Tennessee selected the mockingbird (genus Mimus polyglottos) as official state bird in 1933, selected from an election conducted by the Tennessee Ornithological Society. Northern Mockingbirds are related to the brown thrasher and the catbird.

Mockingbirds have extraordinary vocal abilities - they can sing up to 200 songs, including their own melodious songs, the songs of other birds, insect and amphibian sounds, even an occasional mechanical noise. The mockingbird is considered one of the finest songbirds in northern America.

The northern mockingbird is also the state bird of Florida, Arkansas, Texas, and Mississippi.

Source:

State Symbols and History: Tennessee Dept. of State
Tennessee Symbols: Tennessee Dept. of Education
All About Birds: Cornell Lab of Ornithology

Links:

Sound: A Brief Guide to Common Birds
State Birds - 50 states List
National Bird