Is a female cockatiel able to take care of a fertilized egg by herself?

Just the egg, possibly, but no to the hatched chick, as it takes two birds to raise a chick. Usually the hen does most of the sitting on the eggs, though many pairs will take turns doing it. Once the eggs hatch, the cock or male bird does the majority of the feeding.

It's a big job even for experienced breeder pairs. If something happens to the cock after mating, a hen might keep sitting on the eggs, but it would be better for the chicks to be handraised then if you really want them to survive. Both birds have to be mature enough to breed, and that's at about 16-18 months of age, not at 5-6 months when they first start feeling sexually active.

At that young an age, it's a great time to get them bonded in a pair. Usually a bird won't even want to make a nest and lay eggs without being in a pair. If your bird does lay eggs without a cock bird, then don't take the eggs away, but let her pretend to brood them so she won't be encouraged to lay even more eggs, which will deplete her ... more.

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