Thursday, July 7, 2011

Songbird monitoring in Poopenaut Valley

Sarah Stock, wildlife biologist with the Resources Management and Science Division of Yosemite National Park, discusses recent songbird monitoring conducted as part of Yosemite's Looking Downstream Project.

In order to gain a more complete understanding of the ecology downstream of O'Shaughnessy Dam, we have been studying bird populations in Poopenaut Valley since 2007. Having characterized the breeding bird community and detected about 100 species in total, this year (2011) we are vigilantly observing the timing of when migratory birds arrive, set up and defend their territories, and initiate nesting. In Poopenaut Valley, nest searching provides a more in-depth look at how the changing water level affects breeding birds in terms of their reproductive success and the timing of nest building, egg hatching, and fledging.

NPS biological technician, Matt Brady, conducts a bird survey
on the north side of the Tuolumne River in Poopenaut Valley
(Sarah Stock photo).

Since bird surveys began this year on April 27, we have detected over 80 bird species in the Poopenaut Valley study area. In addition to the regular, resident species that are present year-round, we have observed many north-bound spring migrants. Pulses of Warbling Vireos and Black-headed Grosbeaks, two of our Riparian Focal Species, were noticeable amongst the flocks of migrating Bullock’s Orioles, Yellow-rumped Warblers, Chipping Sparrows, and Red-winged Blackbirds. By the first week of May, the first pair of Yellow Warblers, another Focal Species, had set up territory. Although the nesting season seems to be somewhat delayed this year because of colder temperatures, late May nest searching yielded nests of American Robin, Bullock’s Oriole, Cassin’s Vireo, House Wren, and Yellow Warbler. Further, several pairs of Song Sparrows have been exhibiting nesting behavior, though we have not confirmed any of their nests.

In some regards the delayed nesting season is good; once the spring snowmelt begins and high flow releases from O’Shaughnessy Dam start to flood the riparian vegetation, any active nests along the Tuolumne River would likewise flood. If birds nest after the beginning of the planned high flow releases, they will only be able to place their nests in locations where they will be less likely to flood. Environmental flow recommendations being developed under UTREP for O’Shaughnessy Dam will likely include measures that should prevent nesting in low lying riparian areas that may be subsequently flooded by spring snowmelt releases.

An interesting aspect of early season breeding surveys is the possibility for finding unusual species that are still migrating toward breeding grounds farther north. This year, we have added seven new species to the total number of species detected: American Coot, American Crow, Common Yellowthroat, Gray Flycatcher (six individuals), Virginia Rail, Wood Duck, and Yellow-headed Blackbird. All of these species are considered rare in Yosemite National Park, which suggests that Poopenaut Valley is an important stopover for birds during spring migration.