Effect of vegetation structure on breeding bird communities in the dry zone douglas fir forest of Southeastern British Columbia
Creators: name: Schwab, Francis Edward
Publisher: name: University of British Columbia
Year: 2010

description: This thesis concerns a study into the relationship between vegetation structure and breeding birds in the dry subzone of the Interior Douglas fir BiogeocIimatic Zone. The study was conducted in the East Kootenay region of southeastern British Columbia. Studies of bird/habitat relationships often relate bird densities with successional vegetation; for management purposes it is convenient to view habitat alterations as changes from one successional stage to-another. Theoretical questions concerning the effect of succession and disturbance on species richness can be approached in this way. Also, there has been considerable interest in the effect of snags on bird density. An analysis of the effect of stumps and snags on bird density was incorporated into the study. Three questions guided the study: (1) What is the relationship between breeding birds and successional vegetation? (2) How is vegetation structure related to successional changes in bird species densities? (3) How does the number of stumps and snags affect the numbers of cavity nesting birds? There were two land management practices which altered the vegetation structure in the study area: selective logging and spring burning. This enabled me to determine the effects of these practices on breeding birds and to extend knowledge of the relationship between birds and vegetation structure. Twenty-four 300 yd by 400 yd (274 by 366 m) plots were established representing replications of each successional stage, transitions between successional stages, a selectively logged mature seral forest, a burned scrubby area, and a burned grassland/scrubby transition area. Bird data were recorded during fourteen, 20 minute visits to each plot in May and June of 1977. All birds detected by sight and sound were recorded during each visit to a plot. The vegetation data gathered on each plot included: estimates of the number of stumps and snags and estimates of crown coverage in each height zone of all species of grass, forb, shrub, and tree. The height zones recognized were 0-.5 m, .5-1 m, 1-10m, 10-24 m, and 24+ m. Multiple step-wise correlation was used to relate breeding bird densities with vegetation structure. Data from only the 25 most commonly recorded bird species were used to describe changes in the bird community with changes in vegetation succession. Results showed: (1) Bird species were non-randomly distributed with respect to successional stages. (2) Most successional stages support a unique compliment of bird species. (3) In general bird abundance was greater with greater successional age of a plot. However, dense stands of young conifer and climax Douglas fir forest had lower bird densities than the successional stages immediately preceding them. Mature serai ponderosa pine/Douglas fir forests had the highest bird densities. In some cases the bird densities on scrubby plots were comparable to the ponderosa pine/Douglas fir plots. (4) The relationship between bird species and vegetation structure is somewhat complex, but in general, visually obvious vegetation structures (i.e. height and coverage of grass, shrubs, and trees) are related to the presence of bird species. The species of trees and shrubs on a site can be used as an indicator of the bird species present. (5) The presence of cavity nesting birds was not closely linked with stumps and snags. Stumps and snags may not be a limiting resource. (6) The bird community on a grass dominated site which was burned in April 1977 was strikingly similar to that of another unburned grass dominated site. There were differences, among the bird communities on burned and unburned scrubby areas which were consistent with the recent fire history of the sites. (7) The selectively logged plot had numbers of bird species and bird species diversity similar to those of unlogged mature seral ponderosa pine/Douglas fir forest. The density of individual birds was lower on the logged plot than on the unlogged plots. Some bird species became more plentiful and others less plentiful as a result of the logging, but the bird community on the logged plot was composed of species associated with forested successional stages. A number of recommendations have been made for further study of East Kootenay bird/habitat interactions through a policy of experimental management.