Dierschke, Volker, and Julia Delingat. “Stopover of Northern Wheatears Oenanthe Oenanthe at Helgoland: Where Do the Migratory Routes of Scandinavian and Nearctic Birds Join and Split?”. Ornis Svecica, vol. 13, no. 2–3, July 2003, pp. 53-61, doi:10.34080/os.v13.22807.