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Abstract
Passerine birds need extra calcium during their breeding for developing egg shells
and proper growth of nestling skeleton. Land snails are an important calcium source
for many passerines and human-induced changes in snail populations may pose a severe
problem for breeding birds. We studied from the bird's viewpoint how air pollution
affects the shell mass, abundance and diversity of land snail communities along a
pollution gradient of a copper smelter. We sampled remnant snail shells from the nests
of an insectivorous passerine, the pied flycatcher, Ficedula hypoleuca, to find out
how the availability of land snails varies along the pollution gradient. The total
snail shell mass increased towards the pollution source but declined abruptly in the
vicinity of the smelter. This spatial variation in shell mass was evident also within
a single snail species and could not be wholly explained by spatially varying snail
numbers or species composition. Instead, the total shell mass was related to their
shell size, individuals being largest at the moderately polluted areas. Smaller shell
size suggests inferior growth of snails in the most heavily polluted area. Our study
shows that pollution affects the diversity, abundance (available shell mass) and individual
quality of land snails, posing reproductive problems for birds that rely on snails
as calcium sources during breeding. There are probably both direct pollution-related
(heavy metal and calcium levels) and indirect (habitat change) effects behind the
observed changes in snail populations.
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