“Goldfinches are among the strictest vegetarians in the bird world, selecting an entirely vegetable diet and only inadvertently swallowing an occasional insect.” (says the Cornell Lab of Ornithology — about the diet of the American Goldfinch.) In fact, (the Cornell website suggests), cowbird chicks that hatch in a Goldfinch nest often starve presumably because they can’t digest the highly vegetable diet the parents bring to their own young.
I often observe Goldfinches foraging in the flowers of the Cup Plant, which form a 7-8 foot tall border on one side of my wildflower garden. I always assumed they were harvesting seeds from the spent flower heads, but that was definitely not the case for one Goldfinch that I watched.
So I pulled apart a few of those flower heads myself, and discovered a treasure trove of insect material. Ants, spiders, tiny mites, and…a web of plant material covering a 1/2 inch long larva right at the base of the unopened flowers. This might be unusual behavior for a Goldfinch to harvest insects in this way, but it would certainly be beneficial to provide a little animal protein for their rapidly growing offspring. Perhaps, the “inadvertent swallowing of insects” isn’t really so inadvertent as was once assumed.