Birds Love Chimneys!
I really wish birds didn't love chimneys so much. I mean, it makes sense why they do. They have protection from the wind and from predators (larger birds, etc). They are safe....until you light a fire. Usually (thankfully) the birds will get out safely as soon as that smoke starts going up the chimney.
Birds nesting in chimneys is such a pain for homeowners and for us. Often we have no idea there is a nest in the chimney until it is professionally cleaned or a fire is lit. Often you can light a fire and not even know there is a nest burning up inside the chimney.