The song thrush, the snail, and polymorphism

The Song Thrush, Turdus ericetorum, eats a variety of prey but the clear favourite is the Brown-lipped Snail, Cepaea nemoralis. The shell colour and patterning of the snail is highly variable; it may be brown, pink or yellow and either unbanded or with up to 1-5 dark brown bands.

NHM-DASS.77-2017, NHM-DASS.78-2017

In the early 1950s, scientists from Oxford investigated the relationship between the thrush and the colours of the snails they ate. The bird opens snails by smashing the shell against a hard object (often a stone) called the anvil. Thrushes are territorial and tend to use the same anvil, thus leaving a heap of discarded shells which can be collected for analysis. 

They found that in winter, when the woodland floor is covered by dead, brown leaves, more yellow shells were eaten whereas, in summer, with a green background, pink and brown shells were more frequently found at the anvils. They concluded that thrushes targeted yellow snails on the brown winter background and pink and brown shells on the green summer background. 

Song thrush using a stone as an anvil to smash open snails. NHM.171-2016

They supposed that the polymorphism (varied forms appearing in one species or individual) was stable because the protective camouflage (or crypsis) of the different morphs varied with season. They also found that pink, brown & unbanded shells were more common in woods whereas yellow and banded shells favoured more open habitats.  

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