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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; similar to 17 g), spiny-cheeked honeyeater (Acanthagenys rufogularis; similar to 42 g), chestnut-crowned babbler (Pomatostomus ruficeps; similar to 52 g), grey butcherbird (Cracticus torquatus; similar to 86 g) and apostlebird (Struthidea cinerea; similar to 118 g). At air temperatures (T-a) exceeding body temperature (T-b), all five species showed increases in Tb to maximum values around 44-45 degrees 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 T-a. 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 T-a tolerated) scaled positively with body mass, varying from 46 degrees C in yellow-plumed honeyeaters to 52 degrees 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.