The gular sack of nightjars helps to dissipate heat efficiently by vibrating.
Image: Alan Wilson /

Common Nighthawk, Benson Pond, Malheur National Wildlife Refuge, Oregon. 2008.

Image: Emily Harrington /

The gular sac is rapidly expanded to increase the speed of air moving through the sac and buccal cavity. As the fast air passes, heat from the blood vessels close to the surface (convective heat loss) and also through the moist membranes (evaporative heat loss). Created by Emily Harrington of eh illustration, http://www.ehillustration.com. This work shall be and remain at all times the shared property of Emily Harrington and the Biomimicry 3.8 Institute, and Emily Harrington grants to the Biomimicry 3.8 Institute a non‑terminating, nonexclusive, non‑limiting right to use the materials for educational purposes. Any use by outside parties requires permission from Emily Harrington and can be requested from [email protected] or through the Biomimicry 3.8 Institute.

“An important environmental for many caprimulgiformes is the ability to withstand high ambient temperature (Ta). Birds of this order are most common in warm climates, and frogmouths, potoos, and nightjars all roost and nest in the open where they can be subjected to long periods of direct sun exposure. In these circumstances, they avoid hyperthermia by using evaporative cooling strategies. Nightjars dissipate heat by gular fluttering, during which the mouth is opened, the rate of blood flow to the buccal area is increased, and the moist gular area is rapidly vibrated.” (Fowler and Miller 2003: 225)

“When poorwills are exposed to high temperatures, they increase evaporation of water by initiation of gular flutter and by some increase in breathing rate. Gular flutter supplements evaporation due to respiration, and involves a rapid vibration of the moist membranes of the gular region, driven by the hyoid. The rate of gular flutter in the poorwill is relatively constant and independent of heat load, and evaporation due to flutter is modulated by varying the amount of time spent fluttering, as well as the amount of air moved per flutter.” (Lasiewski 1969:1504)

Watch Video (doesn’t show gular fluttering, but beautiful!)

Watch Video (gular fluttering of a heron chick)


Gular flutter cooling mechanism. The gular sac is rapidly expanded to increase the speed of air moving through the sac and buccal cavity. As the fast air passes, heat moves from the blood vessels close to the surface (convective heat loss) and also through the moist membranes (evaporative heat loss). Artist: Emily Harrington. Copyright: All rights reserved. See gallery for details.

Last Updated October 14, 2016