Before you buy your air rifle, please check with your local police, etc, to see if you can legally shoot where you live. Some states and cities (New Jersey or Chicago, for example) have pretty severe regulations. Also, your local laws stack up on top of state laws. That is, if your state says OK and the country/city says NO WAY, it's NO WAY.
Pellet guns of any kind are much more powerful than airsoft. The only repeater which functions like a semi-auto airsoft that I know of is the Crosman 1077 which looks like a Ruger 10/22. It has a clip magazine and uses CO2. The velocity is kind of on the weak side, although it would do for small birds. I would not use it on squirrels. The two downsides of the 10/77 are 1) It is not too accurate due to the heavy trigger pull. (It is not a true semi-automatic; you cycle the gun by pulling the trigger.) 2) You can quickly go through CO2 cartridges which can be a bit expensive.
That said, the 10/77 can be a fun plinking gun and they don't cost an arm and a leg.