>But why is that necessarily true? Assuming both the Pygmy and Great Horned
>have equally good night vision, why would darkness favor the predator rather
>than the prey? Or do the larger owls perhaps have better night vision than
>the small species?

I think it is reasonable to presume that night vision is less effective
than day vision, no matter how impressive night vision might be. As
a geek, I think in terms of information loss, i.e. fewer photons
flitting about, which after all (as a geeky photographer) is why
photography is difficult when there's no light.

Among other things, contrast is greatly reduced. If a small owl
hoots, beeps, or gives that recorder A note, (varies by species).
a predator might get a very good bearing on where it's at. It
can reduce it's visual search to a narrow area. After all, barn
owls can capture prey by sound alone. And owls in flight are
truly quiet, unlike hawks which tend to sound like 747s in full
stoop. So, the attacker may well be at an advantage. Also, on
moonless (or pre-moonrise) nights there is no shadow, a big
key for detecting surprise attacks (I've actually identified
harriers and redtails by species by shadow alone when blindsided
by raptors from the rear. My pigeon loves me:). So the attacker's
opportunity to avoid detection is enhanced at night.

- Don Baccus, Portland OR <donb at rational.com>