Avian species are highly mobile and usually migrate, thereby lessening the chances of mass starvation. Some species experience mortality of the females due to nesting activities in the early spring, when they are unable to leave the nest to feed. Deaths attributed to malnutrition and starvation are seen in young birds during the hatching season due to parent neglect, or once they are fledged from the nest, the inability to acquire their own food. If an animal is forced into an inadequate plane of nutrition, there are many physiological changes as the animal attempts to satisfy its energy requirements. Reserve stores of nutrients contained in the individual are utilized to compensate for the lack of nutritional intake.
Energy is generated from the utilization of proteins, fats, and carbohydrates.
The most readily usable material, the carbohydrate glycogen, is utilized first. This is derived from glycogen stored in the liver and is exhausted within a few hours. This is followed by stored fat from the various subcutaneous deposits, around the kidney, and in the mesentery and omentum tissue. Fat deposits in the parenchymatous organs are utilized next. The last area of the body to lose its fat deposits is the marrow of the bones.