Mavis

The way this question is worded – including the use of the uppercase M – may give the impression that in the countryside, all song thrushes are called Mavis (in the same way that Rita's sidekick in Coronation Street was called Mavis).

In fact, the song thrush was known as "the mavis" in the same way that the mistle thrush was also known as the throstle. According to Wikipedia, the "first modern use" of the name was in the 1895 novel The Sorrows of Satan, by the English writer Marie Corelli. The name, according to the author, was "rather odd but suitable" for the character, Mavis Clare, as "she sings quite as sweetly as any thrush".

Wikipedia continues: The name was long obsolete by the 19th century, but known from its poetic use, as in Robert Burns's 1794 poem Ca' the Yowes ("Hark the mavis evening sang/Sounding Clouden's woods amang"); and in the popular love song Mary of Argyle (c.1850), where lyricist Charles Jefferys wrote, "I have heard the mavis singing its love–song to the morn."

Note the use of the definite article in both extracts.