Vogeleieren en slakken

Bird eggs and snails. This contribution gives a review of the Ph.D. thesis by J. Graveland (1995). Graveland studied the cause of the poor breeding succes due to eggshell deficits in especially the great tit (Parus major) in forests on calcium-poor solis in the Netherlands. Land snails turned out to be the main calcium source for this bird. Eggshell deficits appeared to be correlated with a low abundance of land snails in such forests. In two forests on poor solis both the abundance and diversity of land snails had declined dramatically in less than three decades, whereas in forests on calcium-rich solis the abundance remained unchanched over this period, and the diversity even increased. The decline in calcium-poor forests. is most likely brought about by acidification of the soil as the result of 'acid rain'. In some forests where artificial liming was applied, the land snail abundance increased significantly. Graveland's study shows the eminent importance of regularly monitoring of land snails in an array of habitats, both quantitatively and qualitatively.