Maggie's Farm

I've lived in my present home for 20yrs. Every year when the robins returned they set up home over a window at the rear of the house. I always tried to discourage them, but they always won out, so we had bugs and a steady stream of robin crap rolling down the window. Last spring I was in the process of putting an extention on the back of the house, covering up their favorite nesting spot. During their nesting period the construction was in the framing stage and I continually had to climb up into the roof trusses to pull their nests down. We didn't want to close in the nesting birds. This year the extention is completed and the robins are still trying to set up a nest on my porch lights. Every morning, at least twice, I go out and pull down a half completed nest.
What is this powerful force that makes the robins return to the same nesting areas year after year? Is it instinctual or genetic? These are many generations of birds over 20 years. I guess only God knows because he designed them.