Overview of Molt Cycle Ageing

Content in Piranga is organized using the molt cycle ageing approach, also known as Wolfe-Ryder-Pyle or WRP after the authors who originally developed it (Wolfe et al. 2010). This system better reflects the actual changes in a bird’s appearance, in contrast to calendar-based ageing schemes in which the category assigned to a bird arbitrarily switches on January 1.

The molt cycle ageing approach has been refined over time, with the latest version outlined in Pyle et al. (2022). We strongly encourage banders to read Pyle et al. (2022), but below we summarize the core concepts that are referenced in Piranga. Visit the Glossary section for definitions of key terms.

Basics

The molt cycle ageing approach uses three-letter codes in which each position has a defined meaning. The first letter indicates the plumage cycle of the bird (e.g., F - first, S - second, D - definitive). The second letter denotes whether the bird is currently molting (P) or in a cycle of plumage between molts (C). The third letter describes the type of plumage that the bird has, or is growing (e.g., J - juvenile, F - formative, A - alternate, B - basic). The table below lists the possibilities for each position in the code.

	First Position Cycle (age)	Second Position Molt Status	Third Position Plumage
Commonly used codes	F (first) S (second) T (third) D (definitive)	C ("cycle" - molt complete) P ("pre" - molt in progress)	J (juvenile) F (formative) A (alternate) B (basic)
Infrequently used codes	4 (fourth) ... 9 (ninth) U (unknown)	U (unknown)	X (auxiliary formative) S (supplemental) U (unknown)

Modifiers

To facilitate greater precision, three prefixes can be added to modify codes in certain situations:

• "M-" is used to indicate that a bird is at minimum a certain age. For example, M-FCA is used for a bird in alternate plumage that cannot be distinguished between FCA (second year) and DCA (after second year); a bird classified as M-TCB is at least in its third cycle (i.e., 2+ years old). Use of this prefix allows the extent of uncertainty to be defined more precisely than by using “U” to indicate the cycle.

• "H-" (hatching season) and "A-" (after hatching season) can be used to modify the code FCF, for species where generations can overlap. For example, Downy Woodpeckers may complete their preformative molt as early as June or July of their first year, and not molt again until as late as September or October of their second year. Therefore during summer and early fall, there may be overlap with young H-FCF birds and A-FCF birds from the previous year, and using the prefix to distinguish between them is important for demographic analyses.

Unknown categories

• For various reasons, it may not be possible to conclusively categorize some individuals. If any of cycle, molt status, and/or plumage are completely unknown, they can be represented by “U”. However, as noted above, use of the modifier “M-” is generally recommended over using U in the first position, as it allows the extent of uncertainty to be narrowed down.

• Use of U in the second position is rare, as it is generally evident whether or not an individual is in the process of molting.

• Use of U in the third position is most common for “Type B” species with a limited to partial prealternate molt, in which there may be insufficient evidence to differentiate whether an individual remains in formative plumage or has acquired alternate plumage (e.g., FCU, DCU, or M-FCU).

Typical sequences

Although many molt sequences are theoretically possible, the majority of species follow one of a few common patterns. In all cases, once a bird reaches its definitive cycle, it continues to loop through this stage for the rest of its life (indicated in molt sequences below as “…”). Stages that may be recognizable only in a subset of individuals are noted in parentheses, indicating that other individuals skip past these to the definitive state beyond. The plumage cycle stages are bolded to indicate that they are the dominant states for banders to focus on, as periods of active molt are relatively brief for most species.

Type A: only preformative and prebasic molts

The three most common varieties are subtypes A1, A3, and A4

Subtype A1: typical pattern for species with a (usually) complete preformative molt, complete prebasic molt, and no prealternate molt

FPJ-FCJ-FPF-(FCF)-(SPB)-M-FCF-M-SPB/M-FPF-M-FCF...

Examples: House Sparrow, Eastern Meadowlark, Northern Cardinal

Note: For species/populations where not all individuals undergo a complete preformative molt, some individuals can be recognized as FCF and SPB based on retention of juvenile feathers, but in all cases, individuals lacking juvenile feathers can only be classified as M-FCF (if not molting) or as either M-SPB (if molting, with retained feathers clearly not juvenile) or M-FPF (if molting, and unable to determine whether any feathers from the previous generation were juvenile, for example late in molt when feathers are still growing in, but old feathers have all been lost).

Subtype A3: typical pattern for species with an incomplete preformative molt, complete prebasic molt, and no prealternate molt

FPJ-FCJ-FPF-FCF-SPB-DCB-DPB-DCB...

Examples: Blue Jay, American Robin, Song Sparrow

Subtype A4: typical pattern for species with an incomplete preformative molt, at least one incomplete prebasic molt, and no prealternate molt

FPJ-FCJ-FPF-FCF-SPB-SCB-TPB-(TCB)-(4PB) … DCB-DPB-DCB...

Examples: Bald Eagle, Herring Gull, Downy Woodpecker

Note: In some species (e.g., Downy Woodpecker), individuals can only rarely be aged precisely beyond the third prebasic molt (TPB), entering their definitive cycle thereafter. If subsequent prebasic molts are also incomplete, it can be possible to age some species to fourth cycle or beyond, perhaps as far as ninth cycle for some albatrosses, with the coding extended as required (TCB-4PB-4CB-5PB-5CB…).

Type B: only preformative, prebasic, and prealternate molts

Subtype B4: typical pattern for species with a less than complete preformative molt, complete prebasic molt, and a partial to complete prealternate molt

FPJ-FCJ-FPF-FCF-FPA-FCA-SPB-DCB-DPA-DCA-DPB-DCB...

Examples: American Goldfinch, Baltimore Oriole, Magnolia Warbler

Note: For species that have a limited prealternate molt (generally restricted to body feathers and sometimes lesser and median coverts – e.g., Gray Catbird or Common Yellowthroat), it may be difficult to tell by looking at an individual whether the prealternate molt has occurred or not. These are instead classified as Subtype B3, with FCU or DCU (“U” representing unknown plumage state) used as a more accurate reflection of what can be observed, rather than FCA or DCA.

Various other molt strategies occur, with some species having only prebasic molts (e.g., Turkey Vulture), or adding a preauxiliary molt (e.g., Indigo Bunting) or presupplemental molts (e.g., Willow Ptarmigan). See individual species accounts for more detail on these sequences.

References

Johnson, E.I., J.D. Wolfe, T.B. Ryder, and P. Pyle. 2011. Modifications to a molt-based ageing system proposed by Wolfe et al. (2010). Journal of Field Ornithology 82:422–424.

Pyle, P., M. Gahbauer, E.I. Johnson, B.T. Ryder, and J.D. Wolfe. 2022. Application of a global age-coding system (“WRP”) based on molts and plumages, for use in demographic and other studies of birds. Ornithology 139:1-12.

Wolfe, J.D., T.B. Ryder, and P. Pyle. 2010. Using molt cycles to categorize the age of tropical birds: An integrative system. Journal of Field Ornithology 81:186-194.