Abstract:
Arthropods are important food resources for birds. Forest management activities can influence shrub-dwelling arthropods by affecting the structure and composition of
understory shrub communities. Changes in abundance and species composition of
arthropod communities in turn may influence the distribution and abundance of
insectivorous birds. I examined relationships among bird abundance, availability of arthropod prey, and composition of understory vegetation in managed and unmanaged Douglas-fir forests in western Oregon. I sampled bird abundance, arthropod intensity in terms of abundance and biomass, and habitat structure in 13 forest stands representing a range of structural conditions. I used fecal analysis to describe the diets of five bird species that forage in the understory of conifer forests, and compared the abundance of food resources for Wilson's warblers among shrub species and silvicultural treatments. I also quantified the foraging patterns of Wilson's warbiers, MacGillivray's warblers, and orange-crowned warblers to determine which shrub species were used for foraging.
Variation in deciduous shrub cover provided the best explanation of variation in the
abundances of Wilson's warbler, MacGillivray's warbler, and Swainson's thrush among study sites. Stands occupied by Wilson's and MacGillivray's warblers had significantly greater cover of deciduous shrubs than unoccupied stands, and both of these species foraged extensively on these shrubs. Their association with deciduous shrubs may be related to prey abundance because tall, deciduous shrubs supported high abundances of arthropod taxa selected as prey by Wilson's warbiers, especially Lepidoptera larvae. Abundance of aerial arthropod prey also was positively correlated with deciduous shrub cover. These shrub species responded positively to partial removal of the overstory by thinning and group selection harvests. Furthermore, small gaps in the canopy of commercially thinned stands and larger gaps created by group selection harvests supported higher abundances of aerial arthropod prey than surrounding matrix forest. I conclude that understory vegetation in general, and deciduous shrubs in particular, make an important contribution to food resources for birds in conifer-dominated habitats. Management activities that promote the development and maintenance of understory vegetation can positively influence songbird diversity by maintaining habitat for shrub-associated species.
