgitate to the young. Although the Brown-headed Cowbird will lay its eggs in goldfinches' nests, it is rare that a cowbird chick will survive to leave the nest, probably because the cowbird chick cannot thrive on a diet of virtually all seeds.