Evaporative Cooling in the Poor-Will and the Tawny Frogmouth

Abstract

The Poor-will is able to maintain a normal level of body temperature at an ambient temperature of 38[degree] C without gular flutter and with mouth closed. When environmental temperature exceeds 39[degree] C, gular flutter begins, and the amount of time spent fluttering increases with increasing temperature. The frequency of gular flutter is independent of heat load, and the movements are apparently driven at a rate determined by the natural resonating frequency of the gular complex. Supporting evidence for this assumption is offered by the fact that adding weights to the gular region leaves the rate of flutter unchanged but causes its amplitude to decrease. Poor-wills adjust rate of evaporative water loss to heat load by varying the amount of time spent fluttering, the amplitude of excursion of the gular region, and the area involved. All 3 parameters increase with increasing heat loads. Heart rate increases during periods of flutter, and there is a conspicuous engorgement of the blood vessels in the buccal region. The gular area may be maintained 3[degree] C below body temperatures, and as much as 9[degree] C below air temperatures, clearly demonstrating the importance of this region as a site of heat loss. The breathing rate of a 675-g Tawny Frogmouth increased linearly with increasing body temperatures as the bird was subjected to heat. Breathing rates varied from 22/min. at a body temperature of 37.6[degree] C to 100/min. at a body temperature of 42.5[degree] C. As body temperature increased, the frogmouth opened its mouth, lowered the gular area, increased the flow of blood to the buccal area, and increased air flow across the moist buccal surfaces by increasing the amplutude as well as the rate of breathing.