Inter- and Intraclutch Variability in Heavy Metals and Selenium Levels in Audouin's Gull Eggs from the Ebro Delta, Spain

Abstract.

Heavy metal (Zn, Cu, Mn, Cd, Pb, Hg) and Se concentrations were analyzed in 57 Audouin's gull (Larus audouinii) eggs belonging to different clutch sizes. Inter- and intraclutch variability in metal concentrations was investigated as a potential source of bias in the assessment of pollution levels. Moreover, we analyzed the relationship between metal levels in the shell and in the contents, to evaluate the reliability of museum eggshells as indicators of historical changes of these pollutants. An outstanding female effect and/or a laying order effect underlies egg Hg levels; the fact that eggs in a clutch are not independent observations needs to be taken into account both when designing sampling strategies and when performing any analysis or interpretation of the results. The relationship between Hg in shells and contents is not sufficiently accurate to allow the use of egg-shell concentration as a reliable predictor of egg-contents concentration. However, if changes in the ecosystems are large enough it could be used to trace gross historical trends of these pollutants.