Ageing adult Reed Warblers (22 September 2011)

For the last twenty years I have been ageing some adult Reed Warblers Acrocephalus scirpaceus as Euring code 5 or 6, since becoming aware of a paper from Falsterbo Bird Observatory, published in Vår Fågelvärld, available at http://www.falsterbofagelstation.se/arkiv/pdf/125.pdf. All Reed Warblers, first-years and adults, undergo a full moult in their African winter quarters, so there are no plumage differences and the key is to use soft-part coloration, specifically a combination of tarsus colour, residual tongue spots and mouth colour, and iris colour.

The photo-montage, of birds in August, shows the features for typical examples of birds aged 3, 5 and 6.

For readers unfamiliar with the Euring codes, 3 is a bird hatched during the current calendar year, 5 is a bird hatched the previous calendar year (SY (second year)) and 6 is a bird hatched at least two calendar years ago (ASY (after second year)).

• Leg colour in young birds (age 3) is bluish-grey, and gradually changes to become pinkish-flesh coloured (age 6). In typical one-year-old birds (age 5) the bluish-grey colour is present especially close to the joints and is less obvious in the middle of the tarsus.
• Tongue spots are prominent in young birds (age 3) with a pale yellow tongue and mouth. The mouth colour of adult birds (age 5 and 6) is a bright orange, usually brighter in 6s than 5s (although this may vary through the breeding season) and almost all one-year-old birds (age 5) show residual smudges on the backward forks of the tongue where the juvenile tongue spots have gradually faded.
• The iris colour of young birds (age 3) is a slaty-grey that gradually becomes more greyish-brown in adult birds, and in some age 6 birds the thin reddish ring is prominent, halfway between the pupil and edge of the iris.

Note that all of these changes are gradual, and not all of the features change at the same rate. The Falsterbo study was based on standardised photographs of known-age retraps, and the colour plates and table in their paper show the range of variation encountered. I recommend careful scrutiny of their paper, which has more detail than I have provided here.

I find the eye colour the most difficult of the three features, which may be because all of my birds have been viewed in natural light - usually early morning or late evening - whereas the Falsterbo Observatory team took birds into their well-lit laboratory. In addition to standardising the viewing conditions, a bright light helps in getting the birds to 'stop down' their pupils and show more of the iris. Some magnification (hand lens/ jeweller's loupe/ headband magnifier) can be helpful.

Note that the first sentence above says some birds: some can appear ambiguous and I would urge caution and that ringers should not strive for spurious accuracy. As in all ageing/ sexing, use a combination of characters, not over-relying on any one; use appropriate judgment and experience and don't expect to assign every bird a 5 or 6 and don't be afraid, if unsure, of ageing some birds as 4 (adults, hatched before the current calendar year). And, as at Falsterbo, any well-ringed site with Reed Warblers has a lot of known-age retraps, giving ringers the opportunity to build experience and hone their technique in applying the soft-part criteria.

For my own sites in north Cheshire, I have had 635 handlings of adult Reed Warblers in 1991-2011, excluding multiple handlings of a bird within a year, and 50% have been adjudged to be age 5, 18% age 6 and 32% were left undetermined as age 4. Two-thirds of those aged as 4 were in fact already ringed and I probably had a subconscious bias towards knowing that their ages had already been determined previously. Six retrapped birds have been aged inconsistently, all of them being thought to be age 5 when they were in fact age 6.

David Norman.