Birds will go there to stay warm, die from Co and fall in and it could
block the chimney. I had a racoon living in my fireplace chimney and
have pulled out 3 live birds, once 2 live pigeons, and many dead ones.
It doesn`t take much on some chimneys to block or reduce airflow
allowing a Co buildup inside fron reduced draw. Old chimneys can be
partialy blocked anyway from years of ofd deteriorating mortar. A cap to
keep out water with screen is needed. Water entering speeds up