I used histograms and box plots to examine the data for errors and check for normality (Figures 11 and 12). Because almost all of the species had a large number of zero counts, the data was not normal nor could it be transformed to achieve normality.
Aspen data
Relationships between mean abundance/density of individual bird species and seismic line density in trembling aspen-dominated stands were examined using line charts (Figures 13 and 14). Seismic line densities were grouped into four categories (0 = no seismic lines, 1 = >0 to 3km/km2, 2 = 3-6km/km2, 3 = >6km/km2) to allow the data to be easily visualized and in a similar fashion to the black spruce data. As seismic line density increased, forest edge species were expected to increase in abundance, whereas forest interior species were expected to decrease. Both black-throated green warblers and red-breasted nuthatches, which are forest interior species, decreased significantly in abundance and density as line density increased. All other species, such as Connecticut warblers (an edge species) and mourning warblers (an interior species), did not seem to be significantly influenced by seismic lines. These results were consistent across both abundances and densities, as would be expected because densities were only adjusted for species-specific differences in detectability rather than differences in detectability due to the treatments. Investigation of forest stands with higher densities of seismic lines than were included in this study would allow an assessment of whether there is a threshold at which a larger number of bird species respond strongly to seismic line density.
Figure 13. Mean abundances of black-throated green warblers, red-breasted nuthatches, Connecticut warblers, and mourning warblers in aspen forest stands in relation to seismic line density categories (0 = no seismic lines, 1 = >0 to 3km/km2, 2 = 3-6km/km2, 3 = >6km/km2). Error bars show standard errors. |
Figure 14. Mean densities per 10 hectares of black-throated green warblers, red-breasted nuthatches, Connecticut warbler, and mourning warblers in aspen stands in relation to seismic line density categories (0 = no seismic lines, 1 = >0 to 3km/km2, 2 = 3-6km/km2, 3 = >6km/km2). Error bars show standard errors. |
Black spruce data
To examine the effects of seismic lines on individual species of songbirds, I again created line charts of mean abundance and mean density for every species at each treatment type (Figures 15 and 16). Chipping sparrows and Tennessee warblers, which are both edge specialists, appeared to increase significantly as line density increased. However, all of the other species which were examined, such as dark-eyed juncos and Swainson's thrushes, did not appear to be affected by seismic lines. Results for abundance versus density again showed the same patterns for all species. In summary, these results suggest that seismic line density has little effect on most species of songbirds in black spruce. In addition to examining areas with higher line densities as suggested above, investigation of behaviour of individual species near seismic lines in aspen or black spruce could allow a greater understanding of whether seismic lines have a significant impact on songbirds.
Figure 15. Mean abundances of chipping sparrows, Tennessee warblers, dark-eyed juncos, and Swainson's thrushes in relation to seismic line treatment (0=no seismic lines, 1=one seismic line, 2=two seismic lines) in black spruce forest stands. Error bars show standard errors. |
Figure 16. Mean density per 10 hectares of chipping sparrows, Tennessee warblers, dark-eyed juncos, and Swainson's thrushes in relation to seismic line treatment (0=no seismic lines, 1=one seismic line, 2=two seismic lines) in black spruce stands. Error bars show standard errors. |
Site-level effects |
Figure 17. Mean abundance of Swainson's thrushes summarized by site and treatment in relation to line density (km per km2). Each site is denoted by a unique colour. |