Throughout the series, Others (humans) are vaguely referred to as an extinct race. This means that the series must be set far into the future, However, churches, paintings, castles, and even books still exist that owls like to collect. If this is set in the far future, though, it would make more sense if the owls found more modern things like wrecked spaceships and robots! 

This is the greatest mystery in the Guardians of Ga'Hoole series. I wonder what happened to us humans!