Changes in the anatomy and microbiology of the gut of many galliforme birds are influenced by the amount of crude fiber in the diet; the response is biphasic. As crude fiber in the diet increases, the bird compensates for the dilution of dietary energy content by increasing the amount it eats, which causes an increase in gizzard size. After exceeding a threshold in fiber content, where the metabolizable energy intake is limited by the sheer bulk of the food consumed relative to the ability of the bird to ingest food, there is an increase in hindgut size and a change in the microflora that allows extraction of energy from cellulose through the production of volatile fatty acids (VFAs). The result is a change in the apparent digestibility of the diet. Altering the energy demands through the application of environmental stressors, such as lowered temperature, may alter a hypothalamic or other regulatory mechanism setpoint at which hindgut hypertrophy occurs. The time frame required for such changes is between 4 and 8 weeks. Studies to determine digestibility of nutrients or the response of the gut of birds to altered diets must allow a sufficient period of time for gut adaptation prior to assessing its function. Other factors, such as age, photoperiod, and environmental factors, may affect the response of the gut.