Laudator Temporis Acti

Edwin Way Teale, Autumn Across America (New York: Dodd, Mead & Company, 1956), p. 22 (on Raymond T. Bond):

Certainly no one else ever proved, eruditely and to our satisfaction, that hell is a place where there are no birds by quoting Virgil's famous line from the sixth book of the Aeneid: "Easy is the descent to Hell—Facilis descensus Averno." Avernus, Virgil's portal to Hades—a noxious lake in the Italian Campania where fumes were supposed to kill all birds flying overhead—received its name from the Greek Aornos, a not, and ornos or ornis, a bird. So it seemed an eminently logical step to Bond, who has been an avid bird watcher since boyhood, that hell should thus be construed to be a place without birds and a place without birds to be hell.

There is little in "facilis descensus Averno" (Vergil, Aeneid 6.126) itself to suggest an absence of birds, but there is a passage later on in the sixth book of the Aeneid that does allude to birdless Lake Avernus (lines 237-242, tr. H. Rushton Fairclough):

A deep cave there was, yawning wide and vast, and sheltered by dark lake and woodland gloom, over which no flying creatures could safely wing their way; such a vapour from those black jaws poured into the over-arching heaven [whence the Greeks spoke of Avernus, the Birdless Place].

spelunca alta fuit vastoque immanis hiatu,
scrupea, tuta lacu nigro nemorumque tenebris,
quam super haud ullae poterant impune volantes
tendere iter pennis: talis sese halitus atris
faucibus effundens supera ad convexa ferebat.
[unde locum Grai dixerunt nomine Aornon.]

Of course it would be easy to argue the contrary, that there are birds (or at least one bird, a vulture) in hell, by reference to another passage in the sixth book of the Aeneid (lines 595-600, tr. Fairclough):

Likewise one might see Tityos, nursling of Earth, the universal mother. Over nine full acres his body is stretched, and a monstrous vulture with crooked beak gnaws at his deathless liver and vitals fruitful for anguish; deep within the breast he lodges and gropes for his feast; nor is there any respite given to the filaments that grow anew.

nec non et Tityon, Terrae omniparentis alumnum,
cernere erat, per tota novem cui iugera corpus
porrigitur, rostroque immanis vultur obunco
immortale iecur tondens fecundaque poenis
viscera rimaturque epulis habitatque sub alto
pectore, nec fibris requies datur ulla renatis.