During the summer of 2014 I was part of a California Spotted Owl surveying team and spent many late nights out in the forest hooting and looking for these medium sized birds.
Surveys are done for a number of reasons including to determine if the birds are present on lands that have logging or fuels projects planned. If spotted owls are present, their nesting site needs to be found, and many acres of prime owl habitat must be given special protection.
Surveying involves humans hooting like owls to attract the territorial birds, and then finding their roosting/nesting sites. Since they are night creatures, hooting takes place in the dark, and many nights we were out until midnight or later.
The owls are very pretty, and if one is nearby, very responsive vocally. They fly close trying to see who has invaded their territory.
If you would like to watch a short video on the science behind the surveying, this video is worth the time.