“Old proverbe says,
That byrd ys not honest
That fyleth hys owne nest.”
John Skelton (1460–1529) English poet
Poems against Sir Christopher Garnesche, probably published c. 1523, reported in Bartlett's Familiar Quotations, 10th ed. (1919). Compare: "It is a foule byrd that fyleth his owne nest", John Heywood, Proverbs (1546) part ii. chap. v.