We have three small owls that share a lot of characteristics. They are the boreal owl (Aegolius funereus) of North America and Eurasia, known as Tengmalm’s owl in Eurasia, and in North America the northern saw-whet owl (A. acadicus) and the western screech owl (Megascops kennicottii). In our area, they are forest birds (although the screech owl also breeds in the dry southwest, and I will not discuss those populations here). None of the three is very common in our area. The quite different pygmy owl is small too, but that one occupied a whole essay by itself, so here I’ll say a bit about three others.
They range in size from about seven or eight to ten inches in length, with wing spans of about 17 to 21 inches. Body weights vary seasonally, so an average isn’t very meaningful, but in general they weigh less than 200 grams. Females are larger than males. The main predators on juveniles and adults are large raptors; marten and squirrels may prey on eggs and chicks.
These three species are similar in many ways. They are mostly nocturnal hunters (except in the far north when summer has no night or if chicks are unusually hungry), able to find prey in the dark and under cover. They can locate prey by sound: their ears are asymmetrical in shape and in location on the side of the wide skull, so sounds arrive differently on right and left sides, allowing them to pinpoint prey location. They have facial discs of specialized feathers that help collect sounds and direct them to the ears. They are generally sit-and-wait predators, waiting for signs of prey presence and then swooping down to grab prey in their talons. Their prey is mainly small mammals, with some birds, insects, and occasionally other things. Prey may be cached on tree branches near roost sites or in cavities, especially during the nesting season.
A saw-whet owl stands quietly, just observing. (Photo by Mark Schwan)
They are cavity-nesters, using old woodpecker holes, wood-rot holes, or nest boxes, and making no modifications or additions of material. They are usually monogamous, but in times of high prey availability, some may have more than one mate. Females incubate eggs and brood the chicks, while males hunt and deliver food to the female, who does not leave the nest during this time except to defecate and cast pellets of undigested food. Eggs typically are laid at two-day intervals, incubation takes about four weeks, and hatching is asynchronous. When the chicks hatch, the female takes food from the male and passes it to the chicks. The nest is cleaned during incubation and brooding of young chicks, at least by screech owls and saw-whets (no data for boreals).Chicks stay in the nest for four or five weeks and are tended by their parents for a while after they fledge. Nest-leaving may be spread over several days, although screech owl chicks are reported to all leave the nest on the same day. They mature and can start to breed when a year old.
Here are some differences: Boreal and screech owls are not migratory, but individuals may move around in search of good prey availability and juveniles disperse from their natal areas. In contrast, saw-whets are partly migratory, many individuals moving south in the fall and moving to lower elevations in mountainous regions.
Saw-whets are the smallest of the three. Judging from various reports, they may lay slightly larger clutches than the others, more often having five or six eggs.
A western screech owl appeared in a previous year near the visitor center. (Photo by Linda Shaw)
In addition to distinctive, species-specific plumage differences, of these species, only the screech owl has “ear tufts.” These are two little peaks of feathers on top of the head that have nothing to do with hearing. Some larger owls have them too but their function is not clear; they may be a form of camouflage in some situations and may be used in communication.
There are records of some special feeding activities for some species. For example: on Haida Gwai’i, saw-whets are known to take invertebrates from the intertidal zone. Screech owls may have a more varied diet than the others; they are known to take fish from shallow water, pick arthropods from foliage, earthworms from soggy soil, and capture flying insects (e.g., moths) and even bats in mid-air. However, more observations might show these examples are not so unusual…or perhaps more observations would disclose still other oddities.
• Mary F. Willson is a retired professor of ecology. “On The Trails” appears every Wednesday in the Juneau Empire.
