Cooper's hawk (Accipiter cooperii). It has been suggested that this may be because the hawk-owl may partially fill an important diurnal niche similar to that of day hunters such as hawks. Vocalization Various calls are used by the different sexes in different situations. When attracting a mate the male usually lets out a rolled whistle of ulululululululul and a sound similar to tu-wita-wit, tiwita-tu-wita, wita, when perching at a potential nest site. The female's call is usually less constant and more shrill. When alerting to danger, the northern hawk-owl lets out a sound similar to rike, rike, rike, rike. It also releases a high-pitched scream followed by a yip when an intruder is near to the nest. To warn of impending dangers to a fledgling, the hawk-owl will let out a noise similar to ki ki kikikikiki. Calls can vary in length from 15 s to 2 min. Distribution and habitat Three subspecies exist across the northern holarctic. The North American subspecies S. u. caparoch spans from eastern Alaska through Canada to Newfoundland and in some areas extends south into northern United States. The other two subspecies are found in northeastern Europe and Siberia: S. u. tianschanica breeds in Central Asia reaching Xinjiang (China); and S. u. ulula resides across Siberia reaching its most eastern range. Occasionally, S. u. caparoch can extend its territory as far south as northern Minnesota and many other states in the northern United States including more central states such as West Virginia, New York, and South Dakota. These southern forays into the northern United States are rare and generally occur during winter, or following an explosion in a population of prey. S. u. caparoch can occasionally be found in more southern areas such as Great Britain, southern Russia and Scandinavia, following explosions of prey. Northern hawk-owls are unevenly distributed and highly variable throughout the boreal forest. They live mostly in open coniferous forests, or coniferous forests mixed with deciduous species such as larch, birch, poplar, and willow. They are found in muskegs, clearings, swamp valleys, meadows, or recently burnt areas, and generally avoid dense spruce-fir forests. Their winter habitat is usually the same as the breeding habitat. Behaviour and ecology Breeding The northern hawk-owl generally starts its mating rituals at the beginning of March. After calling and pairing is complete the northern hawk-owl will build a nest and start to lay eggs. On average the