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25 October 2021 Application of a global age-coding system (“WRP”), based on molts and plumages, for use in demographic and other studies of birds
Peter Pyle, Marcel Gahbauer, Erik I. Johnson, Thomas B. Ryder, Jared D. Wolfe
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Determination of a bird's age or cohort is critical for studies on avian demography, occurrence patterns, behavior, and conservation management. Age designations have largely been developed in north-temperate regions and utilize calendar-based or seasonally based codes; however, in tropical regions and in the southern hemisphere, these coding systems have limited utility at best. To address these issues, we had previously devised the “WRP system,” based on the nomenclature of Humphrey and Parkes (H–P) and Howell et al., which defines molts in an evolutionary context applicable to birds globally. Here we refine and build upon core concepts and definitions of the WRP coding system, resolving key limitations that were identified during its first decade of use. The WRP system employs a three-letter alpha code in which each letter describes a different aspect of H–P terminology: the molt cycle (which informs a bird's age) and molt and plumage status within the cycle (each of which can also inform age). Here we recommend the continued use of most of the original (“core”) WRP coding while augmenting the system with an optional adjunct-code entry for comprehensiveness, clarity, and flexibility, and we clarify a few additional codes to cover less common molting and plumage strategies. For most users, from 7 to 13 core and 1 adjunct WRP code will be sufficient to describe all plumages and provide molt status and ages for demographic studies or other purposes. The revised WRP system is flexible enough to be adapted to the specific goals of programs while also providing core codes that can facilitate the comparison of avian age, molt, and plumage status on a global basis. We anticipate that our revised and standardized version of the WRP system will be easily adopted and could eventually replace calendar-based and seasonally based coding.


  • Age determination of birds is critical for studies on avian demography and conservation.

  • Several age-designation systems currently in use are applicable to the temperate northern hemisphere but cannot be used in tropical regions or the southern hemisphere.

  • The “WRP” coding system was thus designed for global use in demographic and other studies examining the ages and cohorts of birds.

  • WRP coding is based on a rigidly defined molt terminology that designates molt cycle along with molt and plumage status, each of which informs age.

  • Here we make recommendations to enhance and stabilize the core WRP system, providing clarified coding that can be adapted to individual programs while also being comparable on a global basis.

Copyright © American Ornithological Society 2021. All rights reserved. For permissions, e-mail:
Peter Pyle, Marcel Gahbauer, Erik I. Johnson, Thomas B. Ryder, and Jared D. Wolfe "Application of a global age-coding system (“WRP”), based on molts and plumages, for use in demographic and other studies of birds," Ornithology 139(1), 1-12, (25 October 2021).
Received: 16 February 2021; Accepted: 17 September 2021; Published: 25 October 2021
age code
ciclo de muda
código de edad
éxito reproductivo
molt cycle
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