Using
operant conditioning and a progressive ratio schedule of food reward, it was found that,
from 8 to 20 weeks, motivation to eat in R and 2R birds was as great after their daily meal
as before it, and was 3.6 and 1.9 times greater than that of maximally (72 h) deprived AL
birds. Another measure of feeding motivation, rate of eating, produced a similar index of
hunger in 2R birds, but not in R birds, whose food intake was probably constrained by
smaller crop capacity.