FWIW, I recently attempted shooting a stray tom-cat with CCI CB Longs. They are incredibly quiet out of my bolt action remmy--I think the colibri's are even more quiet--but I haven't done a side by side comparison.
At any rate, I nailed the tomcat in the "forhead" at ~20 yards while it laid in the grass. It did a backflip, let out a wait, and ran off. I saw it about 6 hours later back on the farm...didn't appear to be injured at all. My guess is the soft conical ball of the CB LONG simply deflected off it's skull and gave it a bit of a headache.
The CB shorts and CB longs both have a velocity of 710fps with a 29 grain bullet. Most air rifles are shooting pellets ~1/2 that mass. I have shot several cats with my 750fps air rifle...most of which are still running around, but I know of a few that didn't survive. I've quit using the air rifle for cats--cats are fast, and like to run when hit...they'll make it home.
I'll agree with using a good sub-sonic (900-1,000fps) with a 36-40 grain bullet.
I've used aguila super hyper maximum velocity's on cats and rats, they have a velocity of 1,750 fps and do wonders...but my group size almost doubles due to an inablility to stabalize.