Abstract:
Small birds inhabiting northern temperate and boreal latitudes
typically increase metabolic rates during cold winters or acclimation
to low air temperatures (Taccl). Recent studies suggest considerable
variation in patterns of seasonal metabolic acclimatization in birds
from subtropical and tropical regions with milder winters, but there
remains a dearth of acclimation studies investigating metabolic
flexibility among lower-latitude birds. We used short-term thermal
acclimation experiments to investigate phenotypic flexibility in basal
metabolic rate (BMR), thermoneutral evaporative water loss (EWL)
and summit metabolism (Msum) in three populations of white-browed
sparrow-weavers (Plocepasser mahali) along a climatic and aridity
gradient. We allocated individuals to one of three Taccl treatments (5,
20 and 35°C; n=11 per population per Taccl) for 28 days, and
measured post-acclimation BMR, EWL and Msum using flow-through
respirometry. Our data reveal the expected pattern of lower BMR and
EWL (∼12% and 25% lower, respectively) in birds at Taccl=35°C
compared with cooler Taccl treatments, as observed in previous
acclimation studies on subtropical birds. We found no variation in the
reaction norms of BMR and EWL among populations in response to
acclimation, suggesting previously documented differences in
seasonal BMR acclimatization are the result of phenotypic flexibility.
In contrast to higher-latitude species,Msum did not significantly vary in
response to thermal acclimation. These findings support the idea that
factors other than enhancing cold tolerance may be driving patterns of
metabolic variation in subtropical birds.
