Re-examination of the capital and income dichotomy in breeding birds

During egg-formation, energy and protein are deposited in the developing eg gs but are, at the same time, needed by the laying female herself. This has been largely overlooked in the discussion on income and capital breeders ( Drent & Daan 1980, Thomas 1988). We discuss data on exogenous versus endoge nous energy and nutrients used during egg-formation for 12 well-studied spe cies ranging from the Adelie Penguin Pygoscelis adelie (3400 g) to the Blue Tit Parus caeruleus (11 g) and calculate which part of the total energy an d nutrient requirements (of clutch and laying female) originates from direc t food intake and/or from body reserves. Because energy and nutrients are a lso needed by the laying female, some large species breeding in cold region s deposit sufficient reserve that they can fast completely during egg-forma tion (like the Adelie Penguin) and even throughout incubation (like the Eld er Somateria mollissima). However, almost all smaller species must forage f or most of their energy and nutrients during the egg-formation period. For the large species, energy and protein of the clutch represent 30% and 70%, respectively, of the total requirements of laying females, much more than i n small species like passerines (4% and 40%). Therefore, the requirements f or the clutch are much greater in larger than in smaller species, and egg-p roduction is much more limited by protein than by energy. The effects of fo od supplementation on timing of laying (moderate advance), on number of egg s laid (not more, when corrected for date) and on egg size (not larger) of income and capital breeders/layers are discussed. It seems that neither the start of egg-laying nor the number or quality of eggs are directly related to the energetics of the laying female.