Feather wear in warblers (22 June 2005)
The abrasion on birds' feathers can be a helpful ageing tool for ringers. To make use of it, we have to know how old the feathers are (how long it is since they were grown; i.e. the species' moult strategy) and how quickly that species' feathers tend to wear. We have to be careful in using this characteristic because the feathers of different species abrade at different rates, partly because of variations in behaviour - birds that creep through the undergrowth tend to wear more than those that are aerial feeders, for instance - and partly because some species' feathers seem to be higher quality than others'. There are also individual differences between birds.
It can often be helpful to "get one's eye in" during spring, before the young birds are about. All of these warbler photographs were taken in spring at a variety of sites in north Cheshire, England.
Sedge Warblers Acrocephalus schoenobaenus from Britain undergo a complete moult in west Africa, starting soon after they arrive in October or November. All birds, first-years and adults, have a complete moult in their winter quarters, and the species cannot be aged by plumage after their first year. There can, however, be substantial differences between individual birds, as shown for instance in the tertials of these four adult Sedge Warblers caught within a couple of hours of each other on 10 May 2005:
The variation in the wear of the feathers is striking. The right-hand bird is amongst the most worn that can be found whilst the bird at the left seems to have almost-new tertials. The bird third from the left appears to have replaced the tertials on its left wing, but not on the right. The tertials are usually amongst the first feathers to be moulted, so by mid-May they would already be more than six months old, and have experienced several months of bleaching African sun and a 4,500 km spring migratory journey.
From the look of some of these birds, I now wonder if some Sedge Warblers moult their tertials twice. Perhaps there is a difference between some first-year birds and adults? More detailed work on the wintering grounds is needed to discover more about their moult.
The images below are the full wings of the two left-hand birds from the four above. The colour differences are caused by differences in the early morning sunlight, but the main point is to show the moderate wear of the tips of the primary feathers.
First-year and adult Garden Warblers Sylvia borin also moult in Africa, although a few examples have been found of birds starting moult in Britain, and there have been suggestions that this might be becoming more common. Or is it that more ringers are looking more carefully at birds nowadays?
This bird's wing appears to be in almost perfect condition: the tips of the feathers showed almost no abrasion when it was photographed on 15 May 2004.
The moult timing and duration of British Garden Warblers appears to be unknown. They winter in west Africa, probably close to the coast especially in Ghana. The BTO Moult Guide (Moult in Birds, by Howard Ginn and David Melville, BTO Guide 19) summarises the moult of birds wintering in Uganda, from breeding populations in eastern Europe: these birds moult relatively late in winter, finishing from February to early April. They take 60-70 days for the full moult, and this relatively extended period perhaps enables them to grow feathers of especially high quality.
Just to show that feather wear is not simply a matter of abrasion against vegetation, however, the real specialist at creeping through the undergrowth, the Grasshopper Warbler Locustella naevia, has relatively fresh plumage.
It is difficult to believe that this bird's feathers were moulted in October/ November, in west Africa, so, when photographed on 18 June 2005, they were probably eight or nine months old, and had undergone a 5,000 km migratory journey.
This bird appears to have one fresh secondary feather (s5), perhaps grown adventitiously following accidental loss.
Adult Whitethroats Sylvia communis moult in Britain, after breeding and before migrating south. The following two images are of the wings of two adult Whitethroats (Euring age 6, after-second-year) caught on 10 May 2005. The tips of the primaries are abraded and bleached, while the tertials - which often are moulted again in Africa - appear relatively fresh. At this time of year most of their feathers are about nine months old and have undertaken two migratory journeys, as well as all the wear-and-tear of the winter.
Whitethroats occasionally suspend moult and complete in the winter quarters. One that looked as if it might be going to interrupt its moult was depicted on these pages on 20 July 2003.
There is no complete winter moult, so first-year birds keep the feathers that they grew in the nest until they are about 14 months old, after they have bred. Their flight feathers can appear very abraded: unfortunately I have no photograph for comparison.
The Willow Warbler Phylloscopus trochilus is the only British species that normally undergoes a full moult twice a year. The adults moult quickly soon after breeding, in July and August, as illustrated previously on these pages (see index to bird-in-the-hand for details). The feathers grown then are clearly not of very high quality and adults and first-years undergo a complete moult in Africa. This moult usually occurs quite late in winter, finishing in March or April just before the birds start their northward spring migration. But neither are these feathers, apparently, particularly strong and by the time of their arrival in Britain they can already be showing signs of wear, as for instance with this bird caught on 15 May 2004 in which the tertials and primaries show significant abrasion:
Ringers catching birds in spring and starting early in the morning can often encounter frost, as on these Alder Alnus glutinosa and bramble Rubus fruticosus plants on 10 May 2005:
This brings a reminder that some people think of our summer visitors coming here because it is warm - after all, it is the warmest part of our year in Britain; however, for the migrant birds, this is the coldest part of their annual cycle. They come here to exploit the long day-length and flush of insects that are ideal for raising their broods. Enjoy them whilst we may: in a few months' time they will fly south again.