Do acid rain and calcium supply limit eggshell formation for blue tits (Parus caeruleus) in the UK?

The normal diet of most small passerines contains insufficient calcium for effective eggshell formation and birds must often forage specifically for c alcium-rich items in the environment. It has been demonstrated that anthrop ogenic acid precipitation can significantly reduce levels of available calc ium in soil and the abundance of calcareous items such as snail shells. Rec ent research in areas adversely affected by acid precipitation on Continent al Europe has suggested that reduced calcium availability has resulted in a n 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 ac id precipitation and where snail abundance (0.36 snails per m(2)) and excha ngeable soil calcium levels (0.02 mg g(-2)) were very low: lower than value s at sites in the Netherlands where severe eggshell defects occurred. The p rovision of supplementary calcium and the examination of eggshells, however , provided no evidence that blue tits Parus caeruleus were constrained in t heir ability to form effective eggshells as a result of calcium deficiency. We suggest that other factors, besides low calcium availability, may be co ntributory to the high incidence of eggshell defects seen in other areas.