Assessment of within-population variation in the timing and duration of molt is crucial to understanding how molt overlaps and interacts with other important phases of the avian annual cycle, including breeding and migration. We investigated the effects of sex and adult age on phenology of the post-breeding prebasic molt in an assemblage of migratory songbird species captured during banding operations at Powdermill Avian Research Center in southwestern Pennsylvania, USA. Across all species examined, males consistently initiated molt earlier than females (14 of 15 species), and young adults hatched the previous year consistently initiated molt earlier than experienced older adults (13 of 13 species). Sex also had a weaker but significant effect on molt duration, with females completing molt more rapidly than males in 67% of the species examined. Adult age, in contrast, had no significant effect on molt duration. A review of the literature indicates that similar patterns are observed in the post-breeding molt of passerines worldwide, with females showing delayed but more rapid molt in diverse geographic, phylogenetic, and ecological contexts. The delayed onset of post-breeding molt in female passerines probably reflects either (1) their generally greater reproductive effort in egg-laying, incubation, and brooding of young, or (2) sex-specific physiological constraints imposed by the shared neuroendocrine mechanisms that regulate both avian reproduction and molt. The shorter duration of post-breeding molt in females may partially reflect their smaller body size and shorter flight feathers. Earlier onset of molt in young adults probably reflects their worn retained juvenile plumage, lower probability of successful nesting, and early termination of their initial breeding season. Regardless of the causes, strong sex- and age-based differences in the phenology of post-breeding molt are likely to produce cascading trade-offs with other aspects of the songbird annual cycle, including the timing of breeding, late-season parental care, and migration.

Collection of Molt Data

Molt data analyzed for this study were collected from 1986–2004 in conjunction with year-round mist-netting and bird-banding operations at Powdermill Avian Research Center (40.1637°N, 79.2674°W), the field station of Carnegie Museum of Natural History, in southwestern Pennsylvania, USA. After identifying each captured individual to species and banding it with a uniquely numbered U. S. Bird Banding Laboratory band, we determined its sex using either plumage characteristics (for sexually dimorphic species) or the presence of a brood patch or cloacal protuberance. Adults were age-classified as either SY (second-year birds hatched the previous calendar year) or ASY (after-second-year birds hatched two or more calendar years ago), based on wing molt limits (Mulvihill 1993) and other plumage criteria that have since become standard in passerine aging (Pyle 2022). Each individual was then assigned a total flight-feather molt score (0–90) based on the molt stage of the 18 remiges (9 primaries and 9 secondaries) on the right wing. We used the 0–5 molt scoring system of Ginn and Melville (1983): 0 = old feather, 1 = old feather missing or new pin feather, 2 = new feather emerging from sheath up to one-third grown, 3 = new feather between one-third and two-thirds grown, 4 = new feather more than two-thirds grown but still sheathed at base, and 5 = fully grown new feather with no trace of the sheath at its base. All assignments of sex, age, and molt score were performed by a single individual. In the present study we focused on species that undergo a complete post-breeding prebasic molt on their breeding grounds and excluded individuals that showed evidence of partial or suspended molts.

Our analysis of sex differences was limited to species with at least 25 records of known-sex individuals in active molt—i.e., with molt scores 1–89. In addition, we selected species with at least some data from birds recaptured and re-scored during the same molt cycle, as data from recaptured individuals greatly improves estimation of molt phenology (Boersch-Supan et al. 2024, Mumme et al. 2025). A total of 15 species representing 5 families, all passerines that are either completely (13 species) or partially migratory (2 species), met these criteria for inclusion. Total sample size available for analysis of sex differences was 2468 molt records—1253 from males and 1215 from females.

Similarly, the dataset for our analysis of age differences was based on species for which there was at least some recapture data and at least 25 records of actively molting known-age adults classified as SY or ASY. Thirteen species representing 5 families, all passerines that at Powdermill are completely migratory, met these criteria for inclusion, and the final dataset comprised 1552 molt records—749 from SY birds and 803 from ASY individuals.

Statistical Analysis

Flight-feather molt in most temperate-zone passerines is a nonlinear sigmoidal process; it progresses gradually at the beginning and end of molt, when only a few flight feathers are being replaced, but more rapidly during intermediate stages, when multiple flight feathers are being replaced simultaneously (Dawson and Newton 2004, Newton 2009, Mumme et al. 2021, Guallar and Quesada 2023). To help linearize the molt process and make it more amenable to statistical modeling, we followed Mumme et al. (2025) by transforming total molt scores as arcsine (square root[molt score/90]), and then rescaling the resulting transformed values as a proportion between 0 and 1. Using this transformed molt index, we then estimated molt start and molt duration separately for the four sex and age categories—males, females, SY birds, and ASY birds—using the mixed-model extension of Pimm regression (Pimm 1976) developed by Mumme et al. (2025). In this approach, ordinal capture date (the dependent y variable) is regressed on the transformed molt index (the independent x variable), and bird-year—the unique band number coupled with the year captured—is included as a random effect to account for the non-independence arising from multiple measures of molt obtained from the same individual during a single molt cycle. This mixed-model extension of Pimm regression, which allows molt duration and start date to be estimated directly from the slope and intercept of the resulting regression equation, is the most appropriate method for analyzing the Powdermill data; it is unaffected by molt-dependent sampling bias, which is pronounced in the Powdermill data, and produces accurate and unbiased estimates of molt phenology whenever molt data are collected across a broad range of dates (Mumme et al. 2025).

We examined the statistical effects of sex on molt phenology by additional Pimm mixed models in which sex, and the interaction between sex and transformed molt index, were included as fixed effects. We interpreted significant main effects of sex on ordinal date as evidence of sex-based differences in the onset of molt, and significant interactions between sex and transformed molt index as evidence of sex-based differences in molt duration. We used comparable mixed models that included age, and its interaction with transformed molt index, to test the effects of adult age on molt phenology. All mixed models were built using JMP Pro 17.2 (SAS Corporation, Cary, NC USA).

To control for phylogenetic effects in across-species comparisons, we used R version 4.4.2 (R core team 2024) and the R packages ape 5.8.1 (Paradis and Schliep 2019) and phytools 2.4-4 (Revell 2024) to incorporate appropriate evolutionary trees and calculate phylogenetic correlation coefficients and phylogenetic matched-pairs t-tests. Trees were based on data drawn from birdtree.org (Jetz et al. 2012) and a multilocus phylogeny of the Parulidae (Lovette et al. 2010), and branch lengths were estimated using the modified Grafen method (Revell 2024).