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

During egg-formation, energy and protein are deposited in the developing eggs 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 endogenous energy and nutrients used during egg-formation for 12 well-studied species 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 and nutrient requirements (of clutch and laying female) originates from direct food intake and/or from body reserves. Because energy and nutrients are also needed by the laying female, some large species breeding in cold regions deposit sufficient reserve that they can fast completely during egg-formation (like the Adelie Penguin) and even throughout incubation (like the Elder Somateria mollissima). However, almost all smaller species must forage for 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 in small species like passerines (4% and 40%). Therefore, the requirements for the clutch are much greater in larger than in smaller species, and egg-production is much more limited by protein than by energy. The effects of food supplementation on timing of laying (moderate advance), on number of eggs 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