Avian eggshell formation in calcium-rich and calcium-poor habitats: importance of snail shells and anthropogenic calcium sources

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Abstract
Most passerines depend on the intake of calcium-rich material in addition to their normal food for proper eggshell formation and skeletal growth. A large proportion of Great Tits (Parus major) in forests on nutrient-poor soils in the Netherlands produce eggs with defective shells as a result of calcium deficiency. Eggshell defects are much scarcer near human settlements and do not occur on nutrient-rich soils. I investigated this variation in eggshell quality by examining the use of calcium-rich material by the birds. The results show that calcium-rich items in nest material and droppings can be used as a measure of calcium consumption. Snail shells were the main calcium source in forests where eggshell defects did not occur. In forests where the tits exhibited calcium deficiency, snail shells were rarely taken and birds used anthropogenic calcium sources such as chicken grit and chicken eggshells. It was demonstrated that the dependence on snail shells and the use of alternatives such as anthropogenic calcium sources in areas where snails are scarce are general features of calcium intake among birds. Thus, calcium limitation may be a common phenomenon in avian reproduction on poor soils in countries less populous than the Netherlands.