Use of Snags by Birds in Douglas-Fir Forests, Western Oregon

Abstract

The use of standing dead trees, or snags, by birds was investigated in the Douglas-fir (P. menziesii) region of western Oregon in spring and summer, 1975 and 1976. Snags were examined in Douglas-fir forests approximately 10, 35, 75, 110 and 200+ yr of age. The use of snags was quantified on the basis of evidence of past and present use by hole-nesting birds. Hole-nesting birds used Douglas-fir snags over 60 cm in dbh [diameter at breast height] and over 15 m tall for foraging and nesting; these snags usually had broken tops, few or no branches, decayed sapwood and heartwood, and less than 100% bark cover. Snags of this size and type occurred primarily in forests over 110 yr of age; the use of snags by hole-nesting birds was concentrated in older forests (> 110 yr old). Bird censuses were conducted in 1 representative area from each forest age-class. Density and species diversity of hole-nesting birds increased with forest age. Density of hole-nesting birds was positively correlated (r = 0.98, P < 0.05) with mean dbh of snags. The intensive management of Douglas-fir forests does not allow for the production or retention of large snags. A reduction in the number of large snags could reduce populations of hole-nesting birds. Possible means for retaining and producing large snags are discussed.