Abstract:
We examined thermoregulatory performance in seven Sonoran
Desert passerine bird species varying in body mass from 10 to
70 g – lesser goldfinch, house finch, pyrrhuloxia, cactus wren,
northern cardinal, Abert’s towhee and curve-billed thrasher. Using
flow-through respirometry, we measured daytime resting metabolism,
evaporativewater loss and body temperature at air temperatures (Tair)
between 30 and 52°C. We found marked increases in resting
metabolism above the upper critical temperature (Tuc), which for six
of the seven species fell within a relatively narrow range (36.2–39.7°C),
but which was considerably higher in the largest species, the curvebilled
thrasher (42.6°C). Resting metabolism and evaporative water
loss were minimal below the Tuc and increased with Tair and body
mass to maximum values among species of 0.38–1.62 Wand 0.87–
4.02 g H2O h−1, respectively. Body temperature reached maximum
values ranging from 43.5 to 45.3°C. Evaporative cooling capacity, the
ratio of evaporative heat loss to metabolic heat production, reached
maximum values ranging from 1.39 to 2.06, consistent with known
values for passeriforms and much lower than values in taxa such as
columbiforms and caprimulgiforms. These maximum values occurred
at heat tolerance limits that did not scale with body mass among
species, but were ∼50°C for all species except the pyrrhuloxia and
Abert’s towhee (48°C). High metabolic costs associated with
respiratory evaporation appeared to drive the limited heat tolerance
in these desert passeriforms, compared with larger desert
columbiforms and galliforms that use metabolically more efficient
mechanisms of evaporative heat loss.
