Identifying the winter grounds of the recently described Barbary Reed Warbler Acrocephalus baeticatus ambiguus. Jiguet, F., Dufour, P., Kardynal, K.J., Hobson, K.A., Copete, J.L., Arroyo, J.L., Lee, R.W., Rguibi-Idrissi, H., Procházka, P. 2022 Ibis. doi: 10.1111/ibi.13113 VIEW
Identifying the non-breeding grounds of small long-distance migratory songbirds has long been a challenge, and recent technical and conceptual developments allows for the tracking of such passerines. Geolocators are so miniaturized that researchers can successfully track small birds weighing only 10 grams. Stable isotopes are also valuable to determine probabilistic regions where winter-moulted feathers have grown. We used these two approaches to determine the yet unknown wintering grounds of the recently described Barbary Reed Warbler (Acrocephalus baeticatus ambiguus) the common reed warbler breeding in Iberia and Maghreb.
The whole story started in early September 2009, during a ringing camp in Morocco dedicated to tentatively capture Aquatic Warbler (A. paludicola). We captured intriguing short-winged, dark-legged reed warblers, clearly different from in-hand migrant European reed warblers, that our Moroccan colleagues reported as ‘local’ breeders. Some adults were in active primary moult by the end of August. We sampled feathers to perform some mitochondrial DNA analyses. Results indicated that these birds were clearly associated with the African sister species of the European Reed Warbler (Acrocephalus baeticatus).
Figure 1 An adult local reed warbler in active moult of flight feathers, Larache, Morocco, September 2009 © Pascal Provost.
Over the next few years, we organised the sampling of breeding reed warblers in closest Europe, to get ‘scirpaceus’ samples to contrast with the Moroccan birds. Well, warblers from Andalucía classified in the same African clade… The Catalan breeders too, and some southern French breeders displayed the same haplotype… There was a new taxon breeding in southern Europe, of the most commonly captured and ringed warblers in reedbeds, that had gone unidentified for decades! Three museum specimens, collected near Valencia, Spain, and preserved at AMNH in New York have been analysed and confirmed to belong to the same clade. As they were labelled ‘Calamoherpe ambigua’ by Brehm in 1857, the ‘new’ taxon had to be named ambiguus. We proposed to call it the Barbary Reed Warbler, in reference to its geographical distribution.
As reed warblers are migratory, the question quickly arose as to where these birds spend the winter. Some Moroccan birds do moult on the breeding grounds at the end of the summer, though they might still migrate later. There were also regular reports of reed warblers in active primary moult in Spain in summer, but very few – still some – winter captures. We had to investigate where these birds are in winter.
The first dataset which comes to mind are ringing recoveries, especially as the Euring atlas was scheduled. Indeed, thousands of reed warblers have been ringed for decades in Spain and Portugal, but along with continental migrants along their flyways, which could be late and early migrants, so that without genetic identification, most birds could not be retrospectively attributed to scirpaceus or ambiguus. Only proven breeding birds from the ambiguus range could help our quest.
We soon decided to investigate the destination of Spanish reed warblers by tracking some local breeders. We deployed geolocators in Andalucía and Cataluña, the tiny 0.36g provided by Migrate Technology. Breeding adults captured for tagging also provided a winter-moulted tail feather to measure the concentration of deuterium, informing the place where the feather was grown.
Figure 2 The Catalonian reed warbler at recapture in 2019, with the retrieved tag BL955 © José Luis Copete.
We retrieved 5 of the 40 loggers deployed in the large reedbeds of the Delta del Ebro and north of Doñana NP, which was not too bad. All tags recorded light levels during a complete migration cycle. Andalusian birds spent the winter along coastal Western Africa, from southern Mauritania to Guinea-Bissau, while one Catalunian bird wintered in southern Mali / Ivory Coast. The deuterium concentrations of the outer tail feather of 39 warblers breeding in the same Spanish populations confirmed that they have grown in the same West African regions. Iberian ambiguus are trans-Saharan migrants, and winter in West Africa.
Figure 3 Breeding, stopover and wintering locations of three GLS-tracked Barbary reed warblers tagged in Spain.
The case of Moroccan ambiguus was investigated with other pairs of stable isotopes, carbon and nitrogen, by analysing worn innermost primary feathers collected on adults. All adults sampled in autumn had old feathers moulted north of the Sahara – as expected from the field observations reported above. But 2 of 6 breeding adults captured in spring had moulted their primaries south of the Sahara. Whether this difference is age-related is unknown, but it illustrates that at least part of the Moroccan ambiguus are trans-Saharan migrants. This is in accordance with the presence but scarcity of the species in Moroccan reedbeds in winter.
The 2016 publication revealing the taxonomic identity of Barbary Reed Warblers reported an individual collected at Lake Chad in winter as belonging to the ambiguus genetic clade. It is therefore highly probable that at least part of the ambiguus populations breeding in Tunisia and Libya do winter in the central Sahel, and the winter presence of Barbary Reed Warblers in Niger, Chad or Nigeria would not be surprising.
To conclude, by combining data obtained from ring recoveries, stable isotopes and light loggers, the winter range of the Barbary Reed Warbler can be considered to extend from western to central Sahel, with a post-breeding moult occurring north of the Sahara for North African populations, and south of the Sahara for Iberian populations. In the future, deploying and retrieving loggers from across the North African range of Barbary reed warblers could confirm a potential longitudinally structured connectivity, as already reported from ring recovery data in the Eurasian Reed Warbler.
Olsson, U., Rguibi-Idrissi, H., Copete, J.L., Arroyo, J.L., Provost, P., Amezian, M., Alström, P. & Jiguet, F. 2016. Mitochondrial phylogeny of the Eurasian/African reed warbler complex (Acrocephalus, Aves). Disagreement between morphological and molecular evidence and cryptic divergence: a case for resurrecting Calamoherpe ambigua Brehm 1857. Molecular Phylogenetics and Evolution 102: 30-44. VIEW
Procházka, P., Hobson, K.A., Karcza, Z. & Kralj, J. 2008. Birds of a feather winter together: migratory connectivity in the reed warbler Acrocephalus scirpaceus. Journal of Ornithology 149: 141-150 VIEW
Procházka, P., Hahn, S., Rolland, S., van der Jeugd, H., Csörgő, T., Jiguet, F., Mokwa, T., Liechti, F., Vangeluwe, D. & Korner-Nievergelt, F. 2017. Delineating large-scale migratory connectivity of reed warblers using integrated multi-state models. Diversity and Distributions 23: 27-40 VIEW
Top right: A Spanish Barbary Reed Warbler equipped with a 0.36g geolocator © Jose Luis Copete.
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