When I hand raised baby budgies from hatching a couple years ago I was always amazed by how I could see the formula go down the newborn's throat into his crop. The skin is so thin you can clearly ee the contents of the crop right through the skin. In parent fed babies you can see the ehole shelled millet seeds and any leafy greens. Occasional air bubbles in the crop are very common and normal as long as they aren't so big that they take up a lot of crop space.
I saw a video online today of parent insect eating song birds stuffing a big green caterpillar into a little baby's mouth. I guess babies can swallow pretty large food items as long as they're fairly soft. Baby birds are essentially eating machines. Fascinating!