Defining moults in migratory birds: a sequence‐based approach

Abstract

Two broad nomenclatures have emerged to describe moult strategies in birds, the 'lifecycle' system which describes moults relative to present-day breeding and other lifehistory events, and the Humphrey-Parkes (H-P) system which reflects the evolution of moults along ancestral lineages. Using either system, challenges have arisen defining strategies in migratory species with more than one moult per year. When all or part of two moults occur in non-breeding areas, extra moults may fail to be recognized or they may have been discriminated temporally, whether feathers are replaced in fall, winter or spring. But in some cases feather replacement can span the non-breeding period, and this has resulted in an inability to identify inserted moults and to compare moult strategies between species. Furthermore, recent analyses on factors influencing the extent of the postjuvenile or preformative moults have either confined this moult to the summer grounds or presumed that it can be suspended and resumed on winter grounds, which has lead to quite divergent results and interpretations. Evolutionarily, the timing, extent and location of moults show phenotypic lability whereas the sequence in which feathers are replaced is comparatively conserved. As, such, I propose defining moults on the basis of feather-replacement sequences as opposed to timing or location of replacement, including strategies in which discrete moults can be suspended for migration and overlap temporally. I provide examples illustrating the functionality of a sequence-based definition in three migratory North American passerines that can undergo feather replacement twice in non breeding areas, and I demonstrate how this system can effectively apply to moults in many other passerine and non-passerine species. I recommend that authors studying the evolutionary drivers of moult strategies in migratory birds adopt a sequence-based approach and to carefully consider replacement strategies both prior to and following autumn migration.

Moult terminology: Let's make it simpler!

“…Howell et al . 2003, Pyle 2008, 2022, Johnson & Wolfe 2017). This terminology simulates a more exact system, but with the exactness comes increased complexity.…”

“…H-P terminology is commonly used in the New World (Palmer 1976, Ginn & Melville 1983, and it can better describe complicated moult cycles, especially when terminology adjustments that were more recently introduced are used (e.g. Howell et al 2003, Pyle 2008, 2022, Johnson & Wolfe 2017. This terminology simulates a more exact system, but with the exactness comes increased complexity.…”

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“…For example, Pyle et al (2020) found that the ratio of yearlings to older individuals among boreal forest birds indicated the older birds exclude yearlings from preferred breeding habitats through despotism, a dynamic that could inform habitat management. Understanding molt strategies is, in turn, critical to age determination, but for many North American species that molt away from their breeding grounds, these strategies are poorly known (Pyle 1997a, 2022a, Pyle et al 2009, 2018a. Fitness consequences and factors driving the evolution of molt, such as migration distance and habitats, have also been understudied because of lack of detailed information for most species and differing opinions on the definition of molts that occur away from breeding grounds (Pyle 2022a).…”

“…Understanding molt strategies is, in turn, critical to age determination, but for many North American species that molt away from their breeding grounds, these strategies are poorly known (Pyle 1997a, 2022a, Pyle et al 2009, 2018a. Fitness consequences and factors driving the evolution of molt, such as migration distance and habitats, have also been understudied because of lack of detailed information for most species and differing opinions on the definition of molts that occur away from breeding grounds (Pyle 2022a).…”

“…We identified feather generations on the basis of plumage color, degree of feather wear, contrasts between retained and replaced feathers (molt limits), and feather position (see Figure 1). We assumed that replacement of coverts during the preformative molt was proximal to distal within each tract and that the tertials were replaced in the sequence s8-s9-s7, as in most passerines (Pyle 1997a, 2022a, Jenni and Winkler 2020. Alternate feathers located earlier in the above sequence and within a group of otherwise formative feathers (e.g., MOLT STRATEGIES BY AGE AND SUBSPECIES IN THE WILLOW FLYCATCHER inner greater coverts, s8) we assumed also to have been replaced during the previous preformative molt (see Figures 1, 2).…”