Dierschke, Volker, and Julia Delingat. 2003. “Stopover of Northern Wheatears Oenanthe Oenanthe at Helgoland: Where Do the Migratory Routes of Scandinavian and Nearctic Birds Join and Split?”. Ornis Svecica 13 (2–3). Lund, Sweden:53-61. https://doi.org/10.34080/os.v13.22807.