Thank you! But now there is a problem.
My original plan, when the chicks were just born, was to keep a male with the parents because I knew that two males get along better than two females (even if it depends on the budgies' personalities).
However, all their chicks looked like females to me, so I chose to keep a female, Celeste (the sky blue opaline), who had slip claws on both her feet, to help her, and I gave the other three chicks away.
But now, the fact that Sid (the cock) is opaline and Stella (the hen) is "normal" means that all the female chicks should have been opaline and all the male chicks should have been "normal" (but split for opaline) since the opaline gene is carried by the Z (X) chromosome and is recessive.
This explains why Ambra, Celeste and Sunny (the light green opaline) are all opaline females.
But this also means that Oliva, the "normal" grey-green who I thought was olive green, is a male, right?
Oliva has a purple cere, but it is lighter around the nostrils, this is why I thought they were female.
But, apparently, they are not, and this explains why they sing a lot and probably why Stella used to only feed Oliva when the chicks were almost weaned.
Stella is a bit bossy toward Celeste, and Celeste is afraid of her, but she is sometimes bossy towards Sid who, in return, becomes bossy. So, should I bring Oliva back (I guess Stella will be less bossy toward a male, and a male will also be peaceful, not making Sid angry) and give Celeste to my grandparents (who currently own Oliva)?
I know odd numbers are not ideal in a budgie flock, and the best thing to do would be, I guess, bringing Oliva back and keeping Celeste, having two males and two females.
Unfortunately, I think their cage is already too small for three budgies, let alone four, it is 20 x 16 x 21/28 inches (it has two roofs) and even if I was able to tame them to let them out of the cage for a few hours every day it would still be too small, and I can't buy a bigger one for now.
So, I have to make a choice.
Thanks in advance!