Calcium availability limits breeding success of passerines on poor soils

1. The role of food in avian reproduction is generally studied from the perspective of the protein and energy demand of birds, This study provides the first experimental evidence that calcium availability may limit reproduction in wild birds as well. 2. Data are presented showing that a large proportion of great tits Parus major on calcium-poor soils in the Netherlands produce eggs with thin and porous shells and desert the clutch before hatching. About 10% of the females do not lay at all. 3. Free-living great tits were supplied with an additional calcium source, i.e. snail shells and chicken eggshells. This treatment reduced the number of females without eggs, the frequency of clutch desertion, the proportion of nests with defective eggshells and the proportion of non-hatched eggs in clutches that produced young. The calcium supplements did not affect clutch size or laying date. 4. We suggest that possible adaptations to a limited calcium supply are not yet evident because the low calcium availability is a recent phenomenon caused by 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 calcium-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 avian reproduction than is presently believed.