***In 2007 we completed our Yosemite Willow Flycatcher study, and we have recently published our results in the journal Western Birds. Click here to view our study results.***

The Willow Flycatcher (Empidonax trailii) has declined precipitously in the Sierra Nevada since the middle of the twentieth century. Once considered ‘common’ throughout much of the Sierra Nevada, the Sierra population was estimated to have dwindled to just 300-400 individuals in the late 1990s. The population may already have dwindled further, as the most recent population growth rate estimate (lambda) for the central Sierra population was lambda=0.87, a value that suggests the population is now halving every five years.

Willow Flycatcher (Empidonax trailii)
(Photograph courtesy of Bill Schmoker)

A recent review of the ecology and conservation status of Willow Flycatcher in the Sierra tentatively concluded that poor nesting success is primarily responsible for the species’ current decline, and that poor nesting success is largely a result of gradual meadow desiccation, which allows mammalian predators easier access to Willow Flycatcher nests. Primary causes of meadow dessication throughout the Sierra have included livestock trampling, road construction, recreation activities, adjacent timber harvests or fuels treatments, fire suppression, water diversions, mining, and climate change. Many of these factors would appear to play little if any role within the confines of Yosemite, at least in recent decades, making the species’ recent decline in the park somewhat of a mystery.

Yet the species clearly declined throughout the park during the twentieth century, and the decline appears to have continued up to the present. At least into the early twentieth century, Willow Flycatchers commonly nested in Yosemite Valley and were ‘vocal, conspicuous birds’ in suitable habitat throughout the lower elevation regions of the park. However the species apparently has not nested in Yosemite Valley since 1966, and has become increasingly scarce throughout the park’s meadows and riparian thickets, with declines evident even since the early 1990s. The Monitoring Avian Productivity and Survivorship (MAPS) station at Hodgdon Meadow, which captured between 10 and 18 individual Willow Flycatchers per year during the early 1990s, has caught no more than two individuals in a year since 1999, and has caught no Willow Flycatchers in four of the last six years.

This two-year (2006-2007) project was designed to assess the current status of Willow Flycatcher in Yosemite. Our primary objectives included the following:

• determine the precise locations of any remaining Willow Flycatcher territories in the park, and

• characterize the habitat that is supporting them.

Collecting such data may enable park managers to take specific actions to maintain the existing population or avoid taking actions that might jeopardize it. Such actions could involve recreation management, fire management, or management of encroaching conifers at any meadows or riparian areas found to still host Willow Flycatcher territories.

For more information, contact Rodney Siegel.

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