The scientific description of one of the sub-species of this owl is attributed to the Rev. John Henry Keen who was a missionary in Canada in 1896. Adults are long with a wingspan. They can weigh from with an average of around , making them one of the smallest owls in North America. They are similar in size to the American robin. Northern saw-whet owls do not exhibit sexual dimorphism through their plumage and were often sexed by size dimorphism, where females are larger than males. Females on average weight 100 g and males on average weight 75 g. Northern saw-whet owls have porphyrin pigments in their flight feathers. When exposed to a UV light the ventral side of the wing, the feathers will fluoresce a neon pink. This is used in order to estimate molt and age in adult northern saw-whet owls.