OBESITY
When a canary eats more than it requires for energy, some of the excess eventually accumulates as fat. Birds differ individually in the amount of food necessary. Some do much better in a large cage where they have more chance for exercise and all birds benefit by free flying.
A bird usually eats from daylight to dark and in some homes “lights out” comes rather late. Try to let the bird keep a natural day, dawn to dusk, and if he is to be kept awake during the evening, cover his cage part of the day.