They may have a special ability involving sonic, electric, or water attacks.
Real World References
Thunderbird was an automobile released by the Ford motor company in seven different models between 1955 and 2005.
Thunderbirds are also a tv show, with puppets and amazing vehicles. As no artwork of them has yet been shown, they could hypothetically be styled after them as with other units and things in Erfworld (the blimps for example)
The Thunderbird is a significant supernatural creature in the real-world Amerindian mythos, and similar creatures appear in Maori legends.
Thunderbird is also an alcoholic drink of dubious reputation.
Thunderbird is also the free emailclient made by Mozilla
Being 'birds' they fit the theme of Transylvito units being slang terms for women (along with Goyles, Dolls, and Molls.)