Abstract:
Birds in the order Caprimulgiformes (nightjars and allies) have a
remarkable capacity for thermoregulation over a wide range of
environmental temperatures, exhibiting pronounced heterothermy in
cool conditions and extreme heat tolerance at high environmental
temperatures. We measured thermoregulatory responses to acute
heat stress in three species of Caprimulgiformes that nest in areas of
extreme heat and aridity, the common poorwill (Phalaenoptilus
nuttallii: Caprimulgidae) and lesser nighthawk (Chordeiles
acutipennis: Caprimulgidae) in the Sonoran Desert of Arizona, and
the Australian owlet-nightjar (Aegotheles cristatus: Aegothelidae) in
the mallee woodlands of South Australia. We exposed wild-caught
birds to progressively increasing air temperatures (Ta) and measured
resting metabolic rate (RMR), evaporative water loss (EWL), body
temperature (Tb) and heat tolerance limit (HTL; the maximum Ta
reached). Comparatively low RMR values were observed in all
species (0.35, 0.36 and 0.40 Wfor the poorwill, nighthawk and owletnightjar,
respectively), with Tb approximating Ta at 40°C and mild
hyperthermia occurring as Ta reached the HTL. Nighthawks and
poorwills reached HTLs of 60 and 62°C, respectively, whereas the
owlet-nightjar had a HTL of 52°C. RMR increased gradually above
minima at Ta of 42, 42 and 35°C, and reached 1.7, 1.9 and 2.0 times
minimum resting values at HTLs in the poorwill, nighthawk and owletnightjar,
respectively. EWL increased rapidly and linearly as Ta
exceeded Tb and resulted in maximum rates of evaporative heat
dissipation equivalent to 237–424% of metabolic heat production.
Bouts of gular flutter resulted in large transient increases in evaporative heat loss (50–123%) accompanied by only small
increments in RMR (<5%). The cavity-nesting/roosting owletnightjar
had a lower HTL and less efficient evaporative cooling
compared with the species that nest and/or roost on open desert
surfaces. The high efficiency of gular flutter for evaporative cooling,
combined with mild hyperthermia, provides the physiological basis for
defending Tb well below Ta in extreme heat and is comparable to the
efficient cooling observed in arid-zone columbids in which cutaneous
EWL is the predominant cooling pathway.
