ARTHROPODS AND SEEDS ARE NOT SUFFICIENT AS CALCIUM SOURCES FOR SHELL FORMATION AND SKELETAL GROWTH IN PASSERINES

We examined to what extent insectivorous and granivorous passerines ca n obtain the calcium for shell formation and growth of the skeleton fr om their normal food, and to what extent they depend on additional cal cium-rich material, such as snail shells and calcareous grit. We deter mined the calcium content of forest arthropods and seeds on Ca-rich an d 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 ca lcium in their skeleton before egg-laying. Thus, all calcium needed fo r egg-laying must be collected during the laying period. Even on Ca-ri ch soils the calcium intake from arthropods or seeds covered only 5-10 % of the Ca requirements. It is demonstrated that these results also a pply to other passerines, with the exception of a few species that inc lude woodlice (Isopoda) and millipedes (Diplopoda) in their diet. Thes e 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.