BARREN had three major aims: 1) to characterize the energetic environment of the study species; 2) to measure the energetic metabolism at organism and cellular level and 3) to sequence genes associated with energetic metabolism. In order to accomplish these aims I conducted two field campaigns in South Africa. During those expeditions I collected: faecal and arthropods samples to address objective 1; data on the energetic metabolism and muscle samples to fulfil objective 2; and blood samples to attain objective 3. At my host institution, the Natural History Museum of Denmark, under the mentorship of Prof Gilbert I used genomic tools to characterize the diet of the birds, as well as to sequence a selected portion of the genome of 101 individual birds. In addition, the collaboration with Dr. Prats facilitated the quantification of the amount of mitochondria in high-energy demand tissues. Altogether, the data obtained from the different sections of the project, show that these birds feed on the resources available across the gradient and do not prefer any particular prey; birds from the most harsh area (large temperature amplitude) have lower basal metabolic rates, i.e. require less energy to maintain basic metabolism, and have larger density of mitochondria (cellular power houses) in the most demanding muscles; some genes encoding proteins/enzymes involved in the cellular pathway of energy production seem to be the target of natural selection.