John D. Rateliff offers us a lovely glimpse at just exactly what type of thrush the Thrush was which so nobly helped Thorin & Co.:
It is entirely characteristic of Tolkien that, even though he nowhere identifies the specific type of thrush that Bilbo, Bard, and the dwarves encounter, nonetheless he provides enough details of its appearance and behavior to make that identification certain. Out of the many thrush species native to England, the Lonely Mountain thrush is clearly a song thrush (T. philomelos),(15) a species particularly noted for its diet of snails and its habit of crushing their shells on a rock. Many song thrushes in fact choose a favorite rock as their ‘snail anvil’ and return to it again and again, making the clue on Thror’s Map a plausible application of real-world avian behavior to the fantasy story. Song thrushes are also, as the name suggests, noted singers, whose voices can carry a half-mile, and often hold their head to one side as if listening (possibly in fact listening for prey such as earthworms beneath the soil).
(15) For those unfamiliar with birds who wouldn’t know a thrush from a warbler, suffice it to say that the song thrush is about the same size as the (American) robin, a fellow thrush (T. migratorius),† and has much the same habits, except that it prefers snails to earthworms. Despite their particular association with snails, song thrushes are omnivores and also eat worms, bugs, and berries as available. I am grateful to Yvette Waters and especially Jacki Bricker for help in identifying the particular species of thrush Tolkien based his Lonely Mountain thrush upon. † not to be confused with the English robin, which is a different bird altogether, and only about the size of a sparrow.
[John D. Rateliff, The History of The Hobbit, Second Phase, XI, ‘The Lonely Mountain’, ii ‘The Thrush’]
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