The vertical occurrence of small birds in an old-growth Douglas-fir-western hemlock forest stand

Abstract

The vertical occurrence of the small bird assemblage (songbirds, small woodpeckers, nighthawks, and swifts) in a ~4 ha stand within the T.T. Munger Research Natural Area, Washington, USA, a 500-year-old Douglas-fir-western hemlock (Pseudotsuga menziesii-Tsuga heterophylla) forest, was quantified to characterize this assemblage and determine whether birds are vertically stratified within the canopy. We used a gondola suspended from a construction crane to count birds in a weekly series of vertically stratified fixed-area point counts, 5-minute, 30 m radius, in the lower (0-20 m), mid (21-40 m) and upper (41-60+ m) canopy. Data are from 21 March 1996 to 21 March 1999, and included 121 survey days (mean 40 counts/year, March-June 42, July-October 46, November-February 33). Twenty-nine species of birds were detected in the plots; the 20 most common were used for analysis. Fifteen of these species were detected significantly more often in one zone of the canopy. Timber foliage insectivores, air insectivores, timber seed-eaters, and most low understorey herbivore-insectivores were stratified within the canopy. Bark insectivores (except brown creeper (Certhia americana)) and omnivore-scavengers (except gray jay (Perisoreus canadensis)), however, were not stratified within the canopy. The number of bird detections shifted to the upper canopy during winter. One species was generally restricted to the lower canopy, and five species were restricted to the upper canopy, whereas no species were found exclusively in the mid-canopy. The small bird assemblage of this old-growth forest stand was vertically and seasonally patterned and the vertical forest structure, particularly within the upper canopy, reflects these patterns.