Check with your provincial regulations, here in B.C they view it as handgun hunting and is prohibited, also co2 pistols are very weak power wise and even head shots wouldn't work well. If I was to take a grouse with a air pistol the best choice would be the pump up Crossman 1377 or .22 counterpart. They would have the power and accuracy to pull off effective head shots.
We have shot HUNDREDS of grouse and hares with CO2 and Pneumatic pellet pistols in .177, .20 and .22 cals… they are neither "weak" nor ineffective.