Abstract:
Evaporative heat loss pathways vary among avian orders, but the
extent to which evaporative cooling capacity and heat tolerance vary
within orders remains unclear. We quantified the upper limits to
thermoregulation under extremely hot conditions in five Australian
passerines: yellow-plumed honeyeater (Lichenostomus ornatus;
∼17 g), spiny-cheeked honeyeater (Acanthagenys rufogularis;
∼42 g), chestnut-crowned babbler (Pomatostomus ruficeps; ∼52 g),
grey butcherbird (Cracticus torquatus; ∼86 g) and apostlebird
(Struthidea cinerea; ∼118 g). At air temperatures (Ta) exceeding
body temperature (Tb), all five species showed increases in Tb to
maximum values around 44–45°C, accompanied by rapid increases
in resting metabolic rate above clearly defined upper critical limits of
thermoneutrality and increases in evaporative water loss (EWL) to
levels equivalent to 670–860% of baseline rates at thermoneutral Ta.
Maximum cooling capacity, quantified as the fraction of metabolic
heat production dissipated evaporatively, ranged from 1.20 to 2.17,
consistent with the known range for passerines, and well below
the corresponding ranges for columbids and caprimulgids. Heat
tolerance limit (HTL, the maximum Ta tolerated) scaled positively with
body mass, varying from 46°C in yellow-plumed honeyeaters to 52°C
in a single apostlebird, but was lower than that of three southern
African ploceid passerines investigated previously. We argue this
difference is functionally linked to a smaller scope for increases in
EWL above baseline levels. Our data reiterate the reliance of
passerines in general on respiratory evaporative heat loss via
panting, but also reveal substantial within-order variation in heat
tolerance and evaporative cooling capacity.
