Bird nests for a particular species of bird are always having the same shape and size. This is true for simple or complex types of nests. Examining the nests closely, I can't help but became fascinated with the fact that the birds actually weave their own "brand" of nests just like village folks traditionally weaving screw-pine leaves to make mats. This ability is inherited from one generation to another.
Thinking of it for a while, questions such as can you actually imagine how and when the birds learn the trade of making their own characteristic nests, can easily come to the fore. Do they learn to make it from their parents or it is an instinct? If it is an instinct, how is the instinct formed within the mind/brain of the bird? The simplest way to answer this question id to believe that the ability to make the nest comes as a package with the creation of the bird species itself.