I usually don't even end up spelling it correctly, let alone pronounce it correctly. My spelling will range from anything from cucco to cucoo to cuccoo to coccu to every other combination of the letters c, o, and u. Frankly, my pronunciations vary just as much, from kuh-koo to koo-koo to kuh-koh to koh-koo and every other combination of kohs and kuhs and koos under the sun.
I'm probably not helping much...
Based on the spelling it seems like it's supposed to be kuh-koh, but I hear kuh-koo far more often. Frankly, it doesn't matter terribly much since you can always say: "cucco, that crazy flock of maniacally homicidal deathbirds that appear out of everywhere and nowhere all at once". People will know what you're talking about for sure with a description like that.
...but for simplicity's sake, I just go with kuh-koo most of the time.