It's also a place with plenty of water because its volcanic crater reaches deep down to the level of the ground aquifer.
Lush vegetation grows all around and insect life ("Namus" means mosquito) is abundant. Its not surprising it supports some resident bird life and attracts large numbers of wintering birds and huge numbers of passage birds.
We know all this primarily because of two pairs of ornithologists.
Panorama of Waw an Namus by Marta Visentin and Bruno Massa
They are Jens and Heidi Hering who visited Waw an Namus in winter 2007/8 and Bruno Massa and Marta Visentin who visited in late April 2006. So we now have records in winter and for part of the spring passage.
Jens and Heidi Hering wrote "Der Wüstenvulkan Wau an Namus – ein unbekanntes Überwinterungsgebiet in der Zentralsahara (The volcanic desert of Wau Namus - an unknown wintering area in the central Sahara) Der Falke 56, 2009"
Bruno Massa and Marta Visentin wrote "Remarks on the importance of scattered vegetation in desert areas of Libya for migrating and breeding birds" Riv. ital. Orn., Milano, 75 (2): 141-158, 30-XI-2006
a closer look at Waw an Namus by Marta Visentin and Bruno Massa
What is surprising is that their list of birds seen is quite different. They have only a few species in common. I think we can assume that those not in common are mostly wintering birds (from Jens Hering's paper) or on passage (from Bruno Massa's paper).
My translation of German is not fantastic. So I may have misinterpreted one or at most two birds from Jen's paper. However, as far as I can see the only birds in common are moorhen, little grebe, and reed warbler. In each case there is firm evidence they are resident breeders. Burno Massa reports there are tens of moorhen. Bruno also reports water rail as definite breeders.
desert sparrow at Waw An Namus by Jens Hering
Jens Hering says the most abundant bird was coot which Bruno Massa doesn't report at all suggesting that this bird may be a winterer only.
Other wintering birds seen by Jens (but not Bruno since he didn't bird in winter!) are teal, shoveler, black necked grebe, sardinian warbler, bluethroat, stonechat, black redstart, meadow pipit and red throated pipit.
Jens also saw desert sparrow (see his picture from his paper above) which is a nomadic bird and desert wheatear which may be resident and not reported (or seen) by Bruno.
leucistic black necked grebe seen and photographed by Jens Hering
Bruno Massa saw the following birds presumably on passage since they weren't seen by Jens - (8 to 10) marsh harrier, tens of European bee-eater, two European roller, hundreds of yellow wagtail and barn swallow, and a few sand martin. He also reported golden oriole resting on isolated acacia trees to the north.
Clearly this oasis is very important for migrant birds and has been for centuries. However the new huge government farms in the desert have dramatically increased the land area of safe havens for birds. They will make interesting birding this spring.