I doubt anyone here has ever suggested that all birds be kept in separate cages nor have I ever come across that information anywhere else on the www.
Birds kept as pets/companions are most often caged independently. If you are talking about birds meant to be breeding pairs, that's an entirely different thing.
So I'll repeat what has already been asked of you; what do you mean?
I have 3 indian ringneck one of them died due to temperature problem. 1 is 2.25 month old male and the other one approx 1 month old female. This is not first time I am dealing with these birds.
if you allow 2 birds in one cage, they heed each other instead of you. they pay little attention to u. male parrot spend more of his time to female parrot he thinks how to impress her how to get her attention etc.. if they dont heed u than how they gonna be train .
There are a number of reasons why people recommend separate cages for birds especially if they are male and female. Here a few.
1. If you don't want to breed birds don't put a male and female in the same cage.
2. If you want tame birds it is better to keep them in separate cages and to let them come out and play with them. Don't leave them in the cage because they aren't a pet that way they are just a bird in a cage. They need opportunity to come out and be part of the family/flock.
3. Usually people don't buy a big enough cage for one bird let alone two so space is a reason.
4. Unless your birds are bonded (get along with each other) they will need separate cages. More than likely they will fight.
I don't know that these are scientifically documented nor do I know how much data has been recorded but they are observations by many people.