Ageing 'adult' Sparrowhawks (3 April 2010)


A recent posting on the btoringers e-mail forum made me realise that the wording in Baker's Identification Guide to European Non-Passerines (BTO Guide 24, 1993) might be open to misinterpretation on how to age second-winter Sparrowhawks. It says 'In early spring some, but not all individuals may still show some fawn or chestnut fringes to brownish feathering on upper rump, or lesser coverts'. I think that the crucial part of that sentence is 'brownish feathering' (i.e. juvenile feathers) rather than 'fawn or chestnut fringes' (which may be present on adult-type feathers).

An adult male Sparrowhawk, ringed on 3 April 2010 at Oxmoor Local Nature Reserve, Runcorn, Cheshire, helps to illustrate the points, in combination with other images elsewhere on the MRG 'bird in the hand' pages.

Its iris was more orange than that of the second-winter male bird depicted previously, although the eye colour may vary somewhat between individuals, and is not, on its own, diagnostic for ageing.

All of the feathers on his wing and mantle were deep bluish-grey, with all of the secondaries of the same generation (none retained from a previous moult), and none of the odd brownish (retained juvenile) feathers that would indicate a second-winter bird (in this case, a Euring 7, hatched in 2008). These feathers often take some finding, but are most often on the rump or between the coverts and scapulars.

The feature which I suspect can cause confusion is that virtually all of this bird's lesser coverts showed chestnut fringes, similar to the colour of the sides of his neck and the barring across the breast. But the key to the ageing is that the feathers are grey, not brownish: this is an adult, not a second-winter bird.

So, it seemed rather straightforward to age this individual as 'adult', Euring code 8 (hatched before 2008).

However, even this bird had a sting in the tail! Its outermost rectrices, symmetrically on both sides, were from a previous generation. Unfortunately the bird was being somewhat uncooperative when I tried to take these tail photographs, and I could get usable images only of the left half of his tail.

Looking at the underside of the tail can be particularly instructive in identifying different generations of feathers, as the different pattern of barring is rather easier to see than on the upperside of the tail. The outermost rectrix on each side clearly has a completely different pattern from the other ten tail feathers.

I have no explanation for why its left-hand outermost rectrix appears to be pointed while the right-hand one is more rounded, but the key to ageing is that these 'old' outermost rectrices are greyish, not brownish: they are not retained juvenile feathers but were grown in a previous moult. Perhaps the final piece of the ageing jigsaw is that the 'old' rectrices are somewhat longer than the 'new' ones. Juvenile Sparrowhawks, like most raptors, have shorter primaries and longer secondaries and tails than adult birds. Detailed studies (e.g. I. Newton, M. Marquiss & A. Village, 'Weights, breeding and Survival in European Sparrowhawks', The Auk 100 (1983) 344-354) have shown that Sparrowhawks' (primary) wing length increases between year 1 and year 2, and again between year 2 and year 3, and I suspect it is likely that tail length similarly decreases in the first three sets of feathers. Therefore I feel it is likely that the 'old' outer rectrices were grown in the bird's first complete moult in 2008, and are now about 18-21 months old. This deduction would mean that the bird was hatched in 2007, and should be given the age code '9'. But no, I was not that brave and it is entered into IPMR as '8 male'.

David Norman.


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