Does it occur to anyone else that the fable is not a warning against doing favors in general but of siding with "outsiders" against "insiders"? When the farmer protects the venomous snake from the people trying to kill it, from a human perspective he's doing a bad thing. When the heron recommends white fowl as a medicine, even he were not to himself become a meal, he's not doing the bird community any favors. And the farmer's wife, in letting the heron go, is depriving her husband of vital medicine.