Natural Nest-Site Characteristics of Two Small Forest Owls with Implications for Conservation and Management

Denver W. Holt, Matthew Larson, Graham G. Frye, Katherine Gray

Abstract


Natural nest cavities of the Northern Saw-whet Owl (Aegolius acadicus) and the Northern Pygmy-Owl (Glacidium gnoma) were characterized using several variables measured from 79 nests. Northern Saw-whet Owls appear to prefer larger diameter trees, with larger cavity openings, and deeper cavities compared to the Northern Pygmy-Owls. Pygmy-owls also use a higher proportion of living trees with natural, i.e., not excavated, cavities compared to saw-whet owls. Tree height, nest height, and the number of cavities located on a snag were consistent between the two species. Internal examination of hundreds of cavities within owl territories shows that many cavities which appear appropriate for nesting owls are unusable. Leaving dead or dying trees for cavity nesting species is a common practice for forest managers in the West. However, criteria for “wildlife habitat” trees often adhere to a one-size-fits-all approach; retained cavities are selected based on external assessment alone. The dissimilarity in nest-site selection by these two species, and the fact that cavities show great variability in internal condition, underscore the need for forest managers to select a diverse array of trees for cavity nesting birds in western forests.

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