As the other answers mention, birds do not, in general, have much in the way of fat reserves. This is partly because flying takes a lot of energy, and having to lift extra fat only makes the problem worse. read more
Most adult songbirds can last a couple of days without food, as long as it’s not too cold, and larger birds can last longer in the cold than smaller birds. read more
They can live at least 10 days with no water, food or new air. At school we had a cockroach in a sealed jar for that long and it was still going strong. read more
Live 10-days to 3-weeks without food depending upon initial heath. Obese. Can live from 3-weeks up to 25-weeks and more without food depending upon initial health and amount of fat. read more