Tuesday 17 December 2013

Curlews, age and moult

4th-14th December 2013 - East Devon

A Warning: to those who think gulls are beyond the pale, look away now because I'm going to write about ageing and moult in waders. I should also add, in case it's not obvious, that I'm no expert; as with my posts on gulls, you could say that 'I know just enough to be dangerous'.

As ever, what started out with 'I wonder if that's...' turned into a major digression and an excuse to not do anything useful. While waiting for good photo opportunities with my local wintering Water Pipit or a co-operative Yellow-legged Gull, I can always find something of interest. So this week it's been mostly Curlews. I see Curlews everyday in Topsham and only bother to point my camera at them if they're fairly close. Looking at some recent photos I wondered if I could tell anything about age or moult state so I dug out a few books and did a little research. I'm going to give references so: a). it appears more authoritative and, b). it'll be obvious if and where I've gone wrong.

Like most northern hemisphere breeding waders, adult Curlews undergo a complete moult following breeding; this is typically complete by late November. Juveniles have a partial moult which involves replacing more or less all head and body feathers and scapulars as well as sometimes a few tertials and lesser coverts (Cramp et al 1983). So in December there will be two age classes of Curlew, adult and first winter. Even without age-related differences in colour or pattern, it should be possible to pick out birds of the year, which are still carrying worn juvenile coverts and flight feathers, from freshly moulted adults.

To make things a little easier, there are some plumage differences between juvenile and adult, although this could be better covered in even the best field guides. The Collins Bird Guide (Svensson et al 2009) illustrates a juvenile but only mentions 'finely streaked on breast-sides, not so spot-barred as adult'. Lars Jonsson's Birds of Europe (1992) illustrates a first winter but the text does not describe plumage differences. Looking at The Handbook of Bird Identification (Beaman and Madge 1998) actually confuses me; 'juvenile...less striped tertials and scapulars (having wider and less defined dark brown, rather than black, dark centres)'. A juvenile is depicted but lacks detail and appears identical to the adult. Help is at hand though in the BTO Guide to the identification and ageing of Holarctic Waders (Prater et al 1977). Here we're told that juvenile scapulars are 'brown, heavily notched buff'...whereas adult non-breeding scapulars are 'brownish with indistinct grey-brown barring'. There are similar differences between juvenile and adult coverts and tertials. The same authors in Shorebirds (1986) describe juvenile coverts, scapulars and tertials as having 'contrasting brown feather centres and buff notching'.

Here's an adult Curlew, probably a male based on the unexceptional length of the bill. All the scapulars, coverts and tertials are fairly fresh; the feathers are basically grey-brown with darker centres and diffuse bars. The visible primaries show pale rounded tips, these are freshly grown feathers.

Curlew adult - Otter Estuary 14/12/20
Curlew adult - Appledore 11/9/2013

The second bird - from September - is in primary moult so on that basis alone has to be a second year or older. From what I can see, all the scapulars and coverts are grey-brown and the pattern is essentially barred with broad dark feather shafts and some pale and diffuse fringing/notching. As to the bird's sex, the bill is quite long so it's probably a female.

Next is another Curlew, most likely a male, photographed in early December. It looks pretty similar at first glance but a closer look shows a few differences.

Curlew -Topsham 4/12/2013

Here's a cropped enlargement with the relevant feathers and feather tracts labelled.

Curlew - feather tracts

There are a few new and at least one old tertials, at least three new and one obviously old greater coverts, and at least three new and a few old median coverts. The scapulars look quite uniform, all dark centred with distinct buff notches and quite different to the two previous birds. This mixture of old and new coverts in December - when adults should have finished their complete moult - suggests that this bird is a juvenile. The older and more worn feathers (most obvious on the old tertial) show more of a notched pattern than the barred adult winter type newer ones. The scapulars appear to be all of the juvenile type, notched with buff rather than barred grey and brown - this pattern is actually similar to adult breeding plumage but on moult alone this bird must be a juvenile.

There's something else to notice, just visible beyond the longest tertial and above the tail is a blackish primary tip; this wing projection looks much shorter than on the adult above. Is this due to moult - the outer primaries still growing?. Here's a photo of the same bird in flight and P9 and 10 do look a bit broken to me, P8 appears to be undamaged and is quite pointed, a juvenile character (Prater et al 1977).

Curlew -Topsham 4/12/2013

I admit though that I'm still a little puzzled by this bird, there seems to be a discrepancy between the upperparts and the underparts. On balance it's a first winter - partial moult, juvenile type scapulars, some retained juvenile type coverts and tertials, and pointed outer primaries; but it also shows (perhaps newly moulted?) adult type breast and flank feathers which are barred and spotted rather than streaked. And if it's a first winter, why has it apparently moulted no scapulars yet?

As ever, comments welcome; as I keep saying, I'm no expert.


Beaman, M & Madge, S 1998 The Handbook of Bird Identification. London.
Cramp, S et al  1983 The Birds of the Western Palearctic. Vol 3. Oxford.
Jonsson, L 1992 Birds of Europe. London
Marchant, J 1986 Shorebirds: an Identification Guide to the Waders of The World. London.
Prater, AJ, Marchant, J & Vuorinen, J 1977 Guide to the Identification and Ageing of Holarctic Waders. BTO Guide 17: Tring.
Svensson, L et al 2009 Collins Bird Guide. 2nd edition. London.

No comments:

Post a Comment