There are various species of thrush found in Europe that could plausibly be the kind imagined by Tolkien, with perhaps the most common being the song thrush Turdus philomelos and the mistle thrush Turdus viscivorus. Of these it is song thrushes that have the habit of knocking snail shells against rocks that was crucial to Bilbo's finding of the secret entrance to Erebor. The particular thrush seen by Bilbo, however, while it had the pale breast common in thrushes, also had unusually dark feathers (said to be 'nearly coal black' in The Hobbit 11, On the Doorstep). This is a feature that does not quite match typical Eurasian thrushes (which are more usually medium brown in colour) so perhaps the variety found in Middle-earth was of a particularly dark kind no longer found today.