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Résumé :
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Recent studies have demonstrated that wild birds breeding in acidified areas may have difficulties in obtaining sufficient calcium for their eggshells, and that a shortage of land snails may be the cause. Here we present experimental evidence that calcium deficiency may affect reproductive traits also in birds breeding in non-acidified but naturally base-poor forest habitats. The study was conducted in Estonia in 1995 and 1996. We compared egg characteristics and other life-history traits of the Pied Flycatcher Ficedula hypoleuca and the Great Tit Pants major in two habitats differing in natural availability of calcium (base-poor pine forests and base-rich deciduous forests). In both species, egg volume was smaller in the poor than the rich habitat. Eggshell thickness also tended to be less in the poor habitat in 1996, but the difference was significant only in the Pied Flycatcher. An experimental supplementation of calcium was performed in the habitats to determine to what extent differences between rich and poor habitats might be caused by differences in calcium availability. Egg volume and shell thickness were positively affected by calcium supplementation in the Pied Flycatcher. The same tendency was observed in the Great Tit in 1996, although not significant. However, there was no effect of calcium provisioning on hatching success, nest desertion rate and number or quality of fledglings. Our results show that calcium limitation may occur in wild birds on naturally calcium-poor soils. Birds breeding in such habitats may be vulnerable to possible increases in acid deposition.
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