Molt in the Annual Cycle of Ring-Necked Ducks

Abstract

Birds have increased energy and nutrient requirements during molt. Estimation of these costs has proven difficult; consequently, the nutritional significance of molt relative to other annual cycle events remains unclear. Here we describe patterns of molt for Ring-necked Ducks (Aythya collaris) throughout their annual cycle. A method was developed for indexing molting costs of waterfowl based on intensity of molt and proportional mass of feathers in eight feather regions (ADJMOLT). This method was then applied to Ring-necked Ducks so that we might examine molting costs in relation to sex and age of birds, month, and annual cycle events. To assess nutritional stress (demands greater than available resources) associated with molt, we examined the relation between remige length and structural size of adults. Contour feather molt in the head and neck, sides and flank, upper breast and back, lower breast and belly, mid-rump and lower back, scapular, humeral, and rectral feather regions varied by sex and month for first-year and after-first-year (adults) Ring-necked Ducks. Mean molt scores (i.e., molt averaged over all feather regions) and ADJMOLT of adults also varied by sex and month. The pattern of change in ADJMOLT was trimodal in females and bimodal in males. ADJMOLT of both sexes was greatest during the period of remigial molt (August) and was also elevated during winter (December and January). ADJMOLT was reduced during spring (March) and fall (October) migrations. Whereas molt remained at low levels in males throughout the breeding season, females initiated prebasic molt after arrival on breeding areas and were molting intensively while acquiring energy and nutrients needed for reproduction. Thus it was apparent that although molting costs were mostly staggered with minimal overlap with other energy- and nutrient-demanding processes, timing of molt in the annual cycle of Ring-necked Ducks was also influenced by non-nutritional factors (e.g., date). Remige lengths were significantly (albeit weakly) related to structural size of both males and females. We therefore concluded that, although costs of molt were greatest while birds were molting remiges, there was no evidence that molting Ring-necked Ducks were nutritionally stressed