Abstract:
Little is known on the importance of riparian areas to birds near small headwater streams in mesic forests. Progress towards understanding limiting factors that affect bird populations has been difficult because of lack of information beyond the breeding period. I compared bird assemblages between headwater riparian and upland areas throughout the post-breeding period by capturing birds using mist-nets in six paired riparian and upland locations along six headwater streams of the Trask River in northwestern Oregon. In order to assess whether birds prefer headwater riparian areas, I also examined factors affecting habitat selection by juvenile Swainson's thrushes (n=37) using radio telemetry. While riparian and upland locations had similar coarse wood volume and fruiting and tall (> 1.3 m tall) shrub cover, riparian locations had less shrub cover (< 1.3 m tall) and different shrub composition than upland locations. Total capture rate was double that of upland in riparian locations, while bird species richness was similar. Similar numbers of birds were captured in mist-nets oriented perpendicular and parallel to the stream suggesting that birds were not using riparian areas as movement corridors. Adult capture rate was greater in riparian locations than adjacent uplands while results of juvenile capture rates were ambiguous. Riparian locations supported higher capture rates of Swainson's thrushes
and winter wrens than adjacent uplands. Higher arthropod abundance in riparian locations may best explain higher capture rates in riparian locations. Radio-tagged juvenile Swainson's thrushes were more likely to select habitat that was near streams with high proportions of deciduous mid-story cover 1.5 - 15 m in the vertical strata and large volumes of coarse wood; possibly due to cover from predation and sources of food (i.e., fruits and arthropods).
I observed higher survival (97.3%) than reported in previous studies on independent juveniles which suggests that headwater streams contribute positively to the demography of the Swainson's thrush in the Pacific Northwest. In comparison with adjacent upland, headwater riparian areas appeared to be disproportionately used by songbirds during the post-breeding period even though vegetation differences were minimal. To assess if current policies sufficiently protect avian biodiversity, further research should evaluate how manipulation of vegetation affects bird distribution in headwater areas.
