Birds on the Edge Forest edge effects on bird assemblage size and composition in the Chuckanut Mountains Drew Schwitters Department of Environmental Sciences, Western Washington University 516 High St., Bellingham, WA Figure 1. Mean abundance of individual bird species per count at sites located in both forest edge and interior habitats with standard error on in the Chuckanut Mountains, Bellingham, WA. n=50 for each habitat type. Figure 2. Mean total bird abundance per count at sites of varying distance from the forest edge. The equation of the regression line and coefficient of determination are included. n=50. Figure 3. Total percentages of birds with various dietary requirements found at edge and interior habitats. n=50 for each habitat type. Birds play integral roles in ensuring proper ecosystem functionality. They provide seed dispersal, pollination, and insect control services. Because of this, it is important to understand the consequences of land management such as urban development and forestry on assemblages of native bird species. This study examined the differences in bird assemblage size and composition between forest edge and interior habitats in the Chuckanut Mountains. Point counts were used to collect bird and habitat data from 100 sites located in both the forest edge and interior. These data were analyzed with statistical and mean comparison methods. It was determined that overall bird abundance was significantly greater in forest edge habitat (P=1.2x10 -7 ), with a mean of 7.82 birds observed per count in the interior and observed per count on the edge. The composition was also significantly different between the habitats (P=0), with a higher abundance of general omnivores and granivores and a lower abundance of insectivores in forest edge habitats. These results can be used for the selective conservation management of individual species, and suggest that developed areas with native landscaping, medium tree density, and substantial shrub cover can support bird assemblages similar in size and composition to those found in forest interior habitat. Abstract Bird assemblages in the forest edge are significantly larger than those in the forest interior. The structure of these assemblages are also significantly different between habitat types. Abundances of American crow, American robin, chestnut-backed chickadee, house finch, and house sparrow were significantly higher in forest edge habitats. Abundances of winter wren were significantly higher in forest interior habitats. Many OLS regression models between habitat variables and bird abundances fit well, but the explanatory variables were different for each species (I.E. the coefficient of determination between the spotted towhee and low shrub cover was very high). The best habitat predictor for total bird abundance was the distance from the forest edge, which accounted for 35% of the variation in abundance. A significantly higher abundance of granivores and general omnivores and a significantly lower abundance of insectivores were found at the forest edge habitat. Results I wish to thank my professor, Dr. John McLaughlin, and teaching assistant David Droppers for teaching me the skills necessary to undertake a project such as this with enthusiasm and understanding. AcknowledgementsIntroduction An increasing amount of traditional forest habitat is becoming fragmented as human development and forestry intensifies. Birds dependent on forests have been relegated to small forest “islands” that are typically situated in less productive high elevations. Because these forest patches are characteristically bordered by development, they can be split into two separate microhabitats: the forest edge and the forest interior. This study examines if bird assemblage size and composition varies between these two microhabitats in the Chuckanut Mountains. Trends are also analyzed between certain habitat variables and bird abundance by species to examine habitat requirements of each individual species. This study indicates that although bird abundance is higher in forest edge environments, the assemblage composition is significantly different due in large part to invasive species such as the house sparrow and generalists such as the American crow. These differences are most easily observed in areas where bird feeders are present. For native bird species, the largest general influences on abundance are tree density, shrub cover, native plant species presence, and distance from human activity. Future development and forestry around the Chuckanut Mountains can use these results to enact management strategies that mitigate the impact on native bird assemblages (I.E. housing developments could feature native landscaping, selective logging could be favored over clearcutting, etc). Discussion Site Location Data from the forest interior was collected along the Chuckanut Mountain trail system at least 400 meters from the nearest road or housing development. Data from the forest edge was collected in the suburbs bordering the Chuckanut Mountains along Old Samish road and Cuckanut Drive. Data Collection 50 sites from each habitat type (100 total) were selected randomly for sampling. Data were collected from these sites using standard five-minute avian point counts. Only common year-round resident sp[ecies were recorded. Following each count, habitat data were recorded for the site. Data Analysis Compositional differences in bird assemblages between the two habitat types were assessed using Pearson’s Chi-square test of independence. Differences in abundance of individual species were tested for using independent two-sample t-tests. A series of OLS linear regressions were also preformed to determine which habitat variables were good indicators of bird abundance. Methods Photo Credit: