Are Northern Saw-Whet Owls Nomadic?

The first known nesting of a Northern Saw-whet Owl (Aegolius acadicus) in the Snake River Birds of Prey National Conservation Area occurred in a nest box in 1986, 4 yr after nest boxes were constructed in the study area. Occupancy of nest boxes by Northern Saw-whet Owls varied substantially over the next 13 yr (0-8 nests per yr). The number of mice counted on nocturnal surveys fluctuated widely during this same period, and the annual number of Northern Saw-whet Owl nests in the boxes was positively correlated with an index of mouse abundance. Only one of the 52 breeding adults that we banded was recaptured in a subsequent year, and none of the 139 nestlings produced in the boxes was reencountered. A male that we banded at a nest in April 1990 was found dead in British Columbia in January 1993, more than 900 km NNW of our study area. Data from the Bird Banding Laboratory were insufficient to evaluate breeding-site fidelity because few researchers have banded adult Northern Saw-whet Owls at nests. Northern Saw-whet Owls seem to exhibit some of the characteristics associated with nomadism in birds (e.g., high fecundity and low survival), but they differ from typical nomadic species because they do not specialize on cyclic prey We suggest that Northern Saw-whet Owls are nomadic in some parts of their range, settling to breed in areas of high food abundance that they encounter during the nonbreeding season.