Actually, Great-horned Owls can take fairly large birds such as waterfowl. Back in NY at the Iroquois National Wildlife Refuge, they had to install protective roofs over elevated nesting platforms that mallards were using (elevated off the water surface) because Great-horned Owls were preying heavily on incubating hens. Mallards are probably heavier or at least the same weight as a sharp-tail. I vote for the diurnal/noctornal hypothesis that most GHOW that are seen in the day are roosting and probably wouldn't go after prey in the daytime unless they are stressed or forced to due to low prey availability. Also, as I commented to Paul, I have flushed covies of California Quail from under trees that a Great-horned Owls were roosting in. Apparently, the quail were not bothered by the owls since they were foraging under the trees right below them...or didn't notice them.

Jeff Kozma

Yakima