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.