Wolfgang Mieder, A Dictionary of American Proverbs (1992) offers the following entry for "It's an ill bird that fowls [sic] its own nest":
It's an ill bird that fowls its own nest. Var[iation]s: (a) It's a mean bird that dirties its own nest. (b) It's a poor bird that will dirty its own nest. Rec[orded] dist[ribution]: U.S., Can[ada]. 1st cit[ation] ca.1250 Owl and nightingale; US1666 Alsop, Character of Province of Maryland, ed. Mereness (1902). ...
It seems highly likely that "fowls" in this excerpt is a typo for fouls. The circa 1250 (or earlier) instance from "The Owl and the Nightingale" reads as follows:
Thar-bi men segget a vorbsine, / 'Dahet nabbe that ilke best, / That fuleth his owe nest.'
The proverb is certainly very old in English. Morris Tilley A Dictionary of the Proverbs in England in the Sixteenth and Seventeenth Centuries (1950) has this entry:
It is a foul bird that defileth his own nest
1509 A. Barclay Ship [of] Fools, I 173: It is a lewde byrde that fyleth his owne nest. a1529 Skelton Ag[ain]st Garnesche I. 197: Wks., I 125: That byrd ys nat honest That fylythe hys owne nest. 1546 Hey[wood] II v, s. H3: (fyleth). 1552 Tav. f. 58v: It is a foule thyng for a man to slaunder the treasure or thinges of his owne house. We haue a verye prety prouerbe in Englysh which we vse in the same sense. It is an euil byrde that defyleth her owne nest. 1586 Withals, s. A4: It is an ill Birde that berayes her owne nest. 1590 Lodge Rosalynde, p.37. p. 37: Is it not a foule bird defiles the owne nest? ...
Lodge's Rosalynde provided the plot for Shakespeare's As You Like It (1599)—and as Tilley notes near the bottom of the entry excerpted above, Shakespeare has Celia allude to the proverb in Act IV scene 1, 206, when she chides Rosalind (in her guise as Ganymede) for saying too many harsh things about women:
Celia. You have simply misus'd our sex in your love-prate : We must have your doublet and hose pluck'd over your head, and show the world what the bird hath done to her own nest.
Lewis Chambaud, The Idioms of the French and English Languages (1793) offers two "English adapted" versions of the French proverb "Chaque oiseau trouve son nid beau" ["Every bird finds its nest beautiful"]:
It is a bad bird that fouls his own nest.
She's a villainous bird that befouls her own nest.
In footnote attached to the second English phrase he cites, Chambaud writes:
Les Anglois appliquent ce proverbe par manière de reproche à ceux qui parlent mal de leur pays, ou de leurs proches. ["The English apply this proverb by way of reproach to those who speak badly of their country, or of their relatives."]
The English version of the French in this case is essentially the negative of it.
Interestingly, Wolfgang Mieder & Fred Shapiro, The [Yale] Dictionary of Modern Proverbs cites two modern versions of the ill bird proverb:
Don't shit on your own doorstep.
1967 Eric Partridge, Dicitonary of Slang and Unconventional English, 6th ed. (New York: Macmillan) 1356: The proverb is glossed with an allusion to an older proverb: "You don't 'foul your own nest.'" ...
and:
Don't shit where you eat. [Citations, starting with Saul Bellow, Adventures of Augie March (1953) omitted.]
But given the choice, I think I'd go with the older version, perhaps as rendered in Thomas Hoccleve's The Letter of Cupid (1402) and quoted in Bartlett Whiting, Proverbs, Sentences, and Proverbial Phrases from English Writings Mainly Before 1500 (1968):
An olde proverbe seyde ys in englyssh: men seyn "that brid or foule ys dishonest, What that he be and holden ful chirlyssh, That useth to defoule his oune neste."
