Fowl signify spiritual truth; a bird, natural truth; and a winged thing, sensual truth. Fowl of heaven, as in Hosea 2:18, signify the affections of truth; and reptiles of the earth, the affection of the knowledges of truth and good. Fowl signify thoughts, and all that creeps on the ground, the sensual principle. Fowl signify intellectual things.

(Referenties: Arcana Coelestia 777; Hosea 2)

In Matthew 23:23, this is explained by the Lord right there, in the verse.