Many people are under the misconception that as long as the new bird is in a cage, or if they are seperated by a divider, they are safe from harm. This is not true. Birds WILL fight through cage bars!
In addition, loose birds should NEVER be allowed to climb on another cage while there is a bird inside. Birds are very territorial about their space and there is a good chance the bird inside the cage will bite toes, legs, or cause other serious injury to the "intruder" crawling around on their cage.
Here is what I recommend to introduce a new bird:
(1) Quarantine: The new bird should be taken to the vet for a health check and quarantined for a minimum of 30 days. Not only does this protect your existing bird(s) should a health issue arise, it also slowly acclimates the new bird to the environment with minimal stress to his immune system.
(2) The new bird should be housed in a seperate cage in the same room as the existing bird. How close the new bird should be put to the existing bird's cage depends on the comfort level of the birds. Some birds may need to be kept on opposite sides of the room for weeks or even months, to get used to each other.
(3) Gradually move the cages closer together paying attention to both birds' body language. Be sure neither bird is scared or uncomfortable.
(4) When cages are placed side by side, they should be no closer than 3 inches apart. This way the birds can get a good look at each other, but there is enough space between them to avoid physical contact between the cage bars. Some cockatoos can reach quite far outside the cage (laugh) so keep that in mind as well!
(5) Once you see the birds are comfortable with each other and sitting near each other in their seperate cages, you can introduce them in a NEUTRAL area. A neutral area is an area that neither bird has a claim to. For example, you can take the birds into a spare bedroom and let them play on the bed while you supervise. Or put them on a new play area that neither bird has been on before. Watch carefully for signs of aggression. Pinning eyes, raised feathers, lowered head, slightly open beak and slow movements (stalking) are signs of trouble. Have a towel in your hand so you can quickly remove a bird from a bad situation if needed. It is so important that all introductions be positive. If one bird scares or injures the other, the relationship often cannot be repaired after that.
(6) After you see the birds are getting along in a neutral area, you can introduce them to each other in their normal environment. All interactions should be supervised closely. Again, have that towel ready just in case!
(7) If you plan on housing the birds in the same cage, It is best to have a new, larger cage that neither bird has been in. Short supervised interactions in the new cage should be done at first, with both birds then going back into their own cages until you know they are truly friends and able to be trusted with each other. The cage should be large enough to house two birds comfortably. Two birds in a cramped cage is trouble waiting to happen.
For the majority of pet birds, I recommend they be housed in seperate cages. This reduces the tendency for them to form a mate bond once the birds mature (even different species can form a mate bond). This also allows the owner to handle each bird seperately and have a relationship with each bird. This is especially important with two birds of the same species. There are some exceptions to this rule, as nothing is absolute with birds. But in general, this is a good guideline to follow.
Some birds just do not get along and will never be able to share the same space. Or they may get along on a neutral play area but become aggressive once inside a cage. My yellow nape despises other birds (particularly smaller species) and would cause serious injury if I allowed her contact with another bird. In fact, if I am holding another bird, I have to put that bird down out of sight, wait a few minutes then go pick up my amazon. Otherwise she will bite me!
One also needs to take account the size and behavioral differences between species. An amazon could do serious damage to small conure. Caiques and lories, tend to be aggressive towards smaller birds and should not be trusted unsupervised (some cannot be trusted at all with other birds). They also tend to be more high energy, and their play habits are diffferent. A quaker or grey for example, would not appreciate the high energy antics of a caique trying to jump on them and wrestle!
I hope the above tips were helpful and will possibly prevent injuries in the future. There are many other things to consider when introducing birds that are beyond the scope of this article. Should you need additional advice, you may contact me at firstname.lastname@example.org and we can set up a consultation.