The evolution and diversification of symbionts is closely linked to that of the host and thus, the study of host-symbiont associations and their evolutionary patterns can provide important insights into the processes and factors that have generated these interactions. This project intends to investigate the processes of speciation and divergence using feather mites and birds as study models. Among the ectosymbionts living on birds, feather mites are of particular interest because they live permanently on the host body, have the highest number of species and have a high degree of host specificity. Although they have been intensively studied at the taxonomic level, much of their diversity has yet to be described, and essential questions about their biology, ecology and genetic structure remain largely unanswered. Therefore, a better knowledge of the structure of feather mite communities at different levels of host organisation may improve our knowledge of the evolutionary ecology of this interaction and of the evolution of symbiont biodiversity in general. The main objective of this project is to study the diversity and genetic structure of feather mites living on non- passerine birds and to assess the relative importance of host and geography in influencing the evolution and population structure of these ectosymbionts. The study will focus on two species of storm petrels from the north-east Atlantic Ocean and the Mediterranean Sea. The feather mite fauna will be quantified and new mite species will be described by combining morphological and DNA barcoding information. In addition, different molecular markers, such as mitochondrial and nuclear genes, will be used to assess the genetic structure of feather mites between different host populations and between the two host species.