Owls do not actually say "hoo" in the way humans might interpret it. The iconic hooting sound associated with owls is a simplification of the complex vocalizations they produce. Different owl species have unique vocalizations that serve various purposes such as territorial communication, courtship displays, or identifying themselves to other owls.
The typical "hoo" sound often associated with owls is primarily heard from the common Tawny Owl (Strix aluco) found in Europe. Its call has a rhythmic pattern that can be perceived as a repeated "hoo" or "hoo-hoo-hoo" sound. However, it's important to note that even among Tawny Owls, there can be variations in their calls and the specific sounds they produce.
Other owl species produce a wide range of vocalizations that can include screeches, whistles, trills, and various other sounds. Some owls have distinct calls that do not resemble a "hoo" sound at all. For example, the Great Horned Owl (Bubo virginianus) produces deep hooting sounds, while the Barn Owl (Tyto alba) has a distinctive screeching call.
So, while the association of owls with the sound "hoo" has become a common stereotype, it's important to recognize that owls have diverse vocalizations that extend beyond a simple "hoo" sound. Their vocalizations are species-specific and play a crucial role in their communication and behavior.