Postnuptial and Postjuvenal Molts of Red-Winged Blackbirds in Cass County, North Dakota

Red-winged Blackbirds (Agelaius phoeniceus) form roosting and feeding aggregations (flocks) in late summer and autumn. Recent investigations of the flocks have centered on behavior and management (Martin 1977, Weatherhead and Bider 1979, Stone and Danner 1980). Postnuptial (prebasic) and postjuvenal (first prebasic) molts of Red-wings coincide with flocking and, at their peak, have a marked effect on the birds' behavior (Allen 1914, Smith and Bird 1969, Meanley and Bond 1970). Quantitative information on the phenology of Redwing molt is sparse. Dunson (1965) and Payne (1969) studied the timimg of molt in two nonmigratory, southern populations. Meanley and Bond (1970) estimated the duration of Red-wing molt in the Patuxent, Maryland area. We studied the molt (molt refers to the entire process of losing feathers and replacing them) progress of five age-sex classes in a migratory population of Red-winged Blackbirds in southwestern Cass County, North Dakota (46ø08'N, 97ø04'W). Our objectives were: (1) to assess differences in the phenology of molt by age and sex, and (2) to determine the seasonal relations of nesting, molt, and migration. Southwestern Cass County is on the edge of the Drift Prairie physiographic region (Klausing 1968). Numerous wetlands, scattered throughout this agricultural area, provide daytime resting sites and night roosts for blackbirds (Icteridae) and Starlings (Sturnus vulgaris, Sturnidae). The principal roost in our study area is located in the U.S. Fish and Wildlife Service Waterfowl Production Area at Alice, which includes a cattail (Typhus spp.) marsh. Paul Lago (MS) estimated that at peak density in 1972 (mid-October) there were 292,000 birds roosting in the marsh. Density varies from week to week during each season and probably from year to year. We collected 3,795 Red-wings from the study area by shooting from June through October 1980. They were sexed, aged by plumage characteristics and presence or absence of the bursa of Fabricius (Wright and Wright 1944, Payne 1969), and assessed for stage of molt. Plumages of Red-wings have been described by Dwight (1900), Selander and Giller (1960), and Meanley and Bond (1970). All remiges are normally replaced during the postnuptial and postjuvenal molts. Primaries are replaced in sequence from 1 (innermost) through 9; the outer secondaries (1-6) are normally molted from outermost (1) to innermost (6). The birds were divided into five age-sex classes. Standard banding terminology is followed for age . designations. After hatching-year (AHY) females breed during their first full breeding season. Their age cannot be easily determined after their hatching year (HY). Second-year (SY) males have lived for 1 yr and generally do not breed. They can be reliably distinguished from the breeding after-second-year (ASY) males, which have lived at least 2 yr, by plumage characteristics until molt is 60% complete (first week of August). Hatching year males and females are young-of-the-year birds. Each primary and secondary feather of the right wing was assigned a numerical value using a fivepoint scoring system similar to that of Evans (1966), Newton (1966), and Sealy (1979). A 0 was assigned an old feather, 1 a missing or small pin feather, 2 a feather up to 1/3 grown, 3 up to 2/3 grown, 4 up to 3/4 grown, and 5 a nearly complete or complete feather. Our emphasis is on the phenology of primary molt, which is considered a good indicator of overall molt progress (Evans 1966, Newton 1966). The secondaries molted irregularly in some individuals and thus were not ideal for statistical analysis. The functional relationship of the increase in primary scores (dependent variable, Y) with time (independent variable, X) is assumed to be linear (Y = a + bX). The increase in primary scores with time (slope, b) and Y-intercept (a) were determined by means of regression analysis (Evans 1966, Newton 1966, Payne 1969). Regression values were used to estimate the beginning and end of molt for four agesex classes: AHY males (SY and ASY males combined), AHY females, and HY males and females. Potential bias in the regression estimates due to nonlinearity (Evans 1966, Newton 1966) was reduced by only including data collected while 95% of the birds were molting. This occurred from 29 July through 23 September for AHY birds and from 26 August through 7 October for HY birds. Duncan's multiple range test was used to determine whether or not the means of