the same species black headed, I don't think they will mate or anything, they seem to only do that under certain conditions. But the aggression is a factor I would keep them seperated unless they got along.
I don't really have an opinion on the trade as to if it is fair or not but...I am wondering if you plan on breeding or not? If I remember right, that was your goal a few weeks ago. Are you still planning on this?
There is no reason to think that the caiques will not breed or that you can control what they decide to do. You can hope that the conditions you keep them in will prevent them from breeding but if they have access to one another then their is no guarantee.
My biggest concern with keeping a pair of birds who may decide to breed (even if this is not your plan) is their health and whether they are related or not. Do you know the history of your caique or the other caique? Where did they come from? Are they banded? can you find out anything about them? Also, hopefully they will not breed if that is not the plan but if they do are you prepared to care for the babies? Can you find proper homes for any babies that might come along?
Normally, I wouldn't ask but since you have expressed an interest in breeding I think it is important that you carefully plan what you are doing.
While you do get a lot of comments urging you to reconsider I think it is because people here really care about birds (even ones we do not personally know) and we want to help you do what is best for them.
Perhaps, the sennies would have a wonderful home with your friend, perhaps that is why you are considering the trade. I do not know.
I do know though that is difficult for birds to move and adjust. It can be done but it is stressful and stress can lead to disease and honestly, birds can die of stress.
I think you are wise to take some time to think it over and figure out what you can handle, what you ultimately want, how much space you have and how many birds you can competently provide for. If there is any way for you to figure it out and then make adjustments as needed by moving as few birds as few times that will be best.
Breeding or keeping pets really do have very different set ups and requirements, both for you and for the birds and it is an invitation to disaster or at least a lot of heartache to confuse the two.
Best wishes!