Unclean but with a heart of gold

Although the dietary instructions are clear enough - "But these you may not eat: ... the hoopoe and the bat..." the text of Deuteronomy 14 does not explain in detail why the hoopoe is unkosher and should not be eaten. It may have been related to the rather widespread reputation of uncleanliness that clung to the hoopoe (Latin upupa, Greek ἔποψ). It is found with classical authors like Pliny the Elder who calls him obscoena avis and foul-feeding. The medieval Bestiary, on authority of Isidorus of Sevilla, adds that the hoopoe likes to wander around graveyards and dunghills and feeds itself with the unsavoury things it finds there. Contact with its blood will cause nightmares.
John Gotthold Kunstmann's dissertation The hoopoe. A study in European folklore (Chicago 1938) subdivides the characteristics ascribed to the bird in folklore in five groups, one of which he calls 'the bird that fouls its nest'. The largest group of names under which the bird is known in a wide variety of languages is onomatopoeic. The second largest group of names, however, suggest that the bird was seen as a dirtball.

Filial love
A further characteristic ascribed to the hoopoe - its love for its parents - is more often associated with the stork, another unclean bird according to Deuteronomy. While storks are frequently represented carrying and feeding each other - the young reciprocating their elders' care later in life - the young hoopoes are cleaning the feathers and the eyes of their parents as is shown in the bestiary of Museum Meermanno, but also in the earlier copy in Aberdeen.
On the Aberdeen site, the moral lesson that is to be learned from the young bird's behaviour is translated thus: "If birds, who lack reason, do as much for each other, how much more should men, who have the power of reason, support their parents in return; because the law says: 'And he that curses his father, or his mother, shall surely be put to death' (Exodus, 21:17); it is as if he were guilty of parricide or matricide"

Hoopoe and grapes
The properties ascribed to the hoopoe in Horapollo's Hieroglyphes are both related to grapes. It is said to eat maidenhair to cleanse its stomach after having eaten too much of the grapes. Secondly, it is said that the hoopoe can predict a good vintage: if it sings "auant que les vignes bourionnent" the grape harvest will be abundant.

Upupa (hoopoe)

Hoopoe

Upupa (Hoopoes, also: Epops): young taking care of their ageing parents: they clean their parents' feathers so they may fly again and lick their eyes so they may see again

Avivm vivae (serietitel)

vermeld op object prentmaker: Collaert, Adriaen

A hoopoe, having eaten too many grapes, now has a twig of maidenhair in its beak

A hoopoe eating maidenhair represents a man curing himself after having eaten too many grapes

Fowler covering his eyes behind a tree with a woodpecker and the nest of an hoopoe

Foreknowledge of an abundant vintage represented by a hoopoe