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Due to the low detectability of Northern Saw-whet Owls (Aegolius acadicus; hereafter, NSWO) throughout their annual cycle, standardized monitoring during migration allows for population assessments over time. We assessed age-class population trends in NSWO throughout eastern North America using banding data from 7 sites over a 25 year period. Using a mixed linear model, we did not detect any significant trends over time for the total owl count, adult owl count, and juvenile owl count from 1992 to 2017. During the period when all 7 sites were active from 2001 to 2017, trend estimates remained nonsignificant despite showing negative slopes. We confirmed this nonsignificant, negative trend through a similar mixed linear model of NSWO data from Christmas Bird Counts. Our results suggest that NSWO populations across eastern North America have been relatively stable since 1992 throughout their migration and winter ranges and demonstrate the value of standardized banding data for monitoring the regional population status of NSWO.

Use of ultraviolet (UV) light, which causes porphyrin pigments in feathers of some birds to fluoresce, provides a simple, effective means of distinguishing multiple generations of flight feathers in owls. This permits easier and more accurate classification of age of adult owls. This lighting technique has been used extensively with Barn Owls (Tyto alba) and Northern Saw-whet Owls (Aegolius acadicus) and works well on a variety of owl species at night in the field, and should have wide applicability among owl researchers. The relative ages of the feathers can be easily distinguished by the intensity of fluorescence they exhibit when the ventral surfaces of primaries and secondaries are exposed to UV (black) light. This allows rapid and accurate assessment of molt and, in turn, the assignment of an age classification for the owl.

We quantified home range size and habitat selection of seven female Northern Saw-whet Owls (Aegolius acadicus) on Assateague Island, Maryland, during the winters of 1996 and 1997. Home range size (95% fixed kernel) was 103.5 ha (± 50.3 SE). Home range size increased with time spent radio tracking as biweekly home ranges were smaller than those calculated for longer time periods. Home ranges often overlapped in time and space and in one instance the home range for one owl was completely within that of another owl. Northern Saw-whet Owls used primarily pine woods and shrub swamp habitats, with pine woods used more often than any other habitat type and significantly more than expected based on habitat availability.

We characterized 30 diurnal roost sites of five radio-tagged Northern Saw-whet Owls (Aegolius acadicus) in the winters of 1996–1997 on Assateague Island, Maryland and found they preferred thick cover at roost sites. Roosts occurred most often in loblolly pine forest (Pinus taeda) and shrub swamps dominated by wax myrtle (Myrica cerifera). Vegetation was measured at paired roosts and random sites in similar habitats. Distance to nearest tree and average canopy height were significantly lower at roost sites than random sites. Numbers of stems larger than 2.5 cm diameter at breast height (dbh), stems smaller than 2.5 cm dbh, and roost tree dbh were larger at roost sites. Roost height, canopy cover, canopy height, shrub height, and ground cover differed significantly between pine and shrub swamp roosts, although cover above and below the roost site were similar. Higher densities of stems and shorter distances to the nearest tree at roost sites compared to random sites indicated that owls chose sites ...