THE CONTRIBUTION OF INSULATION CHANGES TO THE ENERGY-COST OF AVIAN MOLT

To evaluate the contribution of changes in plumage insulation to the e nergy cost of molt, we measured oxygen consumption by wintering White- crowned Sparrows (Zonotrichia leucophrys gambelii) before and after pl ucking 12, 24, or 36% of their plumage, and when they were replacing t hese feathers. Measurements were made at 20 and 25 degrees C, two temp eratures bracketing the lower critical temperature (ca. 23 degrees C) of wintering Z. l. gambelii, and at 10 degrees C, well below the birds ' lower critical temperature. For comparison, oxygen consumption by na turally molting birds was measured at 25 degrees C during summer. In t hese sparrows, feather loss resulted in increased oxygen consumption o nly at 10 degrees C and when feather loss was moderate (24% plumage; 1 0% increase) to intensive (36% plumage; 24% increase). Regrowth of 24 and 36% of plumage resulted in increased oxygen consumption at 20 degr ees C (10 and 8.5%, respectively) and 10 degrees C (16 and 28%, respec tively). Oxygen consumption by birds was unaffected by loss or regrowt h of 12% of the plumage regardless of temperature, and at 25 degrees C , oxygen consumption was unaffected by the intensity of plumage replac ement (0-36%). Comparison of oxygen consumption at 25 degrees C betwee n naturally molting summer birds and treated winter birds revealed tha t the energy cost of molt and the apparent energy inefficiency of molt result neither from added thermoregulatory costs nor from the costs o f feather synthesis per se, but seemingly from metabolic changes entra ined by molt.