baeticatus-like Warblers at Llobregat Delta

“Dreams are the genus; nightmares the species.” – Jorge Luis Borges

It’s been already 5 years with the same idea hanging around in my head. It was in summer 2008, already in July but after a memorable spring migration plenty of eastern migrants. Joan Castelló caught an Acrocephalus warbler at Reguerons, Llobregat Delta, that immediately struck him as being something different. His first impression while extracting the bird from the mistnet was of a cold-toned reed warbler, very small. Later on, already taking the measurements, he thought about Blyth’s reed warbler, especially due to the position of p2. The only problem he could find was, as always, the lack of emargination in p4. The bird showed a hint of emargination, but this is usually present in the subsequent primary to the one emarginated, due to a higher exposure of this part of the feather and therefore a differential level of wear.

We were afraid of the possibility of a Blyth’s with a non-emarginated p4, so we asked both Killian Mullarney and Anthero Lindholm, who kindly replied giving extra reasons for ruling out dumetorum. Both agreed the edges of GCs and the centre of the alula were too warm, the supercilium too colorful and the primary projection too long.

Fine, the bird was not a Blyth’s, but, then, what was it? Neither Joan nor me were happy with just “2cy+ Reed warbler” and, during the last 4 years I’ve been looking at the pictures again and again, without reaching any conclusions. However, the Acro world has been shaken in the last months, especially in its southern boundary. News coming from North Africa talk about resident populations of African reed warbler Acrocephalus baeticatus breeding north of the Sahara, as close to Llobregat Delta as Morocco and Tunisia. By that time this possibility was not taken into account but it’s now worth-considering.

The overall appearance is indeed that of a cold-toned acro, especially the head and the nape but also the upperparts. After a spring ringing in Falsterbo, Sweden, I am aware about how grey a Reed warbler can be, but Iberian birds are much warmer. In that sense, this bird was quite different from the ones usually caught in spring along the Iberian Mediterranean coast. The eyebrow was quite indistinct, something that we knew didn’t fit Blyth’s, and there is a noticeable open-face impression. The bare parts were also striking. Admittedly, the bill is not as dark as described for nominal baeticatus, especially in the under mandible, but the shape is more triangular than in a typical scirpaceus. The legs are much darker, almost even darker than in a juvenile Reed, with a more contrasted yellow sole. These legs would be, in fact, the darkest I’ve ever seen in a 2cy+ Reed.

acrocephalus sp3

The biometrics are the most striking feature. The wing length was 62, in the lower end of Reed warbler range according to Svenson. P2 is surprisingly short, falling between p5 and p6, far from the typical p3-p4 of scirpaceus. The emargination in p3 is very long, taking up 2/3 of the visible part of the feather. Moreover, the notch in p2 is also long, ending below the secondaries; according to Jiguet et al. 2010, the notch should end below p8 in a Reed warbler.  Finally, the wear of the primaries indicates that the tip is p4 and not p3 as typical in scirpaceus.

acrocephalus sp
acrocephalus sp2

All these features fit African reed and rule out Reed but, as always in the acro world, is not that easy! The most misleading thing is that those short-winged birds are quite common in the Mediterranean and Southern Iberia. I’ve caught birds with a wing length of 58 and there are several claims of Blyth’s reed warbler in Southern Spain that were identified based on biometrics. I’ve heard that some of those birds didn’t match all the features for Blyth’s, but this species was the most similar. I wonder if those ringers, as we did 4 years ago, forgot about baeticatus and, lost in Svenson’s bible, didn’t manage to find the answer.

The moult, the song and of course some feather samples should be the key. Although the moult of primaries in breeding grounds is a criteria for African reed warbler, BWPi already gathers some cases of Reed warblers moulting primaries in the Iberian Peninsula, before departing to the wintering grounds. Marc kindly sent me the photo below of a bird in active primary moult caught at Ebro Delta in early September and a bird with a very advanced primary moult caught in Northern Italy also in early September can be seen by clicking here (log in needed). Note that Marc’s bird also shows a suspicious wing formula.


Some evidences points to the fact that baeticatus, or at least baeticatus-like warblers,do occur at least in the south and east of the Iberian Peninsula but further studies are required to confirm this point. Keeping in mind the short distance from the nearest breeding grounds recently described, it wouldn’t surprise me if some birds are found to be breeding. The taxonomic debate may start soon and we shouldn’t be surprised if the final outcome is the description of another clinal variation from North (or even South) Africa to Eastern Asia, this time with Reed warbler as the principal actor.

Photos by Joan Castello © (1, 2 & 3) and Marc Illa © (4)

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