• bird;
  • egg;
  • calcium;
  • acid;
  • snail;
  • blue tit;
  • Parsus caeruleus


The normal diet of most small passerines contains insufficient calcium for effective eggshell formation and birds must often forage specifically for calcium-rich items in the environment. It has been demonstrated that anthropogenic acid precipitation can significantly reduce levels of available calcium in soil and the abundance of calcareous items such as snail shells. Recent research in areas adversely affected by acid precipitation on Continental Europe has suggested that reduced calcium availability has resulted in an increased incidence of eggshell defects in small birds. In this paper we present data from one of the areas of the U.K. most severely affected by acid precipitation and where snail abundance (0.36 snails per m2) and exchangeable soil calcium levels (0.02 mg g−2) were very low: lower than values at sites in the Netherlands where severe eggshell defects occurred. The provision of supplementary calcium and the examination of eggshells, however, provided no evidence that blue tits Parus caeruleus were constrained in their ability to form effective eggshells as a result of calcium deficiency. We suggest that other factors, besides low calcium availability, may be contributory to the high incidence of eggshell defects seen in other areas.