The owl-hoot is an archaic term meaning dusk or nightfall. The word is a hyphenation of owl and hoot (the call of the owl). It is similar to the term owl-light which also means dusk but also dim or poor light. While owl-light was first recorded in 1599, owl-hoot is a much later term, first appearing in 1832.
The Selah, Bamberger Ranch Preserve is a 5,500-acre ranch in Blanco County, Texas. The Chiroptorium is a bat cave built on the preserve to house Mexican Free-tailed Bats. The artificial bat cave is now home to a few hundred thousand bats. A dusk, these bats emerge, an event that takes over 30 minutes each evening.
This video shows just what it looks like when hundreds of thousands of bats emerge at the owl-hoot from their cave: