Spot the difference

A while ago I threatened to write something about Spotted Flycatchers. The time has come.

I was in the museum today; well, what else can you do when the forecast looks like this?

My visit was largely to look at something other than Spotted Flycatchers, but I had a bit of spare time and couldn't resist a rumage through the Muscicapa tray.

NB the bird front-centre is moulting its legs. More on that later.

But let's start at the beginning. Conventional wisdom states that both age-classes of Spotted Flycatcher (i.e. adults and young birds) have a complete winter-moult; thus, by the time they return to Europe in spring, they can't be aged using plumage. That was the view I held until spring 2010 when I caught a bird in Catalonia that appeared to have retained some wing feathers.

The primary coverts were old, worn and brown; S3 on both wings was completely shagged (technical term); and, critical to ageing the bird at the time, the retained carpal coverts were tipped pale, i.e. juvenile feathers. So it should be possible to confidently age the bird as a 2cy. The winter moult of Spotted Flycatchers might not always be complete after all!

I took a close look at a couple more Spotted Flycatchers that I caught during the same period of ringing. Several other birds also showed primary coverts that seemed old and worn.

A look through the literure showed that an incomplete winter-moult wasn't unknown. Svensson describes the moult of the species as: “Ad.: sp, WC; (SC*WC). – Young: sp, WC; (sp, WC—).” Basically, in English, that means adults usual have a partial summer-moult and a complete winter-moult but that some adults may start their complete moult in summer, suspend it during migration, and complete it in the winter; and most young birds have a partial summer-moult and a complete winter-moult but that some young birds may have a partial summer-moult followed by a "complete" winter-moult that is suspended before it completes (i.e. leaving some feathers unmoulted).

BWP says on the matter: “Adult pre-breeding: Suspension of primary and secondary moult sometimes occurs before spring migration. Some birds complete moult of primaries, secondaries, and/or tertials, suspended during preceding autumn migration" and "First adult pre-breeding: Complete, as adult pre-breeding. In winter quarters, but later, extending to April or May”.

Jenni and Winkler go into even more detail with their assessment of moult on spring birds. Here's what they found:

Extent: usually whole plumage. According to our observations on 145 spring migrants in Italy, MaC, MeC, CC, Al, R and P were always moulted.
S:10.3% retained one to three central S: eight birds S 2-3, four birds S 3 and one bird S 2+4, S 3-5 and S 2, respectively. Sometimes,  S6 appeared older than the other S.
PC: PC appeared older than the P in 54% of the birds and sometimes the difference in wear was very distinct. One bird had PC 2-5 older than the other PC and the P.
GC: one bird retained GC 6 and 8.
T: in many birds, the T appeared more bleached and sometimes distinctly more abraded than the other remiges, in others one or two T appeared newer than the other wing-feathers.

They then go on to discuss the subject: “It appears that S 2–3, which may be retained during the prebr moult, are precisely those occasionally moulted during the postbr moult. Thus it seems that an interrupted prebr S-moult is resumed during the postbr moult. Whether the S renewed during the postbr moult are regularly skipped during the prebr moult remains unclear.
“Those PC that in spring often appear older than the P are more difficult to judge. Since no precise information about the extent and timing of PC-moult is available[*], it remains unclear whether the PC of some birds are moulted much earlier, just after arriving in the winter quarters, than the rest of the feathers or whether they are not moulted at all during the prebr moult as suggested by their heavy wear.”

Williamson (1992) says that “Moult of the primary coverts does not appear to be linked with the corresponding remiges as it is in other passerines”. It's interesting to note that Jenni and Winkler add to this in their publication with “...but probably before or at beginning of P moult”. They do go on to say that “Two birds in P-moult, examined by [Jenni & Winkler] in the Vienna Museum, showed PC already renewed (P 9–7 and P 4–1 growing, respectively)” so it might be unwise to cast any aspersions on their remark; but taking the Williamson statement purly at face value, I think it's best not to make any assumptions — the birds examined by Williamson may well have moulted their primary coverts already, but then again they may not have and they may never had; either would still be classed as "not linked" with the corresponding remiges.

[*] I'll pause briefly there to have a moan about primary-covert moult. Ask most ringers for an assessment of a bird's moult and they'll tell you how many primaries it's growing. Some of the more thoughtful ringers might also tell you about the secondaries but that's generally about it. Even the BTO ringing unit (and I'm going to get my licence taken off me for saying this) seem to have little idea of the value of primary coverts moult; indeed, in the BTO's ringing data programme, when I enter primary moult score I get told the number of primary coverts must equal ten. Well, I challange anyone to find the 10th primary covert on a Greenfinch! But anyway, the point being that the moult of primary coverts is often overlooked; a bird in primary moult will often be noted as being "in complete moult", even if the reality is that the bird is not moulting the corresponding primary coverts. Thus data from African wintering grounds of birds having a "complete moult" may have to be taken with a pinch of salt; the reality may well be that the birds were simply moulting primaries.

And to hammer the point home, Jenni and Winkler finish their paragraph on pre-breeding moult with: “It would be interesting to investigate whether or not the state of the PC in spring is age dependent, because 1y/2y of a number of species (e.g. Lanius senator, Sylvia communis, Carduelis chloris, C. spinus, Loxia curvirostra) which moult P, usually do not renew PC.”

There's an advantage to being at Falsterbo. They have a trick up their sleeve when it comes to ageing: iris colour (a subject that I will say more on another time). There is, with practice and experience, a difference in iris colour between second-year and 3+cy Spotted Flycatchers. Thus, it's possible to age birds; and to age them independent of plumage features. This added some extra interesting information to the Spotted Flycatcher moult story, although it also raises further questions that would be interesting to try and answer. So, think of this as a work in progress; something to check if you're ever in the company of a Spotted Flycatcher; but be aware that some things are still unknown and that I might even change my mind about some of these things in the future!

A lot of the photos I'm going to use come from the Falsterbo Bird Observatory photo project. Many similar photos have been used in publications by FBO, which can be found on their website publication pages.

Bird 1: This bird was aged as a 2cy using iris colour. The primary coverts are brown and worn.

Bird 2: This bird was aged as a 2cy using iris colour. The primary coverts are brown and worn; S2–3 were not moulted during the pre-breeding moult.

Bird 3: This bird was aged as a 2cy using iris colour. The primary coverts are brown and moderately worn; S1–4 and P1–2 were not moulted during the pre-breeding moult. P3 appears fresher/newer than the other moulted primaries; the inner primary coverts appear contrastingly less worn than the outer primary coverts. 

Bird 3: This individual had also retained two tail feathers, R1(left) and R2(right); much of the retained R1 had broken away while the retained R2 is in extremely poor condition.

Bird 4: This bird was aged as a 2cy using iris colour. The primary coverts are brown and moderately worn; the innermost primary covert is blacker and fresher — there was no contrast in the primary coverts on the left wing.

Bird 5: This bird was aged as a 3+cy using iris colour. The plumage is of a single generation; the primary coverts are dark, rounded and fresh.

Bird 6: This bird was aged as a 2cy using iris colour. The primary coverts are moderately brown but do not appear too worn. S1–4 and median covert 3 were not moulted during the pre-breeding moult, though note the contrast between the feather generations in the secondaries is not as obvious as in e.g. Bird 2 or Birds 3.

Bird 6: This individual had also retained at least two upper tail coverts. Pale makings at the tips of the retained feathers have all but worn away. The pattern of these pale tips, and the quality of the retained feathers, points towards them being juvenile feathers.

For reference — moulted adult-type upper tail coverts:

 — fresh juvenile upper tail coverts:

So, what can we say about ageing Spotted Flycatchers? On all of the birds ringed this spring, ageing with iris colour reached the same conclusion as ageing with plumage — note my careful wording of that statement. If we put faith in iris colour as a valid ageing tool, it appears that second-year birds and 3+cy birds can be aged on the basis of plumage. However, I'm reluctant to say it's so simple. There are several issues that need answering before we can say anything for sure.

• Age ratio According to our ageing, only a few % of the 50 or so birds handled were 3+cy; the remainder were aged as 2cy. That seems an awfully skewed age breakdown. Maybe there are other factors at play? Perhaps 2cy are more likely to be grounded on their migration north, or similar, and thus more likely to be trapped.
• Primary-covert moult There was variation in the degree of wear/bleaching to the primary coverts. Is this because some birds keep the feathers in better condition than others? Or do some individuals moult the primary coverts whilst others don't? If the latter, are the moulted primary coverts a distinct and identifiable "first adult" type? There's quite a difference between the primary coverts of e.g. Bird 6 and Bird 2; but, as Björn Malmhagen said to me at the time, the primary coverts of Bird 6 "just don't give the appearance of being adult feathers". And he's right; there is still a clear difference between the primary coverts on Bird 6 and Bird 5.
• Genuine complete 1/2cy moult Even if we establish that we can age some birds as 2cy following an incomplete winter-moult, can we safely age them all using the same characteristics? If some first-winter birds can undergo a complete moult then plumage alone may only be safe enough to age birds as 2cy and 2+cy (though the above comments re: "first adult" plumage may come into play here).
• Incomplete adult moult On a similar note, how often do adults leave feathers unmoulted? Do they ever leave feathers unmoulted?
• Known age birds A critical point worth bearing in mind. So critical, in fact, I'm going to write it in block capticals. NONE OF THE BIRDS IN THIS BLOG POST ARE OF KNOWN AGE. It's easy to create a self fulfilling prophecy, though using two (apparently) independent criteria should at least lessen the chances if that happening. However, in order to come to a definite conclusion, ringed birds of known age need to be studied — a project for the future?

Finally, back to the museum. I hate museum specimens; they smell, they're stiff, they're often contorted or discoloured, and the wings are clamped tight shut. That makes it hard to examine them for signs of moult, especially so if you're checking the outer secondaries. However, there's certainly no easier way to get your mitts on 30 Spotted Flycatchers in the space of an hour or two.

Of the 30 or so skins I examined, I could find only one with any retained feathers; a bird with S2–3 unmoulted.

Can you see it?

That's not to say other birds didn't also have retained secondaries/primaries, just that I didn't manage to find any. For completeness, here's a shot of the same bird's primary coverts.

The other problem with museum skins is that the specimens are often old and brittle. Quite often you take a bird out of the drawer to find its legs fall out, or its wing flops about, or its head drops off and rolls across the floor. Remember the bird in the very first photo that was moulting its legs? It was also moulting its left wing... That did have an advantage, though; I could hold that bird's wing right up close to another birds wing to compare. The "moulting" individual had worn, brown, narrow primary coverts; the bird I chose to compare it with had fresh, dark, rounded primary coverts. Both birds were 'taken' in May so they should be comparable.

And for anyone who's made it this far down the page, very well done!

Many thanks to FBO for use of the images, to Tony Parker at National Museums Liverpool, and to Björn and everyone else at Falsterbo who fuelled my interest in the subject.

Maybe I should just stick with Woodpigeons...