Changes in water and ash contents in the body and calorific values of dry and wet body weight during nestling development of Pygoscelis antarctica and P. papus. were examinted. It was found that water content in the tissues of both species decreases from 85% to less than 65% whereas the content of mineral substances in dry body weight increases from 9<Vo to about 12%, at the time. Caloricity indices are high and increase during the development of nestlings. A particularly intense increase (from 0.8 kcal x g-1 to 2.2 kcal x g-1 ) characterizes calorific value of 1 gram of biomass.