Hummingbirds make for bad spys as they are an hour away from starvation for large parts of their lives.
Well, obviously it doesn’t mess with your brain otherwise if you turned into a cat you’d probably just spend the rest of the day sleeping and licking your butt.
There are stories of druids shifting to wild shape and living among nature for years, so I would assume they need to eat the same way their shifted shape would. It seems unlikely that they could be sustained from the magic of a single spell for that long.
Not in D&D. They just need to eat every other day, like anything else that doesn’t have enough CON to go longer. I’m not sure how much they need to eat. I’m told there’s a table on page 111 of the DMG, but I don’t own one.