Most significant is excessive fat in the diet. Birds like high fat seeds and will often select them over any other seeds in the mix. Heredity appears to be another significant cause of obesity. Birds that are obese often produce offspring that are obese. Many of the large birds, upon necropsy, have been shown to be obese, as have their progeny.
Cockatiels carry fat in two locations in the body.
They carry large fat deposits under the skin on the sides of their body, and inside the abdominal area. Abdominal fat is not always noticeable from the outside. By parting the feathers on the side, one can observe fat deposits under the skin on the sides of the bird. Fat deposits will cause the skin to appear more yellow than the normal pinkish to white. The skin will feel roly-poly rather than firm.