Temperatures Inside the Chimney
What's the big attraction? Why do ten thousand small swifts seem compelled to stuff themselves in an old brick chimney for the evening? Here are some ideas.
1. If the chimney has a small enough opening, predators will not be able to "get" them. Vaux's Happening has documented a Cooper's Hawk going a short distance down the Monroe Wagner chimney attempting to snag a swift. Our security cameras have captured this event at least twice. A Barn Owl was seen flying into and catching swifts at the biggest holed chimney of the three Mcnear Brick and Block roosts. But once they get in the chimney and down a ways, as far as we know, the swifts don't need to worry about predators.
2. The joints between common bricks have just the right spacing, (2.25 inches) for the 4.5 inch long swifts to cling to while overlapping the swift hanging onto the next joint up.
3. Inside the chimney they will be out of the wind.
4. Inside the chimney they will be out of the rain. This is not a sure thing. The swifts will prefer the chimney wall that faces the direction the wind is blowing. The bird above them may also catch some precipitation headed their way. If the chimney has a taper, that may make it more difficult to cling to, but all walls could be out of the rain if there's no wind.
So inside the chimney the Vaux's Swifts are safe and dry and......WARM?