Protein intake and the dynamics of the postnuptial molt in White-crowned Sparrows, Zonotrichia leucophrys gambelii

Abstract

Nutritional constraints have often been invoked as either ultimate or proximate agents that account for variation in the schedule and intensity of molt and the quality of the new plumage. We examined this hypothesis by analyzing the dynamics of postnuptial molt and the condition of the new feathers in seven groups of White-crowned Sparrows (Zonotrichia leucophrys gambelii) in which the mean daily consumption of high-quality protein in otherwise balanced diets ranged from subadequate (0.20 g/bird) to superadequate (3.23 g/bird). Mean body mass during molt did not differ among the six groups consuming 0.46 g/bird-day or more (0.69, 0.97, 1.76, 1.99, and 3.23 g/bird-day) but was significantly less throughout molt (e.g., ca. 20 vs. 26 g in mid to late molt) in the group subsisting on 0.20 g/bird-day. The mean date of molt onset (27 June – 2 July) was statistically the same in all the groups, as was the duration of molt (51–57 days) in the six groups consuming 0.46 g or more of protein per day. In the lowest-protein group (0.20 g/bird-day) the molt lasted > 111 days. The protraction of molt resulted from both a slower growth rate of flight feathers and longer shedding intervals between them. The new primary remiges of the birds in this group averaged 4–9% shorter than in the six other groups, which were statistically indistinguishable from each other. These results are consistent with those of earlier experiments on the effects on molt of shortages of nutritionally balanced diets and of diets deficient only in sulfur amino acids. Molt is very resistant to the kinds of malnutrition that free-living birds may encounter. Only very severe levels of privation that sharply depress body mass and potentially threaten life significantly slow the rate of molt or reduce the quality of plumage.