Very interesting! There's a lot of information here about the eating and digestion habits of birds. I've had a good vantage point to watch baby birds eating in the nest. It's really true, they eat like there's no tomorrow.
The robins, which are a very common bird in our area, eat a lot of worms and bugs. Their digestion in the proventriculus must be easily done, so their gizzard is probably quite small.
It's surprising that some birds eat sand, and small rocks that are used in the gizzard to help digest the undigested food. I don't know what happens to the sand and rocks after that. Do they sit there for the next meal or are they eliminated?