Ageing Nuthatches (6 November 2004)
The last (4th, green cover) edition of Lars Svensson's Identification Guide to European Passerines was published in 1992 and is now out of print. He is working on a new, fifth, edition which will surely contain even more information as our knowledge of birds' plumage and of techniques for ageing and sexing continues to advance. One species which is often easier to age than Svensson states is the Nuthatch Sitta europaea. Indeed, the text in Svensson says 'no plumage differences discovered which are valid post-juv. moult of 1Y in Jun.-Aug.', so, based on that, almost all Nuthatches are aged as Euring 2 (age completely unknown) during September to December, and age code 4 (hatched before this calendar year) from January onwards.
In fact, many Nuthatches show a moult limit in their median coverts, and sometimes in the lesser (marginal) coverts, by which they can definitively be aged as 3/ 5 until they undergo their first full moult when they are a little over one year of age. Some ringers take Svensson to mean that ageing 'cannot' be done, so they do not look hard at Nuthatch plumage; indeed, for some birds the effect is quite subtle, and takes some experience to discern.
As with many plumage characteristics, the appearance changes during the year. Earlier in the year, when the feathers are fresh, the juvenile median coverts can often be seen to have buff tips, but they usually wear away during the winter. Two birds caught on 3 October 2004 in Delamere Forest, Cheshire, illustrate the effect. The first one has four old median coverts, with buff tips visible at least on the two outermost:
Most of us are not used to looking at the median coverts, moult limits usually occurring in the larger feathers of the greater coverts. Most passerines have eight median coverts visible. Structurally, they overlay each other in the opposite direction from the greater and primary coverts and the flight feathers: in the median coverts, the outermost - distal, farthest away from the body - feathers lie on top of the innermost - proximal, closest to the body - ones.
The second bird caught on 3 October 2004 has three old median coverts and one old lesser covert. As often happens when trying to see a moult limit, it can be easier to distinguish the contrast when the wing is tilted at a different angle to the light. As with this bird, the juvenile ('old') coverts do not always have buff tips, but often can be distinguished because the colour and texture of the feathers is a duller grey:
In spring the buff tips have usually abraded away and the moult limits are hard to find, but every now and again a bird comes along that is very obvious, such as one caught on 27 March 2004, also in Delamere Forest. The photo below shows that it has four old median coverts and some old lesser coverts, greyish rather than bluish in colour and much more worn than the innermost median coverts that were grown in the bird's post-juvenile moult in July-August 2003.
All of this is described, with photographs, by Lukas Jenni and Raffael Winkler in their classic book Moult and Ageing of European Passerines, published by Academic Press in 1994 and now, sadly, out of print. Not for nothing is Svensson's book known as "The Ringers' Bible", but Jenni and Winkler's book, for the more limited number of species that it covers, is probably even more useful.