Body moult (25 July 2004)

Much of the emphasis in studying post-juvenile moult is focussed upon the wing feathers, especially the coverts. Most species undergo an extensive body moult which can be a helpful guide to ageing birds, but is often overlooked. Of course, the most obvious examples of body moult are where a species has a distinctive juvenile plumage, such as the Robin Erithacus rubecula. At Woolston on 24 July 2004, in one round of the mist-nets I caught three Robins showing the range of body plumage: one that was fully juvenile (left), another roughly half-way through post-juvenile body moult (centre) and a first-year bird that had completed its body moult (right):

Such an extreme range of plumages can probably only be found simultaneously in a species like the Robin which has quite an extended breeding season, where most pairs are double- or treble-brooded. Although the first bird was wholly in juvenile plumage, both of the others had almost completed the post-juvenile moult of their greater coverts, retaining 5 and 6 old greater coverts respectively:

Both of these birds have grown 'adult' greater coverts that are plain brown, unspotted, contrasting obviously with the buff-tipped 'juvenile' coverts. However, some Robins grow 'adult' greater coverts that also have spots at the tip. Both of the "Ringers' Bibles" urge caution in use of this character. Svensson (Identification Guide to European Passerines) warns 'yellow tips to greater coverts variable', while Jenni and Winkler (Moult and Ageing of European Passerines) show that some adults have lighter coloured tips to all of their greater coverts. There are subtle differences in shade and shape of the tips between adults and immatures, but it is certainly not true that the presence of any buff tips has to indicate a first-year Robin.

Today (25 July 2004), at Oxmoor Local Nature Reserve (Runcorn), I caught a close relative of the Robin, also in juvenile plumage - a Redstart Phoenicurus phoenicurus. This is a new record for the site, although they breed about 10 miles away, as described previously. The bird provided a nice treat for the 20 members of the public attending our ringing demonstration. Its resemblance to the Robin is clear from the upper photo:

but even the juveniles have the eponymous red tail, and rump:

Before migration to Africa, this bird will moult its head and body plumage, the lesser and median coverts, and probably a small number of greater coverts.

Another facet of post-juvenile moult can be illustrated by the Wren Troglodytes troglodytes, known to North Americans as the Winter Wren. Amongst the groups of feathers replaced in the post-juvenile moult are the undertail coverts, and these can often be easy to distinguish. Unfortunately, some ringers home in on the undertail coverts as if they are the only diagnostic feature for ageing Wrens, and - as with spotting on the Robin's greater coverts - caution is needed in using just this one character. The following two photographs illustrate the differences. The upper image is a juvenile before its post-juvenile moult, photographed on 24 July 2004 at Woolston; the lower photograph, taken on 1 April 2004 at Norton Priory, Runcorn, shows the underparts of an adult Wren, Euring age code 6 (after-second-year):

The juvenile's undertail coverts are loose, fluffy, and a relatively dull brown in colour, with the tip a slightly lighter shade of brown; those of the adult are a more rufous brown with obvious white spots at the tip of most feathers. Svensson's statement that 'juvenile has undertail coverts almost uniformly brown, spotted white' is possibly confusing: amongst British Wrens, at least, white spotting of the undertail coverts is characteristic of adult birds, or immature birds after their post-juvenile moult.

In most passerines in Britain, the juvenile birds fledge with a minimum covering of feathers. Most will leave the nest with no underwing coverts, as shown in the image below of a House Sparrow Passer domesticus, a species which British ringers are encouraged to ring because it is on the BTO's Red List of conservation concern, having shown a UK population decline of more than 50% in the last 25 years. This bird, ringed on 16 May 2004, is part-way through growing its underwing coverts. It can also be seen that the second-from-outermost primary is still growing, with the sheath visible near to my finger-tip. This characteristic led to my ageing it as a '1J' - fledged, but flying so weakly that it is obviously incapable of having flown far from the nest - the category known as 'Local' in North American banding.

One further aspect of body moult is the bare belly of juvenile birds. Typically for the first month or more after the bird leaves the nest, its belly is bare, rather hard and shiny. The composite image below shows the underside of three Blackcaps Sylvia atricapilla, with the feathers parted to expose the belly. From left, they are an adult female (17 May 2004), an adult male (17 May 2004) and a juvenile (23 July 2004).

The Sylvia warblers can be amongst the trickiest of species in which to distinguish the sexes/ ages from their bellies, although the cloacal shape is diagnostic, because the males take a significant part in incubation whereas in most species the female alone broods the eggs or chicks. These photos show clearly the heavily vascularised brood patch of the female, the most efficient at getting the necessary heat to the growing embryo in the egg. The male's bare patch is less extensive and less well supplied with blood vessels near to the surface, and shows his prominent cloaca. A few tufts of feathers are usually visible on the belly or chest. The juvenile's belly resembles that of an adult male but is less wrinkled. The appearance of the adults' bellies changes through the breeding season, and this characteristic needs some practice and experience in its application.


David Norman.