Fonte: Swifts in a Tower (1956), p. 108, 2nd edition, 1973
David Lack: Frases em inglês
“... Female song is uncommon in birds.”
[The Life of the Robin, https://books.google.com/books?id=wZs_AAAAYAAJ, 1943, H.F. & G. Witherby Limited] p. 28
The Life of the Robin (1943)