Molt Chronology of Northern Pintails in California

Abstract

Intensity and chronology of molt in 10 feather groups (head, neck, breast, belly, back, rump, side, scapular, tertial, and tail) were measured using percentages of developing feathers (pinfeathers) converted to molt scores (0-100) in samples plucked monthly from northern pintails (Anas acuta) collected by shooting from August through March 1980-82 in the Sacramento Valley, California. Data to supplement sample size of immatures were obtained from October 1982 through January 1983. Molt scores of the prealternate molt peaked (30-40% pinfeathers) in immature and adult females and adult males in October when nearly all feather groups were molting. The prealternate molt in immature males did not peak until November. Adult males nearly completed this molt by December; no new feather growth occurred after January. Immature males were still growing new neck, side, rump, scapular, and tertial feathers in December. Molt scores of the prebasic molt among adult and immature females peaked (30-40% new growth) in February in both years. The prealternate molt of breast and belly feathers (both sexes) peaked earlier (Sep) than molt of most other feathers (Oct). Increased molt scores in peak months were a function of a disproportionate increase of molt activity within each feather group and the number of feather groups with molt. Molt scores of the prebasic molt in adult females were less during December and January of the dry (198081) than the wet (1981-82) winter. Restriction or delay of molt may conserve energy or other nutrients during poor habitat conditions. J. WILDL. MANAGE. 50(1):57-64 Annual molt cycle and plumages have been described for individual redheads (Aythya americana) (Weller 1957) and gadwalls (Anas strepera) (Oring 1968) and for a large sample of greater scaup (Aythya marila) (Billard and Humphrey 1972). Heaviest molt in scaup occurred in November for males, and in November and February-April for females. Weller (1970) and Paulus (1984) used subjective molt status categories and found similar patterns for redheads and gadwalls, respectively. However, detailed quantitative information on the intensity, duration, and variation between years of molt in dabbling duck (Tribe Anatini) populations during winter is unavailable. Molt is a nutrient-costly process (Payne 1972, King 1980) and required nutrients are supplied in the diet (Ankney 1979, Young and Boag 1982, Murphy and King 1984b). Since nutritional deficiencies may delay molt in birds (Payne 1972), variation between years in the food supply available during winter could affect adaptive molt patterns in waterfowl populations. Food supply may be a function of proper habitat management (Chabreck 1979) and annual rainfall (Chabreck 1979, Fredrickson 1980, Heitmeyer and Fredrickson 1981). Female dabbling ducks acquire basic plumage from late winter to early spring before nesting (Palmer 1976). Sequential, rather than synchronous, timing of molt with nesting and migration minimizes daily energy expenditures This content downloaded from 207.46.13.114 on Thu, 26 May 2016 05:57:45 UTC All use subject to http://about.jstor.org/terms 58 PINTAIL MOLT * Miller J. Wildl. Manage. 50(1):1986 (Payne 1972, King 1974). Delayed molt, resulting from food shortage, would overlap spring migration and the period critical to accumulation of body reserves important for egg production (Krapu 1981). Northern pintails begin to arrive in the Sacramento Valley in August, >300,000 are present by mid-September (LeDonne 1980), and about 1 million are present in mid-winter (Bellrose 1980:267). They are the most abundant waterfowl species in California during winter (Bellrose 1980). Because little information on their winter molt existed, I studied timing, intensity, and variability of molt among age and sex classes of pintails in the Sacramento Valley from August through March. These molt patterns were examined during a winter of high rainfall with abundant wetland habitat and a winter of low rainfall and restricted wetland