New insights into the morphology and taxonomy of the Acrocephalus baeticatus / scirpaceus species complex based on a newly found West African syntopic population

A precise knowledge on biodiversity in tropical developing countries is pivotal to address proper conservation guidelines and policies, especially when natural habitats are strongly jeopardized by land use changes due to agriculture or industrial issues. In West Africa, Burkina Faso is a typical example of this emerging trend, and its natural or semi-natural wetland areas are of great importance as breeding or wintering sites for many passerine species, including the taxonomic complex group of unstreaked reed warblers (genus Acrocephalus). The use of morphological characters to distinguish between A. baeticatus and A. scirpaceus occurring in syntopy, is often subtle. To shed more light on the taxonomic status of a newly found Acrocephalus community in South Burkina Faso and on the reliability of the available criteria adopted to identify members of this species complex, we here compared the morphological features of some specimens collected during different field expeditions and supported our phenotypical assessment with a molecular identification approach. Sixteen Acrocephalus specimens were collected at a recent wetland area located South of Burkina Faso during winter. Specimens were measured and assigned to the species level based on the published and conventionally adopted morphological criteria. A COI-based DNA barcoding approach was used to confirm identification. Our integrated identification approach confirmed the occurrence of the first population of A. baeticatus for Burkina Faso. It is geographically isolated from other West African known populations. Interestingly, our results highlighted that the traditionally adopted identification trait, based on the emargination on the 7th primary, is not always valid to distinguish A. baeticatus from its conspecifics, which also partly overlap in biometric measurements. We therefore support the greater reliability of the ‘wing length / P9 notch’ criterion recently proposed for the identification of Palaearctic unstreaked Acrocephalus species and its importance to separate the wintering species of Palearctic origin from the African resident ones.