| Abstract/Description or Keywords |
Human activity has resulted in dramatic declines in the amount of riparian habitat across North America. Parallel to this loss have been widespread population declines of bird species that use riparian habitat. Conservation of riparian dependant bird populations may be facilitated by minimizing any further impacts on birds using remnant patches of riparian habitat, but are also likely to require active restoration effort to ensure populations remain viable. Studies evaluating the success of restoration efforts have often compared bird densities in restored habitat with those under “natural” (i.e. reference) conditions. However, it is far from clear that bird abundance or density is related to the reproductive success and survival of individuals and/or the growth rate of bird populations. We studied yellow warblers (Dendroica petechia) breeding in three types of riparian habitat adjacent to the Upper Arrow Reservoir near Revelstoke,BC. We assessed the impact of dam operations on the breeding performance of the yellow warbler and evaluated whether yellow warbler density provides a reliable measure of riparian habitat quality. We found that current dam operations did not have a major impact on the breeding success of yellow warblers in Revelstoke Reach in either 2005 or 2006, even though water levels were close to the maximum currently allowed in 2006. Nest mortality due to flooding was low because native riparian vegetation is no longer found below 435m, yellow warblers place nests 3m above the ground, and the majority of nests fledged or failed prior to increases in the water level. However, changes in the timing of reservoir fill of even 2 weeks, while maintaining the current maximum allowable fill, would have increased yellow warbler nest mortality due to flooding from 6 to 16%. We also found evidence that rising water levels may have an indirect effect on yellow warbler productivity as broods raised on territories that were partially flooded tended to be lighter than broods on territories that were not impacted by rising water levels. This is likely to reduce juvenile survival since lighter nestlings have lower survival in many species of birds. Yellow warblers appeared to prefer riparian habitat that included mature cottonwoods bordered by willow since they consistently arrived and initiated reproduction at Machete Island, the site representative of this habitat type, before starting to breed at other sites. Riparian habitat at Machete Island is therefore apparently perceived as being of higher quality than the patchier riparian habitat at Drimmie Creek or restored riparian habitat at Illecillewaet. Yellow warbler densities provided some insight into what defined high quality riparian habitat for this riparian dependent songbird, but were not closely related to productivity. Machete Island had the highest density of breeding pairs in 2005 and 2006 and pairs breeding at this site fledged more young than pairs at either of the other two sites in both years. However, the density of breeding pairs at Illecillewaet varied two-fold between years and although densities approached that of Machete in 2006, productivity at this restored site was consistently far lower than that of the reference site. These results suggest that concerns raised about the validity of using avian densities to evaluate habitat quality and the success of restoration activities are warranted. However, the use of data on avian densities from multiple years may reduce the potential for error since densities at poor quality sites are likely to be more variable than densities at high quality sites. |
