UK Government. ↑ Eric Hosking; Harold Lowes (1947), Masterpieces of Bird Photography, William Collins, Sons, p. 7, ASIN B000O8CPQK, Wikidata Q108533626 1 2 3 Parry & Greenwood (2020) pp. 68–69 ↑ Pycraft, William Plant (1920). "Some neglected aspects in the study of young birds". Transactions of the Norfolk and Norwich Naturalists' Society.