Poor reproduction in forest passerines from decline of snail abundance on acidified soils

ON poor, acidified soils in The Netherlands, an increasing number of great tits, Parus major , and other forest passerines, produce eggs with thin and porous shells 1 . Here we show that the egg-shell defects, and the related high incidence of clutch desertion and empty nests, are caused by calcium deficiency, that snail shells are the main calcium source for the laying female, but that snails are scarce on poor soils. Similar laying irregularities in birds are reported from acidified regions elsewhere in Europe 2–4 . We provide evidence that snails declined by a decrease in soil calcium on poor soils. Acid deposition is the main cause for decreasing calcium levels in such soils 5–7 . To our knowledge, this is the first experimental evidence for calcium limitation in wild birds and it reveals a previously overlooked mechanism by which acidification affects higher trophic levels of the forest ecosystem.