CALCIUM AVAILABILITY LIMITS BREEDING SUCCESS OF PASSERINES ON POOR SOILS

1. The role of food in avian reproduction is generally studied from th e perspective of the protein and energy demand of birds, This study pr ovides the first experimental evidence that calcium availability may l imit reproduction in wild birds as well. 2. Data are presented showing that a large proportion of great tits Parus major on calcium-poor soi ls in the Netherlands produce eggs with thin and porous shells and des ert the clutch before hatching. About 10% of the females do not lay at all. 3. Free-living great tits were supplied with an additional calci um source, i.e. snail shells and chicken eggshells. This treatment red uced the number of females without eggs, the frequency of clutch deser tion, the proportion of nests with defective eggshells and the proport ion of non-hatched eggs in clutches that produced young. The calcium s upplements did not affect clutch size or laying date. 4. We suggest th at possible adaptations to a limited calcium supply are not yet eviden t because the low calcium availability is a recent phenomenon caused b y acid deposition and because a large part of the breeding population consists of immigrants from calcium-rich areas. 5. We provide evidence that calcium limitation in avian reproduction may be widespread on ca lcium-poor soils. 6. The results imply that the costs of egg formation in calcium-poor areas can be much higher than is currently estimated and that food conditions during egg-laying have a greater impact on av ian reproduction than is presently believed.