We came across this little scenario yesterday at our fish pond. This small bird was feeding her ‘baby’. There is obviously a size discrepancy. I haven’t identified the young bird but I know cowbirds and starlings will sometimes lay their eggs in other birds nests and let the new parents raise their chicks. I thought the mother may have been a Swainson’s Thrush but the ‘eyebrow’ didn’t look right. I’m now leaning toward some variety of Vireo.