In late summer and fall, there may be confusion about whether observers are seeing bats or Chimney Swifts coming or going from a chimney. Typically, swifts will be seen swirling near and then entering a chimney near dusk to roost for the night. Bats would be leaving their roost, rather than entering it at this time of day. While on rare occasions bats can use chimneys to roost in during summer months, we don’t often see them in large numbers during the Chimney Swift migration period because they are centered around their hibernacula (overwintering locations).

The typical Chimney Swift fall migration in the state runs from mid-August – mid-October. During this time, they congregate in large numbers in chimneys throughout Wisconsin in both rural and urban areas. They roost in these chimneys at night and spend the daytime hours exclusively on the wing foraging for insects. The numbers of roosting Chimney Swifts can vary nightly. Some roosts have small numbers, but others can have several hundred to a thousand or more individuals!

When trying to differentiate these two, here are some important clues to keep in mind:

Clue	Bats	Chimney Swifts
Air space	Bats tend to fly much lower in the air than Chimney Swifts do. Think of the bats you may have seen fluttering low over your backyard searching for insects.	Swifts will fly higher, up in the upper portions of the air column while they hunt for insects.
Sound	Bats utilize echolocation that is largely inaudible to the human ear but may produce some ‘clicks’ or ‘chirps’ that can barely be heard.	Chimney Swifts make a distinct audible twittering sound as they fly above either foraging for insects or swirling as they prepare to enter a chimney for the night.
Timing of Activity	Bats can be active before sunset, they are usually most active 20-30+ minutes after sunset. Bats that may be roosting in or around chimneys will typically leave the area when they emerge in the evening.	Swifts “swarm” as they return and descend into their chimney roost for the night at or prior to sunset.
Flight/Appearance	Bats have a fluttery, sometimes erratic flight. As well, bats cannot soar, or glide well and typically need to flap their wings to stay aloft	Chimney Swifts are distinctive flyers and have a unique wing appearance. Swifts have uniformly long, skinny wings in a curved boomerang shape and coming to point. They fly with short, stiff wingbeats that are very recognizable

 

 

 

More Information on Nesting or Roosting Swifts in Your Chimney

The birds cling to the sides of the chimney and huddle in there for warmth during this cooler migration period. They will eventually depart in a few months for South America, so be patient if you can!

These same chimneys can potentially be used as nesting sites too, during the summer months. In those cases, it would only be one pair utilizing the chimney. They do not nest communally. We recommend folks close their damper (which they typically do during summer anyway) if they suspect they have nesting swifts. This works well for the birds as it creates a safe and secure nesting location while keeping them out of the dwelling when the homeowner wouldn’t usually be using their fireplace or chimney.

Information on Bats and Chimneys

It is unusual for bats to roost inside chimneys because they cannot fly vertically up to emerge. Usually if bats are found near chimneys, they are roosting under flashing, around the chimney cap, or between the chimney and the side of the building. Rarely bats may roost between bricks if the chimney is deteriorating and cracks form.

Additional Resources

Watch videos of Chimney Swifts swarming and entering chimneys from Swift Night Out:
https://www.wiswifts.org/videos-from-swift-night-out-events/