Well, this question really depends on how large your cage is. If you have a very, very large cage, maybe you should fit about five at the MOST. You don't want to overcrowd them. This leads to fighting, and even in the worst case-death. In a medium/large cage, you should put two, at the most.
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Cockatiels do not need anything to have a mate. Just put him/her in the cage with your cockatiel, however, if you want them to mate, they will not mate if you do not have a nesting box. In order to breed cockatiels, you will need a nesting box.
Answer: Sure, they can. When cockatiels are white, they are either of the Lutino breed, or they are albino.
You can, but it really depends on the personality of the breeding pair. Sometimes, the breeding pair will be so territorial that they will not breed with other cockatiels in close proximity, and will toss the eggs out of the nesting box. Others have no problems at all, and several pairs of cockatiels will be breeding at the same time. It is important to make sure the aviary is large enough to accommodate more birds, and that each of the breeding pairs has sufficianet privacy, for example, placing nesting boxes at opposite ends of the cage.
To sell cockatiels, you need to breed them first. If you have a permit for breeding, you can. If you have a bunch that you didn't breed, than you might be able to, I'm not sure.
Cockatiels are very territorial and are okay either by themselves or one other but I don't know about seven. For one cockatiel you at least need a cage were it can stretch out it's wings and fly around in so for seven, get an aviary cage.
Not necessarily. You can breed them in any cage.
I'm pretty sure that cockatiels should be about 18-24 months when they start to breed. In other words, cockatiels mate after they age about a year or two.
Yes. They will breed best in a flight cage, a cage big enough for them to fly from perch to perch.
Cockatiels do not become aggressive unless they are treated badly.
Cockatiels eat paper to keep themselves entertained. Usually they don't actually eat it, they just tear it up and chew on it. If you are uncomfortable with your cockatiels eating paper, you can try to distract them from the habit. Putting more toys in the cage might help.
No it's fine to feed cockatiels in their cage. I feed mine in their cage as they will not eat out of my hand. Just make sure you clean their cage thoroughly every day, including perches, dishes, toys etc. Also make sure your birds have vegetables and fruit to eat aswell as seeds or pellets to keep them healthy.
to spread there wings often when they don't get to fly around or don't have a big enough cage.