Expanding the traditional definition of molt-migration

The occurrence of molt during migration, known as “molt-migration,” has increasingly received attention across many avian taxa since first being described in waterfowl in the 1960s. However, despite the many different types of molt stages and strategies, most, if not all, uses of the term “molt-migration” apply to the definitive prebasic molt of flight feathers in post-breeding adults, whereas fewer studies address migration for body-feather molts. Here, we argue that the current definition of molt-migration, as applied, is limited in focus relative to the diverse ways in which it can manifest in avian populations. We suggest a new, broader definition of molt-migration and highlight examples of molt-migration as traditionally defined, and the many examples that have not been defined as such. We propose a new, 2-tiered typology for defining different forms of molt-migration, based on (1) its progression relative to stationary portions of the annual cycle, and (2) the stage of molt involved. In order to advance our understanding of the ecology and evolution of this increasingly documented phenomenon and apply this knowledge to conservation and management, avian researchers must begin to utilize a common framework for describing molt-migration in its various forms.