John Arvin's comments on Stygian Owl from a letter to the discoverers...
Dear John [Wright],
Congratulations on a truly outstanding record. Stygian Owl is a species that no one would have predicted would ever occur in the Lower Rio Grande Valley. The Chisos Mts. of Big Bend would have been a much more likely site. You are correct in your assumption that Stygian Owl is very poorly known in life. I am familiar with most of the ornithological literature from Mexico south (I am almost an exclusively "south of the border" birder, except for southern Texas of course), and there is little besides the (known) distributional records in the various regional works. As far as I know there are no papers dealing exclusively with any aspect of Stygian Owl biology.
The species has a huge, if seemingly discontinuous, range from north central Mexico to northern Argentina. Across this vast area the bird occurs in a great variety of habitats and elevations. In Mexico it seems to be primarily a montane forest species except for a population of unknown size on Cozumel Island off the Yucatan Peninsula. This population is probably of Greater Antillian affinities as are a number of other Cozumel species that are found only there in Mexico (Stripe-headed Tanager, the local endemic race of Bananaquit, Smooth-billed Ani, Caribbean Elaenia, Caribbean Dove, etc.). David Wolf, a friend and former associate, found a recently dead specimen there back in the early 80's and it has been seen there since by others (but not by me although I have searched for it at night there on several occasions). I can find out what race the specimen represents if it has been so identified.
For what it is worth (probably not much given the very low density in which the species seems to occur throughout its range) the closest point Stygian Owl occurs to Texas is southwestern Chihuahua (Barrance de Cobre area). Here as elsewhere in Mexico the habitat is mountain pine, oak, and fir forest. The range of the species in Mexico north of the isthmus is probably continuous from southern Chihuahua south in the Sierra Madre Occidental to the region where the two chains of the Sierra Madre essentially join in the transverse volcanic range south of Mexico City. The disjunct range shown by Howell and Webb in western Veracruz is where the Sierra Madre Oriental joins the transverse range. The range is probably continuous across the latter range. There is no evidence that Stygian Owl occurs northward in the Sierra Madre Oriental, which lies adjacent to the Texas border, except the verbal comment of Frank Harrison, founder of Rancho del Cielo (presently a biological station) in the humid cloud forest in southwestern Tamaulipas that there was a "big black owl" there that he was never able to collect. This area is still extensively forested and is very rugged and inaccessible so a cryptic species like Stygian Owl could easily escape detection by the few ornithologists who have collected in the area. I have spent a great deal of time there myself without any hint of the species.
The vast range of Stygian Owl, including several of the Greater Antilles (mainly Cuba and Hispanola), indicates that it is an exceptionally good disperser. Away from Mexico it is found in a great variety of habitats, from humid montane forest in the Andes to the hot, semiarid scrub of the Chaco (very like the brushlands of South Texas) in Paraguay and northern Argentina. I have seen it in tropical dry gallery forest in the low lying llanos of Venezuela (a new locality for it just 4 years ago). According to Sick it also occurs in Amazonian rainforest in Brazil. This seems not to be the case in western Amazonia (Peru, Ecuador, Bolivia, and Colombia) where I have spent much time and which is perhaps better known than much of South America.
I have mostly seen the bird in eastern Sinaloa/western Durango. I did a number of trips to the area in the first half of the 80's and we always made a major effort to see Stygian Owl as it was one of the few more or less reliable sites for it. I made a few observations that give tiny glimpses into its natural history. For one thing it seems to prey largely on flying prey. We first found Stygian Owls when one flew in in response to me playing a tape of Whiskered Owl in an effort to see that species. As this happened more than once it seems that the Stygian was attracted to the call of a smaller owl to prey on it. Hilty and Brown (citing Carlos Lehmann) report a remarkable 8 collected in the main plaza of Popayan, Colombia, as they flew in to feed on roosting Eared Doves. Popayan is at the head of the Cauca valley between the central and the western Andes. A native collector in Chihuahua noted "persigue murcielagos" (catches bats), which requires quite a bit of aerial dexterity.
Another habit we noted (after we were able to tape record its voice-there was no tape at the time) was a loud, sharp crack like a .22 shot just overhead when the bird flew in in response to playback. Apparently the birds make this noise by bring the wing tips sharply together on the downstroke. This noise was heard virtually every time we tried to call the owls in over a period of several years.
I know little about the geographic variation in Stygian Owl. None of the sources I have consulted give any criteria for separating the species racially, and I don't expect that any geographical variation would be detectable from photos regardless of how good. De Schauensee doesn't give any racial variation information for the South American races which I take to mean that there isn't any detectable in the field. I do know that the race in Sinaloa/Durango/Chihuahua is lambi and that from southern Mexico south through northern Central America, including those from the "mountain pine ridge" (not really mountainous) of Belize where we have been finding them at Hidden Valley for a few years, and in the Andes at least in Colombia (all the Andes?) are robustus. I would expect the lowland population in South America to be racially distinct from the Andean one and the Antillies to have endemic races on the various islands, but this is just guessing.
Sorry I couldn't put you onto any real Stygian Owl literature. There aren't any Stygian Owl "experts". Those of us in the birding tour business have more field experience with them than anyone else, but that isn't saying much. I probably have seen them 10 or so times, but it has always been birds that have occupied the same territories in 3 or 4 places scattered around Mexico and South America.
I hope that your experience with the owl will turn your sights southward. It is a whole different ball game (and a much better one).