The Attic Nights of Aulus Gellius, Vol. 2

The augurs indeed call certain places præpetes, and Ennius says in the first book of his Annals:

They commit themselves cheerfully to the præpetibus (favorable) and beautiful places.

However, Nigidius Figulus, in his first book of Private Augury, says that the aves inferæ (low-flying or unfavorable birds) are the opposite of aves præpetes; for instance: "The right differs from the left, the præpes from the infera." From this, we may conjecture that they were called præpetes because they fly at a greater height, since Nigidius says the inferæ differ from the præpetes.

When I was a young man in Rome, while attending the lectures of the grammarians, I heard Apollinaris Sulpicius—whom I particularly admired—speak to Enucius Clarus, the prefect of the city, during a discussion about the office of the augur. When mention was made of aves præpetes, he said that these birds seemed to him to be the same as those Homer called tanupterugas (long-winged); because the augurs particularly regarded those birds that flew with broad and outstretched wings. He then recited these lines from Homer:

But you order me to obey the birds with outstretched wings,
Which I do not mind nor regard.