Scale Perspectives in Habitat Selection and Reproductive Sucess for Willow Flycatchers (Empidonax trillii) in the Central Sierra Nevad, CA

MASTERS THESIS: The Willow Flycatcher (Empidonax traillii) is a neotropical migratory species that has been extirpated from most of its range in California. Historically, the E. t. brewsteri and E. t. adastus subspecies were locally common in the Sierra Nevada mountain range of California. At this time it is believed that as few as 200 pair of this State Endangered species remain within the region. Because there is little information available on the current Willow Flycatcher population within the Sierra Nevada I examined habitat selection and animal performance at the meadow, territory and nest scales. During 1997 and 1998 survey efforts in 104 meadows documented Willow Flycatcher presence or absence, as well as broad scale vegetative and hydrologic condition. Six new willow flycatcher breeding sites were discovered, and 11 historically occupied sites were found to be vacant. Through intensive monitoring I followed reproductive success on 66 territories, and located and monitored 94 nests. Vegetative and hydrologic variables were measured in association with all territories and nest sites. I used multivariate and logistic regression to determine which variables best predicted animal performance at each of the three spatial scales: meadow, territory, and nest. IN addition, I used logistic regression to compare nest sites, territories, and occupied meadows with unused areas at each scale. Results suggest that shrub cover is the primary predictor of habitat selection at all three spatial scales. Of additional importance is a negative relationship between willow flycatcher presence and the dominance of forbs within the herbaceous layer, suggesting the need for meadows with elevated water tables and low disturbance levels. Shrub cover was again important in defining animal performance at the meadow and territory scales. Nest success improved with distance from trees. Management implications include the need to ensure that occupied meadows maintain elevated water tables, that shrub regeneration and recruitment occur, and that management activities do not disturb nesting birds. Unoccupied meadows should be restored to resemble occupied sites, particularly at historically occupied meadows, and meadows within dispersal distance of current breeding sites.