Bioenergetics of Molt in the Chaffinch (Fringilla coelebs)

The annual molt of adult Chaffinches begins in the third week of June and is completed by the end of September. In first brood juveniles, molt begins in early July, in second brood juveniles in early August, and in both molt is completed about the same time as in adults. Metabolized energy, measured as the difference between food intake and excreta, increased during the first part of molt and then decreased to a level following molt completion of$1.5\ {\rm kcal}\ \text{bird}^{-1}\ \text{day}^{-1}$lower than before it began. Metabolized energy increased with a drop in ambient temperature at the same rate in all stages of molt$(0.44\ {\rm kcal}\ {{}^\circ}{\rm C}^{-1}\ \text{day}^{-1})$. Total productive energy (metabolized minus existence energies) used for molt under natural conditions totaled$240\ {\rm kcal}\ \text{bird}^{-1}$in adults and$147\ {\rm kcal}\ \text{bird}^{-1}$in juveniles. Net productive energy at a constant temperature of 26°C was$140\ {\rm kcal}\ \text{bird}^{-1}$, while the amount used at a constant temperature of 7°C was 95% greater. Catabolism of resting, postabsorptive birds at night, calculated from oxygen consumption, was$1.1\ {\rm kcal}\ \text{bird}^{-1}\ \text{day}^{-1}$higher at thermoneutrality during molt than before molt began. This represents a minimum estimate of the net energy cost of plumage replacement. Below thermoneutrality, the cost of molt in adults changed from$0.384\ {\rm kcal}\ {{}^\circ}{\rm C}^{-1}\ \text{day}^{-1}$before molt to 0.596 kcal at mid-molt to 0.332 kcal after molt. In juveniles the rate of increase at mid-molt was$0.504\ {\rm kcal}\ {{}^\circ}{\rm C}^{-1}\ \text{day}^{-1}$. The caloric equivalent of weight loss at night was lower in molting than in non-molting birds and lower at high than at low ambient temperatures, due largely to the greater amounts of water loss involved. Protein catabolism, as shown by the amount of nitrogen in the urine, increased during molt to the extent of 8.33 g. The nightly respiratory quotient was 0.88.