Plumage, Age and Moult Terminology
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British Birds VOLUME 78 NUMBER 9 SEPTEMBER 1985 Plumage, age and moult terminology he ability to tell the age of a bird by its plumage and other features is, Tin itself, an interesting and challenging part of identification. Also, for some difficult-to-identify species, it is often an essential starting point in the identification process. In these and other studies related to plumage, a clearly defined terminology is important, and it would seem useful, there fore, to define that used in British Birds. 'Calendar-year' terminology is useful as a simple age-categorisation in some cases, and is defined and discussed separately. Other systems of plumage, age and moult terminology are used elsewhere. These are also discussed, and the equivalent terminology for two important ones (the 'Humphrey & Parkes' system, and that used in BWP) are given in table 1. 'British Birds' terminology DISTINCT PLUMAGES Juvenile (abbreviation: juv) The first set of true feathers, which replaces the down plumage. Juvenile plumage is that in which the bird first flies, although some species (e.g. some auks) are flightless until they acquire first-winter plumage. In the great majority of species, juvenile plumage is distinguishable from other immature or adult plumages. Juvenile plumage is retained until the moult to first-winter plumage (also called the post-juvenile moult). Depending on the species, this moult can be anything from partial (involving at least most of the head and body feathers), as is the case with most species, to complete. Depending, again, on the species, the moult to first-winter starts at any time from shortly after fledging to winter. [Bril. Birds 78: 419-427, September 1985] 419 420 Plumage, age and moult terminology First-winter (abbreviation: lst-w) In some species, especially those few which have a complete post-juvenile moult, the first-winter plumage may be similar to the adult winter plumage. In others, juvenile features are retained (especially distinctive patterned juvenile wing and tail feathers) which enable first-winter plumage to be identifiable. First-winter plumage is retained until the moult to first-summer plumage, which, depending on the species, starts in late winter to spring. This moult is partial in most species, complete in very few. Some species do not moult at this time, in which case first-summer plumage is either little different from first-winter, or is acquired by loss of feather-fringes which reveals the underlying first-summer coloration. First-summer (abbreviation: lst-s) It should be firmly noted that 'first' summer here refers to the first full summer after the year of hatching, not to the summer in which the bird was hatched: misunderstanding of this simple point can lead to a great deal of confusion. First-summer plumage is identifiable only on those species which acquire distinctive first-summer features (including through differential feather-wear) or, much more commonly, by the continued retention of juvenile features (especially, distinctive juvenile wing and tail feathers). First-summer plumage is retained until the moult to second-winter plumage, which, depending on the species, starts in early summer to early winter. This moult is complete in most species, partial in rather few. Second-winter (abbreviation: 2nd-w) Second-winter plumage is identifiable only on those species which acquire distinctive second-winter plumage patterns or other features, and is retained until the moult to second-summer plumage, which, depending on the species, starts in late winter to spring. This moult is partial in most species, complete in very few. Second-summer (abbreviation: 2nd-s) Second-summer plumage is identifiable only on those species which acquire distinctive second-summer features, or, more commonly, by the retention of second-winter features (especially, distinctive second-winter wing and tail feathers). Second-summer plumage is retained until the moult to third-winter plumage, which, depending on the species, starts in early summer to early winter. This moult is complete in most species, partial in rather few. Third-winter (abbreviation: 3rd-w) Some species take a further year or more to acquire adult plumage, and follow a sequence of plumages after second-summer, thus, 'third- winter', 'third-summer' (3rd-s), 'fourth-winter' (4th-w), 'fourth-summer' (4th-s), and soon. Adult (abbreviation: ad) The ultimate plumage of a bird, which does not change further through age. It should be noted that, in some species, 'adult' plumage does not necessarily indicate sexual maturity. Many species have adult plumage which changes twice a year: 'adult summer' (ad. s), which results from a moult which starts in late winter to spring (the 'spring moult'), or from the loss of fringes on winter plumage feathers; and 'adult winter' (ad. w), which results from a moult which starts in early summer to early winter (the 'autumn moult'). GENERAL TERMS Immature (abbreviation: imm) Any plumage other than adult. The length of recognisable immaturity varies from a few weeks to several years, depending on the species. It should be noted that 'immature* here refers only to plumage, since some species can be sexually mature while still in an immature plumage. First-year (abbreviation: lst-y) A general term covering the plumages of approximately the first 12 months of life, thus referring jointly to juvenile, first-winter and first-summer plumages. Second-year (abbreviation: 2nd-y) A general term covering the plumages of approximately the second 12 months of life, thus referringjointly to second-winter and second-summer plumages. Similarly, 'third-year' (jointly third-winter and third-summer plumages: abbreviation 3rd-y), 'fourth-year' (4th-y), and soon. We consider that use of the vague term 'sub-adult' should be avoided. There is no widely accepted definition of the term, and, in any case, it seems to be a less descriptive alternative for terms which have already been Plumage, age and moult terminology 421 defined here. If vagueness over the age or plumage of a bird is unavoidable, it would be better to describe it as, for example, 'immature', 'first- or second-year', 'immature other than juvenile', or 'near-adult' as appro• priate. TRANSITIONAL PLUMAGES Transitional stages are referred to as, for example, 'juvenile moulting to first-winter', or 'first-summer moulting to second-winter'. For species which acquire summer plumage by loss of feather fringes rather than moult, phraseology such as 'first-winter in transition to first-summer', or 'adult winter in transition to summer' is more appropriate. The duration of a moult can vary from a few weeks in the case of small species, to several months in the case of some large species. Calendar-year terminology A simple terminology for age-categorisation (for immatures or known-age adults) is based on the calendar year, and is useful in some cases. Calendar- year terminology has been rather little used in Britain, but is increasingly 199. Ring-billed Gull Lams delawarensis near end of complete autumn moult from adult summer to adult winter plumage. The dark-marked winter head plumage shows that these feathers have already been renewed: only the outermost primary on each wing remains to be dropped, and the end of the autumn moult will come with the full growth of its replacement. USA. September 1984 {Richard Chandler)' 422 Plumage, age and moult terminology popular abroad, especially in Europe. It is especially well-suited to those species which have lengthy or overlapping moult periods and thus have no long-lasting or static winter or summer plumages. It is also useful when age-groups need to be referred to generally, or when precise plumage definition is inappropriate or not determinable. 'First calendar-year' (abbreviation: 1st cal-y) refers to individuals up to 31st December of their hatching year. 'Second calendar-year' (2nd cal-y) refers to individuals from 1st January to 31st December of the year after hatching. Similarly, 'third calendar-year' (3rd cal-y), and so on. When necessary, further precision can be added to the calendar-year terminology by the addition of the date, month or season, for example 'first calendar-year (December)' or 'third calendar-year (autumn)'. The calen• dar-year terminology as defined here is, of course, unsuitable for use in equatorial regions or in the southern hemisphere. Discussion Several other systems of plumage and moult terminology are in use elsewhere. It should be noted that our attempts to find a single, inter• nationally acceptable teminology were unsuccessful. Such consistency would be highly desirable, but a draft of this editorial sent to five inter• national consultants attracted five different proposed alternatives! In deciding to retain the existing British Birds terminology, we are conscious that it can be criticised on mainly semantic grounds. For example, 200. First-winter Dunlin Calidris alpina. The post-juvenile moult of small waders does not include the wing feathers, and the neatly pale-fringed, retained juvenile wing-coverts and tertials enable this first-winter individual to be distinguished from adult winter plumage (which would have plainer coverts and tertials, matching the rest of the upperparts). Essex, October 1984 (R. Glover) Plumage, age and moult terminology 423 'summer' and 'winter' plumages do not necessarily fully coincide with those seasons; migrants occur in the southern hemisphere's summer in 'winter' plumage; season-based terminology is inappropriate for equatorial species; and it is rather inappropriate for species which have a complete, once- yearly moult (e.g. the adult winter and adult summer 'plumages' of Starling Sturnus vulgaris involve the same set of feathers). 201. Juvenile Dunlin Calidris alpina moulting to first-winter plumage. The progress of the post-juvenile moult on this individual can be clearly seen from the mixture of old juvenile mantle feathers and scapulars (dark with sharply defined pale fringes) and new, plainer, first-winter ones.