Before this continent was settled, however, they nested in hollow trees, which were everywhere. Trees such as sweetgum, sycamore and basswood especially, since these trees, when they die naturally, die from the inside out, and naturally hollow out.
These days trees are chain-sawed down, and most homes have their chimneys capped. Many chimneys are made of sheet metal on the inside. (Chimney Swifts can't get a grip) As a result, the species is declining; a shame because it's one of the most aerobatic fliers around!
Some people have built artificial chimneys to attract them (see pic); both for their in-flight beauty and their insect-decimating appetite.
My what big eyes you have! Birds have huge eyes in comparison to their heads. Far more than humans! The relatively large pupils let them see in much lower light than humans. That's a big reason why birds are especially active in low light (like dusk and dawn). Owls, Nighthawks, Swifts, Cardinals and Whipoorwills have the largest eyes of birds in north Texas, and can see in the least light.