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Bird Populations in Montana Linear Riparian Areas at 12, 28, and 40 Year Intervals

Abstract In 2008, I surveyed bird populations in two riparian areas in western Montana using territory-mapping and compared the results with surveys done in 1968 and 1980. Bird populations were assessed by mapping territories of singing males. Overall vegetation height of the ground layer increased between 1980 and 2008. A major vegetation component, mountain alder (Alnus tenuifolia), suffered massive mortality due to a freezing event in February 1989. The death of the main trunks and re-sprouting of mountain alder changed the structure of the riparian vegetation. Mean height of alder was 1.4 m less in 2008 than in 1980 along Elk Creek. It was 3.4 m lower along the North Fork. Based on male habitat-use information collected in 1968, these structural changes may have negatively impacted the Hammond's Flycatcher, Warbling Vireo, Yellow-rumped Warbler, and American Redstart since males of these species were documented in 1968 to primarily use the upper level of the alder canopy for singing. The MacGillivray's Warbler may have benefitted from the change in alder structure. Bird species richness was similar over time. On Elk Creek, the Willow Flycatcher, Veery and Red-eyed Vireo disappeared, whereas the American Redstart declined from 19 territories to two in 2008. On the North Fork, the Warbling Vireo and American Redstart disappeared and the Hammond's Flycatcher declined from 10 territories (in 1980) to 2.5 in 2008. Bird population size and species composition varied between the two riparian areas. Riparian width of Elk Creek was significantly larger than the North Fork and this corresponded with higher bird species richness and abundance.