ESTIMATES OF THE MASS OF STRUCTURES OTHER THAN PLUMAGE PRODUCED DURING MOLT BY WHITE-CROWNED SPARROWS

Analysis of the caloric or nutritional demands of molt requires a thorough inventory of the mass and composition of all molted structures and of the ephemeral nonmolted structures (e.g., feather pulp) that accompany the process. Only plumage mass and composition are adequately known in a selection of species. We reported previously that the air-dried plumage mass of a 27-g White-crowned Sparrow (Zonotrichia leucophrys gambelii) is about 2,000 mg at the end of the postnuptial (PN) molt, during which about 400 mg of feather sheaths were grown and shed. In this report we show that the stratum corneum (air-dried mass = 88 mg) of captive Z. l. gambelii is totally shed and replaced during the PN molt, and that the podotheca (19 mg, both legs) is molted about 10 weeks later, in November. The rhamphotheca is not shed during feather molt, but appears to be renewed continuously in response to wear of the tomia. It is unlikely that the molt of the podotheca is delayed because of nutritional stringency during the PN molt, since its mass is only about 1% of the combined mass of feathers, sheaths, and stratum corneum. Very little is know in other species about the renovation of corneous structures other than feathers. It is thought that skin, beak, and claws grow continuously in response to wear, and that the skin also molts totally during feather molt. In some species, claws and parts of the beak may also be shed episodically during or near the time of feather molt. The podotheca is probably shed annually in all species, often during feather molt but sometimes earlier or later.