Meeting Abstract
In the Sonoran Desert, where diurnal surface temperatures can reach 70°C and air temperatures can reach 50°C, it is imperative for birds to defend body temperatures from lethal hyperthermia. Nocturnal activity buffers nightjars and owls against some temperature extremes. When environmental temperatures exceed body temperature, evaporative cooling is essential but carries the risk of dehydration. During the breeding season, Lesser Nighthawks (Chordeiles acutipennis) and Common Poorwills (Phalaenoptilus nuttallii) nest on the soil surface and must cope with very high diurnal temperatures, while Western Screech-owls (Megascops kennicottii) and Elf Owls (Micrathene whitneyi) use cavities in trees and cacti for nesting. As a consequence, owls and nightjars are subjected to very different diurnal heat loads, which affects their costs of thermoregulation and ability to balance their water budgets. More frequent and severe heat waves will raise nocturnal temperature minima and may affect activity periods or costs. An understanding of the thermoregulatory capacity across species may offer some prediction of their ability to adapt to changes in climate. We measured resting metabolic rates, body temperature, rates of evaporative water loss and thermal tolerance using flow-through respirometry. Nightjars and owls show some similarities as well as some differences in their capacities for thermoregulation in the heat. Overall, nightjars tolerate air temperatures as high as 64°C and owls, in contrast, showed thermal tolerances more similar to passerine birds (ca. 50°C).
