Is a protective shell formed around bird egg after fertilization?

Here's why:

* Shell Formation: The shell is formed in the oviduct, a tube in the female bird's reproductive system, as the egg travels down. It's made of calcium carbonate and other minerals.

* Fertilization: Fertilization occurs later, when the sperm from the male bird reaches the egg. This happens inside the oviduct, but after the shell has formed.

So, the shell acts as a protective barrier for the developing embryo, but it's not a result of fertilization itself.