To study bird migration at sea, MARECO operates a bird radar on an offshore platform. Radar observations greatly contribute to the understanding of the spatial and temporal patterns of bird migration because of their ability to register birds continuously at a large spatial scale and at high altitudes. We make use of a Merlin bird radar (DeTect-inc., Florida, USA). The radar antenna is rotating in the vertical pane, creating a vertical ‘radar screen’ that registers all the targets moving through that screen. Every registration is one or a group of birds passing through that area. The flux of birds is expressed as migration traffic rate, i.e. number of birds that pass across a one kilometer line during an hour.