I have 2 cockatiel I was told I have a male and female. They have not been mating but one of them sits in the nest box all the time my question is why do they do that
Whether you want the birds to breed or not, I would take the box down for right now. If you don’t want them to breed, then they do not need a nest box. Birds only need a nest or nest box when they are actively breeding. The rest if the year it should be removed, or in the case of pet birds they do not need one at all.
If you want them to breed, you need to give them a chance to bond first. Signs of bonding are spending their time side by side, feeding each other, grooming each other and of course mating. You definitely want to replace the box when you see signs of bonding, especially if they do start to mate. It could be the male is immature as most likely it is the female sitting in the box. Both birds need to be ready when you set them up for breeding. Otherwise the female could end up laying infertile eggs and tending to them herself. When that happens, sometimes she will reject the male in favor of her eggs.
And just because you have a male and female does not mean they will successfully breed. Breeding birds is not easy and takes a lot of patience on the part of the owner. Also, the birds need to be a couple of years old before you start breeding them. While they might lay eggs at an earlier age, they are not emotionally mature enough to raise chicks. Birds tend to get sexually mature at too young of an age in captivity so it is up to you to make them wait until they are old enough, by not providing a nest box until then.
Thank you for asking Lafeber,