Although the Northern Saw-whet Owl (hereafter Saw-whet Owl) is one of the most common owls in forested habitats across southern Canada and the northern United States, much remains to be learned about its populations, distribution and movements, behavior, and breeding biology. It is one of our smallest northern owls; males, for example, weigh about as much as an American Robin (Turdus migratorius), females only about 25% more. Saw-whet Owls are found in most woodland habitats, with densities highest in coniferous forests at moderate elevation and latitude. Throughout most of its range, this species preys on small mammals, primarily deer mice (Peromyscus maniculatus) and white-footed mice (P. leucopus), which it hunts at night from low perches. Like many other owls, it has asymmetric ears, which allow prey to be accurately located by hearing alone.
Many Saw-whet Owls move southward in winter, with large concentrations in spring and fall around the Great Lakes. Males provide almost all of the food for the female and young, whereas females incubate and brood the young. Normally monogamous, polygyny may occur where prey are abundant. This is a hole-nesting species, so destruction of habitat, particularly nesting snags, represents its greatest threat, although individuals take readily to nest boxes.
Studying Saw-whet Owls is difficult because they are secretive, nocturnal, and have irregular movement patterns. Consequently, work by many researchers has been necessary to better understand Saw-whet Owl biology. As all researchers that have contributed to our current knowledge about Saw-whet Owl biology cannot be cited, only those that have most recently contributed to the areas of breeding biology, diet, communication, migration, genetics, and evolution will be mentioned. The rest can be seen in the reference list.
Matthew P. Rowe and his graduate students have contributed immensely to the understanding of Saw-whet Owl habitat use (see Cockerel 1997 , Milling et al. 1997 , Milling 2000 ), as well as highlighting a population in the Southern Appalachians that may be a separate, undescribed subspecies (see Tamashiro 1996 ).
In his studies of the Saw-whet Owl in British Columbia, Richard J. Cannings has published most of what is known about Saw-whet Owl breeding biology ( Cannings 1986 , Cannings 1987a , Cannings 1993 ; Gill and Cannings 1997 ). By studying breeding biology as well as conspecific and interspecific interactions, Jeffrey S. Marks and John H. Doremus ( Marks and Doremus 1988 , Marks 1997, Marks and Doremus 2000 ), as well as Charlotte Rains ( Rains 1997 , Rains 1998 ), have contributed greatly to our understanding of this species' ecology.
David F. Brinker and co-workers ( Brinker and Dodge 1993 , Brinker et al. 1997 , Brinker 2000 ) have studied Saw-whets for over two decades, initially in Wisconsin, then Maryland. They developed a discriminant function for sexing Saw-whet Owls based on mass and wing chord that was accepted by the Bird Banding Laboratory of the North American Bird Banding Program. Brinker also is founder of “Project Owlnet”, a non-profit organization devoted to conservation of Saw-whet Owls that encourages, facilitates, and coordinates migration monitoring as well as facilitates communication between a network of Saw-whet Owl banding stations across North America.
Ken Otter's research led him to discover the individuality of the male Saw-whet Owl's advertising call (see Otter 1996 ). More recently, his graduate student, Carmen Holschuh, analyzed vocalizations of the subspecies brooksi and found that calls varied among individuals as well as among habitat types (see Holschuh 2004 ).
Spencer G. Sealy added greatly to knowledge of the diet, endemicity, and sedentariness of brooksi, previously essentially unknown (Sealy and Hobson 1991, Sealy 1998 , Sealy 1999a ). He has also brought attention to the occurrence of acadicus on the Queen Charlotte Is., BC.
These studies have greatly advanced our knowledge of Saw-whet Owl biology, but much remains to be learned.