discouraging breeding

I recently adopted two cockatiels to join my flock, I already had one male cockatiel and the previous owners believed these two to be male as well. However after they had settled in I was confident from their body language that I'd adopted a male and a female. Today I spotted him trying to mate...

A friend of mine has just inherited two Parakeets - a bonded pair (male and female). She was told that the 'keets are about one year old. Both ceres are dark in their respective colors (very blue and pinkish brown). She has them in a 30x18x18" flight cage with food, water, and 3-4 toys. That's...