Yup, goldfinches, like most of the finches (House, Redpoll, Crossbill, etc.)
feed their young almost entirely on regurgitated seeds. No insects for
their babies. Evening Grosbeaks however do feed more on insects, taking
advantage of spruce budworm outbreaks especially to feed their young. But,
goldfinches and the like feed their young a regurgitated mass of half
digested seeds...yum. Hence, goldfinches time their breeding to take
advantage of the late summer abundance of seeding plants, particularly
thistles which they also use the white fluffy part of the seed for lining
their nests.

Jeff Kozma
Yakima