We examined to what extent insectivorous and granivorous passerines can obtain the calcium for shell formation and growth of the skeleton from their normal food, and to what extent they depend on additional calcium-rich material, such as snail shells and calcareous grit. We determined the calcium content of forest arthropods and seeds on Ca-rich and Ca-poor soils and the calcium content of females, eggs and nestlings of the Great Tit, Parus major, and calculated the calcium budget for females and nestlings. We found that female Great Tits do not store calcium in their skeleton before egg-laying. Thus, all calcium needed for egg-laying must be collected during the laying period. Even on Ca-rich soils the calcium intake from arthropods or seeds covered only 5–10% of the Ca requirements. It is demonstrated that these results also apply to other passerines, with the exception of a few species that include woodlice (Isopoda) and millipedes (Diplopoda) in their diet. These arthropods contain 10–13% (dry matter) calcium; a hundred times more than other arthropods. However, most passerines need Ca-rich material, in addition to their normal food, to meet their calcium demand.
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