May 2018: Nature Watch

April 29 2018

We make no secret of the fact that it was the garden that sold our house to us.

 Having turned a blind eye to the shoddy Victorian construction, just imagine our delight when, shortly after moving in, we found a thrush’s anvil under the apple tree! This unusual avian tool consists of a large stone (or sometimes the sharp edge of a wall) against which a song thrush breaks open the shells of snails to get at the soft morsel inside. 

Song thrushes prefer banded snails. Just two species occur locally yet they have a great range of toothsome colours including yellows, browns, greens, purples, pinks and oranges, often with one or more coloured bands following the spiral neatly around the shell. The resulting pretty litter of glossy, round, colourful objects by the thrush’s anvil looks like a minor accident in a sweet shop. 

This amount of natural variation is very rare, even amongst the gastropods which are a notably variable group (including, for instance, the gem-like seaside periwinkles that come in many varieties including red, orange and yellow). The vast range of banded snail shell design is hard to explain. After all, if their shells are cryptic (to provide camouflage) then why does the most effective pattern not prevail? Well, it has been shown that different patterns act as effective disguise in different circumstances. Also that the variation helps fool predators: the song thrush might fix on one single search image, perhaps a pink shell with a brown stripe, but fail to spot the other variants (just as tasty). But these factors must also be true of the camouflage of many other prey species, the brown garden snail for instance, which are rather uniform in design.

Come this time of year I know that many of us will be cursing the snail. Lettuces are reduced to rags and new treasures (even those marked as “snail-resistant” by the garden centre) can disappear over-night  - shining trails reveal the culprits. Unfortunately for the gardener our area is really good for snails! We have a damp climate (have you noticed?) and much of Bristol has a limey soil which snails need for making their shells. The larger gardens also suit them as they can always find cover in an old wall or shrubbery. In fact, the banded snails, though numerous in our area, do very little damage to most garden plants: their favourite foods include nettle and ragwort and they do not much trouble the gardener. They also have a taste for algae and will climb surprisingly high up our walnut tree, patiently cleaning the smooth bark with their toothed tongues.

So when you do see them you can celebrate them whole-heartedly for their beautiful shells, for the fact that they tempt the soulful song thrush into our gardens and for the fact that their wildly varied appearance is still an ecological mystery. It’s a wonderful world!