Most owls are nocturnal, and they use their voices to establish territories and attract mates in the dark. Several species begin nesting during as early as midwinter, which is one reason why you can often hear them in fall and winter, when most other birds are quiet.
For more owl sounds, check out Voices of North American Owls, http://macaulaylibrary.org/guide/voices-of-north-american-owls available through our Macaulay Library. This comprehensive audio guide is highly sought after and no longer available on CD. The digital download includes nearly 200 audio recordings of North America's 19 regularly occurring owl species, plus two rarities. The accompanying 28-page booklet contains detailed text and color photographs about each of the owls featured in the audio download.