Are eggshells and egg contents of Great and Blue Tits suitable as indicators of heavy metal pollution?

We examined whether eggs of the Great Tit (Parus major) could be used as indicators for lead, arsenic, cadmium, copper and zinc pollution. We collected eggs from two sites with different pollution levels, and measured heavy metal levels in the egg content and eggshell separately. At the polluted site, situated near a metallurgic factory in Hoboken (Belgium), eggshells contained significantly higher concentrations of arsenic, cadmium and lead than did eggshells at the reference site on the campus of the University of Antwerp. Egg contents also contained significantly higher concentrations of lead at the polluted site than at the reference site. Both at the polluted and reference site egg contents contained higher concentrations of zinc and copper than did the eggshell. The eggshell contained higher arsenic and lead concentrations than did the egg content at the polluted site but not at the reference site. There was a clear discrepancy between essential and non-essential elements. Copper and zinc, two essential elements, were highest in the egg content, while arsenic and lead were higher in the eggshell. Moreover, the concentrations of essential elements did not differ significantly between the two sites. We also collected eggs from Blue Tits (Parus caeruleus) at the polluted site, and compared metal levels between the eggshell and egg content. Differences between the shell and content were similar to those for the Great Tit. At the polluted site we found no significant differences in metal levels between the two species. Our study indicates that Great and Blue Tits sequester non-essential heavy metals in their eggs, especially in the eggshell. Therefore the eggshell is suitable as an indicator for heavy metal pollution.