To evaluate the contribution of changes in plumage insulation to the energy cost of molt, we measured oxygen consumption by wintering White-crowned Sparrows (Zonotrichia leucophrys gambelii) before and after plucking 12, 24, or 36% of their plumage, and when they were replacing these feathers. Measurements were made at 20 and 25 °C, two temperatures bracketing the lower critical temperature (ca. 23 °C) of wintering Z. l. gambelii, and at 10 °C, well below the birds' lower critical temperature. For comparison, oxygen consumption by naturally molting birds was measured at 25 °C during summer. In these sparrows, feather loss resulted in increased oxygen consumption only at 10 °C and when feather loss was moderate (24% plumage; 10% increase) to intensive (36% plumage; 24% increase). Regrowth of 24 and 36% of plumage resulted in increased oxygen consumption at 20 °C (10 and 8.5%, respectively) and 10 °C (16 and 28%, respectively). Oxygen consumption by birds was unaffected by loss or regrowth of 12% of the plumage regardless of temperature, and at 25 °C, oxygen consumption was unaffected by the intensity of plumage replacement (0–36%). Comparison of oxygen consumption at 25 °C between naturally molting summer birds and treated winter birds revealed that the energy cost of molt and the apparent energy inefficiency of molt result neither from added thermoregulatory costs nor from the costs of feather synthesis per se, but seemingly from metabolic changes entrained by molt.
Facultative Altitudinal Movements by Mountain White-Crowned Sparrows (Zonotrichia Leucophrys Oriantha) in the Sierra Nevada