Which warbler species - or even genus?

By Dominic Mitchell

Mystery warbler on Corsica: can you name the species?

Sometimes it’s impossible to be certain of an identification in the field, even when prolonged views facilitate careful observation of the bird in question. In such situations, obtaining a comprehensive series of images may allow the ID to be resolved subsequently.

An excellent recent example of this involved an intriguing warbler found during a Bird Holidays tour to Corsica in May. The group was birding at Ile Rousse on the north-west coast of the island on 28th when they came across the mystery migrant. According to one observer, “there was no tail pumping and no call (although we only had 15-20 minutes on the bird). [It] always looked large, beaky and pallid, and we didn’t really pick up any rufous tones in the upperparts. It was quite a showy bird (for a warbler), with fairly clumsy movement through a small area of bushes and limited skulking – unlike most ‘Acros’ out of habitat on passage.”

He continued: “Although our initial thoughts were of an Olivaceous, there have been plenty of niggling doubts (especially with the long undertail coverts), and none of us who watched the bird, or any who studied the photos subsequently, have been 100 per cent happy with any firm ID. Western Olivaceous must be a vagrant to this area …

“A couple of things niggle me slightly; the photos seem to show a slight olive tinge to the upperparts, although this could be just the light conditions. The wings do seem to have some pale edging to the secondaries. However, comparing it to a definite Eastern from the same time of year … the bill shape does look better for Western (I think!).”

These comments and images were sent to Lars Svensson, who responded: “I, too, felt it looked plain and rather pale at first, leading my thoughts towards the Iduna species, in particular to Isabelline Warbler (‘Western Olivaceous’, this patently dull alternative name). And there seemed at first to be more than one primary emarginated on its outer web (which would make Reed and Marsh Warblers rather unlikely). But the more I look at it, the more I feel that other clues are more important.

“I can see no hint of paler outer edges or tips to the outer tail feathers, features which I think should have been visible to some extent on at least some of the images. This would speak against Iduna. I also sense there is a little too vivid tawny-ochre or even pale rufous tinges in the plumage for Iduna opaca. This would fit better with Reed Warbler. (But rules out a May Marsh Warbler, then invariably showing subtle olive tinges on mantle.)

“The primary projection is also a bit long for any of the Iduna spp. And the wing-tip seems to be made up of p3, with p4 a little shorter. In opaca this would be wing-tip pp3-4 with p5 slightly shorter. Also, an Isabelline would have had deeper emargination of p3, halfway in from tip to primary coverts, but on this bird the emargination falls opposite the tips of the tertials and secondaries, which fits best really with Reed Warbler. I don’t think there is more than a hint of a narrower web on p4, thus not a proper emargination (but this particular feature is difficult to assess) ... All considered, probably a slightly odd Reed Warbler.”

The Corsica warbler: annotations © Killian Mullarney

Eastern Olivaceous Warbler for comparison. © Killian Mullarney

The same images and observer comments were sent separately to Killian Mullarney who, having studied them, replied: “The impression I had of this bird was that it looked more like an Acrocephalus than an Iduna sp. However, pin-pointing precisely what creates this impression is the difficult part! I spent quite a bit of time analysing the photos … and comparing some of the critical details with my own photos of Eastern Olivaceous Warblers (a species I have gained a lot of good experience with in the course of regular trips to Lesvos over the past decade or so) and Reed Warblers.

“It is more difficult to resolve the really fine detail of primary edges and emarginations on the off-net photos than in my own shots [of other birds] because of the big difference in resolution. Still, I think we can make out enough to be reasonably confident of the bird's wing formula. Before I go into this I'd like to make clear that my instant impression that the bird looked better for an Acrocephalus had very little to do with the tiny details that I have concentrated on in the annotations. In reality, just as generally happens in a field situation, it is an accumulation of numerous (mostly indefinable) subtle and subjective features that make me think something is an 'X', or a 'Y'. However, as so many of these features are somewhat variable, subjective and show a considerable degree of overlap it really isn't much help to try to 'deconstruct' the bird in this way and make a case for an identification. It might work a little better in conversation, while looking at photos together, but it just takes too long to do in correspondence.

Problem birds require critical analysis. Annotations © Killian Mullarney

Looking good for Acrocephalus ... Annotations © Killian Mullarney

“I believe the Haute-Corse bird's wing structure clearly points to it being an Acrocephalus, as opposed to any species of Iduna. The original photos would probably establish this even more conclusively, but note that the wing point is p3 (it would be 4, or equal 3 and 4, in Eastern Olivaceous), and only one primary (apparently p3) is emarginated. In Eastern Olivaceous, 3, 4 and 5 are emarginated. Incidentally, with these small Acrocephalus especially it is often very difficult to see outer edge of p2, as it tends to lie very close to p3 and have a less pronounced pale edge that is easily lost if the resolution isn't pin-sharp. Likewise, p1 is often very difficult to see on Acrocephalus, because it is so short, whereas in good photos of Eastern Olivaceous it can be quite prominent and easy to see.” Killian’s annotations to the photos of the Haute-Corse bird illustrate the key ID features.

He added: “In addition to the wing formula, I think the overall tone and areas of contrast in the wing are distinctly Acrocephalus-like, lacking Eastern Olivaceous's somewhat lighter-edged greater coverts and remiges. Also, to my eyes, the undertail coverts are rather long and seem buff washed, pointing to Acrocephalus.”

Note: Iduna is a resurrected genus supported by some authorities for certain warbler species traditionally placed in the genus Hippolais.

Acknowledgements
Thanks to Paul Willoughby of Bird Holidays (see birdholidays.co.uk) and participants in the Corsica May 2011 tour for permitting the use of the images and comments featured in the post.

Identifying juvenile Yellow-legged Gulls

By Keith Vinicombe 

Juvenile Yellow-legged Gull (Rainham landfill, Greater London,
16 August 2011). Photo by Dominic Mitchell.

Identifying immature large gulls is not everybody’s cup of tea, but there is an annual influx of Yellow-legged Gulls Larus michahellis in July, mainly into southern Britain. The species was finally split from the familiar Herring Gull L argentatus in 2006, although most birders probably had it on their lists for a couple of decades before this!

Yellow-legged Gull is a common and familiar sight in the Mediterranean, and these large gulls have spread up the west coast of France and the odd pair now even breeds in southern England. If seen well, adults are readily separable from Herring Gulls by their obviously darker grey mantle and yellow legs. Before tackling juveniles, it is advisable to get to know the adults, so that you gain a good ‘feel’ for the species.

The Yellow-legged Gulls that arrive here in July are usually large, heavy and sturdy birds, with a large bill with a distinct gonydeal angle on the lower mandible. The males in particular can be very large and often approach Great Black-backed Gull in size and overall bulk. In fact, a few days ago, I actually watched one persistently displaying to a Great Black-backed! When scavenging on dead fish – a favourite food source – they are very much second in the pecking order behind Great Black-backed and dominant over both Herring and Lesser Black-backed Gulls.

Like Great Black-backed Gull, large male Yellow-leggeds appear heavy and ponderous in flight. Females, however, are smaller, as are Yellow-legged Gulls from western Iberia, which appear in south-western England later in the autumn.

In flight, Yellow-legged Gull has very long and pointed wings compared with Herring and, on inland reservoirs, they can often be picked out at some distance – even with the naked eye – as they patrol up and down on arched wings, about 30-50 m above the water’s surface, looking for fish.

Juvenile Yellow-legged Gull

Juveniles are much more difficult to identify than the adults, but the following photographs should shed some light on their identification. Simply look at the images and soak in the plumage and structural differences.

Juvenile Yellow-legged Gull (Chew Valley Lake, Somerset, 21 July 2011).
Photo by Keith Vinicombe.

Plumage-wise, the following are the main points to note:
• Yellow-legged is more similar to Herring than Lesser Black-backed in overall tone, being brown with whitish feather fringes.
• Note the very white background colour to the head, neck and breast.
• Note also the grey ‘mask’ extending back through the eye. This can be very obvious on some individuals and, when combined with the white head and neck, it can create an impression distinctly reminiscent of a juvenile Mediterranean Gull.
• The entire upperparts are mid-brown, with clear-cut whitish feather edges. In particular, there is little chequering compared with juvenile Herring Gull (see photo below).
• Note the plain brown tertials, with a neat white fringe to all the feathers. The tertials are much more similar to those of juvenile Lesser Black-backed Gull than juvenile Herring, which shows heavy chequering around the edges. Note, however, that some juvenile Yellow-leggeds do show pale ‘notching’ or chequering towards the tips of the tertials.
• The greater coverts – the band of large feathers that runs along the bottom of the closed wing – have plain dark bases, particularly towards the front of the bird, this forming a dark bar. Two points to be aware of: firstly, it should be noted that the greater coverts are often hidden by the fluffed-up flank feathers; and secondly, many juvenile Yellow-leggeds show quite noticeable chequering on the inner greater coverts (those towards the tail end of the bird).

In flight, three features are worth remembering: first, the pale ‘window’ on the inner primaries is less obvious than on Herring, but much more obvious than on Lesser Black-backed; second, the upperwing coverts contrast quite strongly with the black secondaries and there is often a ‘second’ darker bar along the outer greater coverts, in front of the secondary bar; and third, the underwing coverts are dark brown, more like Lesser Black-backed Gull than Herring. The following photograph of a first-winter Yellow-legged Gull (taken in March) illustrates these points.

Yellow-legged Gull (Chew Valley Lake, Somerset, 30 March 2011).
Photo by Keith Vinicombe.

It is also worth remembering that, being a more southerly species, Yellow-legged Gulls breed earlier than Herring and Lesser Black-backed Gulls. Consequently, being more advanced, they moult earlier at all ages. The settled bird shown in the first image above (photographed on 21 July) has already acquired a few darker and greyer first-winter scapulars.

Juvenile Herring Gull

The juvenile Herring Gull below (photographed in September) is similar in plumage tone to Yellow-legged Gull, but note in particular the heavily chequered wing coverts and the chequered fringes to the tertials. Also, it lacks the dark bar across the outer greater coverts, shown by Yellow-legged.

Herring Gull (Praa Sands, Cornwall, 15 September 2009).
Photo by Keith Vinicombe.

Herring Gull is much more heavily and uniformly streaked across the head, lacking the striking white background colour and the dark mask of Yellow-legged. In addition, it is a slightly smaller-billed and more compact bird, its shorter primaries producing a somewhat truncated rear end compared with the long-winged Yellow-legged Gull. In flight, it is a rather stocky, relatively short-winged gull with noticeable pale inner primary ‘windows’.

Lesser Black-backed Gull

Young juvenile Lesser Black-backed Gulls are dark and rather smoky looking compared with both Herring and Yellow-legged. They are densely streaked and the feather fringes are darker and buffer, contributing to the overall darkness and uniformity of their plumage.

Lesser Black-backed Gull (Chew Valley Lake, Somerset, 29 July 2011).
Photo by Keith Vinicombe.

Note in particular that, like Yellow-legged Gull, Lesser Black-backed has plain tertials with a clear-cut pale fringe (if present, any ‘notching’ is slight and confined largely to the tip). Like Yellow-legged, Lesser Black-backed also tends to show a dark bar across the outer greater coverts.

On average, Lesser Black-backed Gull is the smallest of the three species, with a shorter, weaker bill and a long, slim and rather attenuated appearance; the latter is caused by the long primaries – a consequence of the fact that, unlike Herring Gull, Lesser Black-backed is a long-range migrant, with most heading down to western Iberia and north-west Africa for the winter.

In flight, Lesser Black-backed Gull has long, slim wings – producing rather a slim, narrow-winged, rakish impression – and juveniles appear particularly dark and smoky. Juvenile Lesser Black-backed is, in fact, much easier to identify in flight than at rest. Note in particular the very dark underwings (with only a faint pale window) and very uniform upperwings. Most importantly, the black of the upper secondaries extends right across the inner primaries to the outer primaries, forming a completely dark rear to the upperwing. Juvenile Lessers are also darker and more heavily marked on the rump.

Common and Arctic Terns in spring and summer

By Keith Vinicombe

The separation of Common and Arctic Terns has traditionally been regarded as problematical, despite some landmark identification papers in the 1960s and 1970s.

Common Tern (location unknown, 14 July 2007). Think of Common as slightly larger, sturdier and longer winged than Arctic, without the very long-tailed impression of the latter. Photo by Steve Young.

Arctic Tern (location unknown, 23 June 2009). Arctic is narrower in the wing and longer in the tail than Common, and the underwing is very white. Photo by Oliver Smart.

Plumage
One of the easiest ways to separate adults is by the pattern of the upper primaries, this being dependent on two facts: first, on Common Terns, as the grey primaries wear, they become darker; in fact they are quite blackish when heavily worn; and second, Common’s inner primaries are moulted twice a year, whereas the outer primaries are moulted only once. This means that, in late summer, most adult Common Terns show an obvious contrast between the old, dark, blackish outer primaries and the newer, fresh, pale grey inners.

In contrast, adult Arctic Terns moult all their primaries only once a year (on their winter quarters) so that, when in Britain, the upper surface of all their primaries is uniformly pale grey.

The problem is that this difference is much less obvious in spring and early summer, when Common Tern shows much more uniformly pale grey upperwings. However, even then, there is still a contrast between the darkest of the old outer primaries and the adjacent grey inners. This manifests itself as a small dark wedge in the centre of the primaries.

This may be obvious, facilitating easy separation, but it must be stressed that it can be hard to see, particularly at a distance or in bright light. Furthermore, the problem of seeing the wedge can be particularly acute on spring seawatches, particularly when distant migrating terns are whipping past at some speed low over the waves.

But it can also be surprisingly difficult to see on mid-summer Common Terns flitting around over inland lakes and reservoirs. The upshot of this is that the potential for misidentifying adult Common and Arctic Terns is far greater in spring and summer than it is in the autumn.

Size and structure
The best way around this is to really look closely at any close-range spring or summer ‘Commic’ Terns that you see, so that eventually you develop a good ‘feel’ for them. As you become familiar with them at close range, you will gradually become much more confident with them at long range. At Chew Valley Lake, we regularly separate feeding birds at ranges of half a mile to a mile. However, we do have the advantage that inland birds usually come closer, so that their identification can be confirmed.

Compared with Arctic, I always think of Common Terns as slightly larger, longer-necked, heavier, sturdier, longer-winged birds with an easy, languid flight action. Although long tailed, they do not have the very long-tailed impression of Arctic Tern. To put it simply, Arctic Tern – with its shorter neck, rounded head, shorter, narrower, more sharply pointed wings and long tail – really does look Swallow-like in shape, an analogy that is not nearly as appropriate for Common Tern.

Another useful difference is that the narrower-winged Arctic Tern really does look ‘Persil white’ on the underwings, with a thin, but clear-cut black trailing edge to the primaries. In addition, when viewed against the light, all the under primaries are translucent. The underwings of Common Tern are a less pure white, with something of a slight silvery effect. Most importantly, the dark trailing edge to the under primaries is obviously thick and diffuse, readily apparent at some distance. When viewed against the light, only the newer inner primaries appear translucent.

Flight action
When feeding over fresh water, Common Tern has an easy flight action, whereas, to quote Rob Hume (British Birds 86: 217): “Arctic is more bouncy and at the mercy of the wind. Common has a fast, powerful downstroke, but a faintly lumbering look. Arctic is fluttery and butterfly-like, with a quick, snapped upstroke and a slower downbeat – it is easier to see the downstroke than the upstroke on an Arctic instead of the other way around.

For birders in the south, it must also be stressed that Arctic Terns are much less regular than Common and, as a consequence, it is a fundamental mistake to assume that you will be encountering the two species in equal numbers. Although in spring and autumn some Arctic Terns may head overland or tag on to flocks of migrating Commons, their more pelagic lifestyle means that they often turn up after strong westerly winds, particularly in September.