1911 guy is on the right track. They are bred to be treeing dogs - typically used for squirrel.
If you've never hunted squirrel this way, I'd suggest you try it, as it's a lot of fun. My dad and I had a squirrel dog when I was a kid, and I have two tree dogs now. Kids love it.
Most people who hunt squirrel with dogs nowadays are divided into two camps - Feist and Cur. The two types of dogs are a lot alike, although feists may have a greater diversity of color, size and general appearance. Feist are smaller than curs, and do not range as far when hunting.
Both types of dogs are bred from mixed breed dogs from the mountains - hence Mountain Feist and Mountain Cur - and both were used to tree or bay game (not birds) usually on their own, as a multi-purpose farm dog. Poor people could not afford to own a pack of hounds.
Lots of people use curs and occasionally larger feists to coon hunt too... (I do) and they differ from coon hounds in that coon hounds will pick up a trail and methodically work it out, and eventually close on the coon and tree it. They are large dogs, and the process could take hours and cover miles. Cur / Feist are ADHD types by comparison, they are looking for some quick action. They will usually not hunt with nose to ground, but will move fast, and when they cross a hot trail they can wind, they press it. A hard running coon that won't tree quick will usually lose a cur on it's trail, but on the other hand, Curs work a trail fast, and put too much pressure on the coon. So they are sort of opportunistic dogs - in coon hunting anyway. If conditions are right they can excel at it, but if not, hounds would be better.
Squirrels go to tree much quicker, and so Curs / Feists make great squirrel hunters.
Both breeds can be very smart dogs. I've seen a top squirrel dog that retrieved doves too. Mine could care less about birds.
They don't mind lighting out after a rabbit once in a while, but since rabbits don't bay or climb trees, they give up on this quickly.