Abundant Natural Cavities in a Deciduous Forest Leads to a Lack of Nest-Box Occupancy by Northern Saw-Whet Owls

Abstract

Nest boxes are commonly used for species that utilize tree cavities, including small owls. Boxes were installed in 2011 to facilitate study of Northern Saw-whet Owls in Pennsylvania. We checked the 11 boxes that remained in 2020 after a hiatus of 8 years and found 3 rodent nests, but no evidence of owl usage. Playbacks revealed owls were present at 8 of 11 boxes checked. Cavity surveys yielded an average of 7.7 large cavities within 50 m of each box; extrapolation of cavity densities to a minimal territory size of 150 ha suggested that suitable cavities are an abundant, non-limiting resource here. Cavity abundance was likely a consequence of beech bark disease complex having recently top-killed most large American Beech trees locally, providing abundant resources for excavating woodpeckers. Assessing cavity abundance should be an essential first step for any management project involving cavity-nesting birds, and nest boxes should be used only in situations where cavities appear to be limited.