How olde proverbys says,
That byrd ys not honest
That fylythe hys owne nest.

III, lines 196–198, quoted in The Complete English Poems of John Skelton: Revised Edition, ed. John Scattergood (2015), p. 113. Compare: "It is a foule byrd that fyleth his owne nest", John Heywood, Proverbs (1546) part ii. chap. v