PHYSIOLOGICAL PREPAREDNESS. “Migratory disposition, i.e. a state of readiness for prolonged flights, comprises hypertrophy of flight muscles, fat deposition in the adipose tissue, integrated changes in enzyme activities involved in the energy metabolism, hyperphagia, dietary changes and the development of migratory activity.”
http://rspb.royalsocietypublishing.org/content/early/2011/02/11/rspb.2010.2567