A few weeks ago, my wife told me some birds were building a nest in the tail pipe of my pick up truck, so I went out and took a look and sure enough they were.
The reason being they had a baby bird fall out of their nest and being it was an extremely windy day, they could not do anything to help it,,,so i suspect they were trying to build a quick nest down low to the ground.
I told my wife and she grabbed a ladder and promptly returned the baby bird to it's nest and that ended the nest building in the exhaust pipe as well as all the other overly excited and stressed actions of the bird parents.
I fired up the truck later and blew out all the sticks and grass. LOL