This is a set within the main walk-through aviary. I'm not sure if any were actually tethered - certainly some were moving from perch to perch of their own accord. I'm presuming they are wing-clipped.
The owls displayed in here are:
*Barn Owl
*Oriental Bay Owl
*Brown Wood Owl
*Spotted Wood Owl
*Buffy Fish Owl
*Barred Eagle Owl
@Great Argus - I have also wondered that before, especially when seeing a comparison such as in the photo below (taken by myself at the Penang Bird Park):
Wow, that photo seems like predation waiting to happen. (Nice photo by the way!) I wonder if they're kept sated so maybe not worth the hassle to the bigger species? I've noted when the local pair of wild Great Horned Owls start calling, the Barn and screech owls silence very quickly. But maybe if there's enough food and perching to go around it's a truce? Either way I've seen enough squabbles and fights between birds of prey it doesn't seem wise to house such a size variation together.
