The gular sack of nightjars helps to dissipate heat efficiently by vibrating.

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“An important environmental adaptation 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)

Zoo and Wild Animal MedicineME Fowler, RE Miller

“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)

Journal article
Physiological responses to heat stress in the poorwillAmerican Journal of PhysiologyNovember 1, 1979
RC Lasiewski

Sturkie's Avian Physiology, Sixth EditionSeptember 15, 2014

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Nightjars And AlliesCaprimulgidaeFamily

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