Nest building

With everything modern science understands about evolution, DNA, and the general process of life, its still hard for me to grasp how such complex behavior as nest building can become hardwired into an entire genus of species, rather than being a learned behavior. And obviously this goes to all complexbehaviors. I guess its just hard to get into an instinctive mindset like that, and imagine what its like not having higher cognitive thought.

Who says we really have higher cognitive thought???

How exactly do we know what we do that is and isn't instinctual???

How do we know we aren't overthinking things???

How do I know I didn't just type up a bunch of BS???

I'm trying to do a little research on the development of nests (many mammals and reptiles also have some form of 'nesting' behavior. Give me a little time and I'll do my best to give a scientificey response. That said, biology really never was my thing.