Over 90% of our passerine birds must feed their developing young, a diet of over 90% insects….or the young die. It is that simple; i.e. no middle ground, no compromise. The nestlings MUST have a diet of almost 100% insects. The American Goldfinch is an exception to this rule. AG's are granivores, almost exclusively. The young are fed mostly grains and some insects, regurgitated from the adults. The 20-30 I have living at my Maine home go absolutely nuts for fresh, "on the stem", sunflower seeds. Every day they tear my sunflower heads open and ravenously rip at the head to pluck the seeds.