Reference

John Lawton Roberts (1980): Observations on birds of the Bulgarian seaboard, with new breeding records for S. W. Bulgaria of Masked Shrike (Lanius nubicus), Bonelli's Warbler (Phylloscopus bonelli), and Blue Rock Thrush (Monticola solitarius) – Bonn zoological Bulletin - früher Bonner Zoologische Beiträge. – 31: 20 - 37.