Question

how the birds know to build the nest

Answer 1

In birds, it is all about wiring. They are “wired” for these things. A weaver bird will always weave a similar nest to its conspecifics because that behavior is “wired” into the bird’s genome, and gets “wired” into its brain when it is developing in the egg. So, really, they do not “know”, because it is already there, no more than a computer “knows” to start Windows when you turn it on.

Answer 2

Answer:

Birds are born with the skill and instinct to build their own nests.