Use of Arbutus menziesii by Cavity-nesting Birds

As part of a larger study of wildlife habitat associations in Douglas-fir (Pseudotsuga menziesii) forests of northwestern California, I recorded characteristics of nest sites used by 16 species of cavity-nesting birds. Pacific madrone (Arbutus menziesii) contributed only 8% of the basal area of the stands I studied, but 24% of all cavity nests were in madrone. Although nests were distributed among 17 tree species, only madrone was utilized by birds at a rate greater than predicted from availability. About 75% of available madrone trees were <30 cm diameter at breast height (DBH), but only 11% of the nests were in these smaller trees. Larger than average madrones seem to be an important habitat component for cavity-nesting birds in California’s Douglas-fir forests. Madrones are also a prime fuelwood species; thus, a potential conflict exists between commercial use of madrone and its value for wildlife. The importance of Pacific madrone as nesting habitat is less well known. The objective of this study was to evaluate the use of madrone as a nesting substrate for cavity-nesting birds. Mixed-evergreen forests of northwestern California support one of the most complex vegetation patterns in North America (Whittaker 1961). Much of this complexity results from the diversity of hardwood species comprising the lower overstory canopy. These hardwoods are recognized as a potentially rich resource for wood products and energy (McDonald 1983 and Zerbe 1985). Pacific madrone (Arbutus menziesii) is a dominant tree species in this complex. It is heavily used locally because of its value as pulpwood and fuelwood. Pacific madrone is also an important food source for birds and other species that feed on its berries. For example, Hagar (1960) found that varied thrushes (Ixoreus naevius) were more abundant in a winter when there was a large berry crop than in years of poor crops. I observed a similar response of varied thrushes and American robins (Turdus migratorius) to changing berry crops in that both species were at least twice as abundant in the winter of 1980–81 when the madrone berry crop was heavy compared with the next 2 winters when berries were much less numerous.