The correct ageing of a bird is often fundamental to its identification. To facilitate this, an understanding is required of the precise terminology that describes the various ages, which are related to a bird’s moult cycle.
Juvenile plumage Newly hatched birds fall into two categories: (1) naked and blind, remains in the nest and is fed by its parents (altricial), or (2) covered in down, has its eyes open and is soon able to leave the nest and feed itself (precocial). All young birds must grow as quickly as possible and acquire the power of flight. In some small passerines this can take as little as ten days, which is an extraordinary physiological feat. In that time, they must grow their first covering of feathers, which is known as juvenile plumage. In order for it to grow so quickly, something has to be sacrificed: the strength and durability of that plumage. Consequently, the juvenile feathers of many species, particularly passerines, are weak and this is why young birds have an endearing fluffy appearance. The parents continue to feed them once they have left the nest but, gradually, the young become independent. The feeding is good in the long, warm days of late summer, making this the ideal time for young birds to quickly lose their inferior juvenile plumage and grow a stronger set of feathers that will see them through the winter. Thus, in late summer and early autumn, most young birds undergo post-juvenile moult. This usually involves just a body moult in which all the head and body feathers are replaced. The juvenile flight feathers (primaries and secondaries) and the tail feathers are retained. A young Robin, for example, loses its spotted brown juvenile body plumage – which made it difficult to detect during its inexperienced days in the shady undergrowth – and acquires its red breast and plain brown upperparts. By late autumn, it resembles its parents and, to all intents and purposes, is in adult plumage.
Immature plumages While a Robin may be adult-like by late autumn, other birds take much longer to acquire adult plumage, and larger birds may pass through a number of immature plumages before reaching adulthood. As a rule, the larger the bird, the longer it takes to mature and the more immature plumages it has. A group that demonstrates this particularly well is the gulls. Larger gulls, such as Herring Gull, usually take about four years to reach maturity, the medium-sized Common Gull takes three and the small gulls, such as Black-headed Gull, take two. To take Common Gull as a straightforward example, its juvenile plumage is essentially brown, but it gradually moults much of its body plumage during autumn, acquiring a whiter head and underparts, and grey back, mantle and scapulars. This new plumage is given a specific name, which is, quite logically, first-winter plumage. In spring, it has another body moult into first-summer plumage. It is not until late summer, when the bird is one year old, that it has its first complete moult, replacing all of its wing and tail feathers for the first time and its body feathers for a third. Its second-winter plumage is much more adult-like but still shows traces of immaturity, such as brown in the primary coverts. This is followed by another spring body moult into second-summer plumage, which is then followed by another complete moult into adult plumage, when it is just over two years old. Each plumage is more adult-like than the previous but remember that the different plumages may vary individually, particularly in larger birds (such as Herring Gulls) so do not expect all birds to conform exactly to those illustrated in field guides. Bare parts too – eyes, bill and legs – also change as a bird gets older and, although their progression is roughly in sync with their plumages, there is great individual variation in the acquisition of bare-part colours.
Juveniles and immatures At this point, it is essential to clarify one particular area of confusion. Any bird to that is not mature (i.e. not an adult) is an ‘immature’. Literally, ‘immature’ simply means ‘not mature’. Thus, a juvenile bird is an immature, as is a first-winter, a second-summer and so on. The term ‘juvenile’ should be confined to a bird’s very first plumage worn for a short time after leaving the nest. Although many larger birds migrate whilst still in juvenile plumage, most passerines do not. Therefore, you will not see juvenile Redstarts or Yellow-browed Warblers in September falls on the east coast. If these birds are not adults, then they are ‘first-winters’. Similarly, subadult is also to be avoided if possible, as again more accurate ageing can usually be determined.
Spring body moult This has an important and obvious function both for adults and many first-years: it enables them to change from their drab, functional winter plumage into brightly coloured breeding or summer plumage.
Calendar-year ageing Some large birds, such as cormorants and large birds of prey (e.g. eagles) have a fairly continuous moult in which several generations of feathers are present simultaneously. Precise ageing terms, such as ‘first-winter’ and ‘second-winter’ then become redundant so, to circumvent this problem, larger birds are often aged in relation to the year in which they hatched. A bird hatched in May 2013 will be called ‘first calendar year’ up to 31 December 2013, ‘second calendar year’ from 1 January to 31 December 2014, and so on.
Feather wear When identifying birds and assessing ageing, bear in mind the effects of wear, abrasion and bleaching. Old feathers wear and fade, and significant plumage features (such as wing-bars) can disappear through wear, while some species alter plumage tone (for example, fresh autumn Meadow Pipits possess a green tint, whereas worn breeders are much browner). Immature gulls are notoriously prone to wear and bleaching, and may look particularly ‘tatty’ and faded in summer, when their wing and tail feathers are nearly a year old. Some species acquire summer plumage not by moult, but by the wearing away their duller feather fringes (see below).
Juvenile waders As noted in part 1, most passerine migrants have a body moult before they head south in autumn, but what happens if there is not time to complete it? This is the problem that faces all birds that nest in the Arctic. Although food there is abundant, summer is short, so young birds do not have time to complete a post-juvenile body moult. They therefore have no option but to head south while still in juvenile plumage. Consequently, nearly all southbound autumn waders are in full juvenile plumage. In many ways, this plumage is a halfway house between adults’ summer and winter plumages. Like summer adults, juveniles tend to be brown and patterned on the upperparts, providing good camouflage when viewed from above, but the underparts are mainly white, like the adults’ winter plumage. Many species really do look intermediate. For example, juvenile Black-tailed Godwits, particularly those of the Icelandic race islandica, are usually quite bright orange on the underparts, while juvenile Spotted Redshanks appear rather dusky grey-brown, somewhere between the smart black of summer plumage and the grey and white of winter. It was no doubt this juvenile plumage that gave rise to its now redundant alternative name of ‘Dusky Redshank’.
Adult waders in autumn Because young waders’ feathers are all grown at the same time and because they are all very fresh in late summer and autumn, juvenile waders always look neat. In contrast, adults in late summer often undertake body moult while on migration, so they look tatty and ‘moth-eaten’ compared to their immaculate offspring. Most young waders do not moult until they arrive on their wintering grounds, after which they look like winter adults. There are, however, a number of notable exceptions, the most obvious being juvenile Dunlin, which often acquire grey scapulars on migration. Grey Phalaropes are also transitional by the time they reach Britain, with a patchy grey-and-black appearance to their upperparts (Red-necked Phalaropes moult somewhat later). For two rarities, Semipalmated and Western Sandpipers, moult timing can be fundamental to their separation.
Other Arctic migrants Birds such as divers, geese, ducks, skuas and terns also migrate in juvenile plumage and gradually start to moult into first-winter plumage once they have reached their winter quarters. Interestingly, whereas most mid-latitude gulls soon start to acquire fresh first-winter feathering, Arctic gulls such as Glaucous and Iceland retain their juvenile body plumage well into the winter. Another useful example is provided by Common and Arctic Terns: any ‘Commic Tern’ showing primary or secondary moult in autumn will be a Common Tern; the more northerly breeding adult Arctics do not normally commence wing moult until they reach Antarctica.
Tail and primary moult With a few exceptions, juveniles do not moult their primaries and secondaries or tail feathers in late summer. Therefore, almost any bird showing wing and tail moult in autumn is an adult. This means that, for example, distant Curlews, Common Buzzards and Ravens with symmetrical gaps in their flight feathers can be aged at considerable distances, even in flight.
When Lars Svensson first published his iconic Identification Guide to European Passerines in 1970, the ageing of passerines was purely the domain of ringers. In recent years, however, improved optics and the advent of digital photography has meant that field birders are in a much better position to try their hand at the subtleties of ageing. This may seem like a step too far, but the accurate ageing and sexing of a bird is as much a part of the identification process as its specific or racial identity. The following provides some tips on the moult and ageing of passerines, but it must be stressed that this is a difficult and complicated subject, bedevilled by significant individual variation.
Single moulters (acquisition of summer plumage by feather abrasion) Several passerines do not moult twice a year, only once. The most obvious examples are some of the chats and finches. In late summer, they lose their colourful summer plumage and acquire a plumage that is much drabber. This is because the colourful bases of the new feathers are obscured by paler and browner fringes. As winter progresses, these dull tips gradually wear off so that, by spring, their bright and colourful ‘summer plumage’ is revealed. Most of these species are resident or relatively short-range migrants, e.g. European Stonechat, Common Starling, Linnet, Chaffinch and Brambling.
Species that migrate in juvenile plumage A small number of passerines migrate while still in full juvenile plumage, their post-juvenile moult not commencing until arrival in the winter quarters. Some warblers fall into this category, including Sedge and Aquatic (but not Reed) and Hippolais warblers (Melodious and Icterine). Another bird that migrates in juvenile plumage is Common Rosefinch. Peculiarly, Red-rumped Swallow migrates in juvenile plumage, whereas ‘Barn’ Swallow has a body moult before it heads south.
Species that migrate in juvenile or first-winter plumage Some species vary in the acquisition of first-winter plumage. For example, young Richard’s and Tawny Pipits from early broods have time to moult into first-winter plumage prior to migration, whereas later broods may not. This means that many later-hatched individuals migrate while still in juvenile plumage. Others may suspend moult and migrate in a mixture of the two plumages.
Adults that moult in their winter quarters Some long-distance adult passerine migrants do not moult until they arrive in their winter quarters, the most obvious being hirundines (although some start their moult in Europe). Other species that follow this strategy include Reed Warbler and Spotted Flycatcher. Adult Spotted Flycatchers are in fact surprisingly rare in Britain in autumn, when they look decidedly worn and abraded compared to the more pristine first-winters.
Juveniles that have a complete post-juvenile moult A final group of exceptions includes a disparate bunch of unrelated species in which juveniles undertake a complete post-juvenile moult. One thing that they have in common is that most are sedentary, and because they do not migrate, they have time to renew all their feathers. These include the larks, Bearded and Long-tailed Tits, Common Starling, House and Tree Sparrows, and Corn Bunting.
Ageing first-winters Following post-juvenile moult, most young passerines are basically adult-like. Indeed, many, such as Phylloscopus warblers, are virtually impossible to age even in the hand. Some families of birds are easier to age than others and there are some useful pointers to remember. 1 BUFF TIPS TO THE GREATER COVERTS Many first-winter thrushes (and related species such as nightingales) retain buff tips to their juvenile greater coverts, forming a narrow wing-bar. It should be noted, however, that whilst the presence of such tips indicates a first-winter, their absence may not prove the reverse since first-winters may lose them through abrasion. Remember though, that Song Thrushes and Robins can show buff tips even in adult plumage. 2 DIFFERENT LENGTH GREATER COVERTS Some young birds moult their greater coverts in autumn and winter, the consequence being that their new adult inner greater coverts are longer than their old juvenile outer feathers, so there is a distinct ‘step’ between the two ages (often referred to as ‘moult limit’). This is particularly useful with thrushes and pipits. On thrushes, there is an additional difference in that the adult inners are fairly plain, whereas the juvenile outer feathers show distinct pale fringes. On larger pipits, such as Richard’s and Tawny, the adult inners are diffusely brown-fringed, whereas the juvenile outers are sharply fringed with white. 3 TAIL FEATHER SHAPE In some passerines adults and first-winters have different-shaped tail feathers: adults have broad tips with rounded corners, whereas first-winters retain more pointed juvenile feathers. This can be particularly useful for ageing well-photographed vagrants. 4 FAULT BARS Young birds in the nest sometimes undergo periods of food shortage, particularly during bad weather. This may manifest itself in ‘fault bars’ across the tail, which are inconspicuous but distinctive narrow bars across each feather, representing structural deficiencies brought about by poor diet. As adults re-grow their tail feathers in sequence, they do not show fault bars across the entire width of the tail (unless they re-grow a new tail after losing it through an accident).