Egg size is a basic species-specific life-history trait in birds which influences offspring performance through the nestling stage and, therefore, fitness. Shells of snails are the principal source of calcium for female parids and some other passerines during egg formation. Shortages in the availability of snail shells may lead to laying smaller eggs with thinner shells. In urban green spaces that require protection against pests, the numbers of snails may become greatly reduced in response to molluscicide treatments. The use of a molluscicide treatment against pest slugs in the Botanic Garden, Łódź, Poland, in 2013 and 2014 provided a unique quasi-experimental setting to study the effect of molluscicide on both non-target snails and Blue Tit Cyanistes caeruleus eggs. Using a forest study site where no treatment was applied as a control area, we tested for effects of the molluscicide treatment in the Botanic Garden on the density of snails and on the egg volume of Blue Tits. We found that after the molluscicide treatment a dramatic drop in snail numbers occurred, which likely restricted the availability of snail shells for egg laying Blue Tits in the Botanic Garden. Correspondingly, egg sizes significantly decreased in the Garden, while they stayed constant over time in the forest. Eggs in bigger clutches were more strongly affected by the molluscicide treatment than eggs in smaller clutches. Our quasi-experimental study suggests that Blue Tits are sensitive to poor availability of snail shells. This is manifested by laying smaller eggs, which require less calcium to be incorporated in the eggshell, in response to a dramatic drop in the availability of snail shells. This result is expected to raise awareness of potential indirect and inadvertent effects of park pest management on biota other than molluscan pests.