Nest Success of the Yellow Warblers (Dendroica petechia) and Willow Flycatchers (Epidonax traillii) in Relation to Predator Activity in Montane Meadows of the Central Sierra Nevada, CA.

MASTERS THESIS: Willow flycatchers (Empidonax traillii) and yellow warblers (Dendroica petechia) are both riparian-dependent neotropical migrant songbirds that have declined throughout much of their former range in California. Population declines have been primarily associated with the loss of riparian breeding habitat; increases in brood parasitism and nest predation have also contributed to declines and may be limiting population growth. IN this study, I determined the effect of potential nest predator activity on the nest success of both species, and identified potential willow flycatcher nest predators using yellow warbler nests as a surrogate. I used track plate transects, point counts, and time-constrained visual encounter surveys to assess potential mammalian, avian, and reptilian nest predator activity, respectively. I used simple linear regression and multiple linear regression analyses to determine the relationship between nest predator activity and nest success. Nest predation was the major cause of nest failure in both species. Only a small subset of the potential nest predator species detected in the meadows had activity indidices that were associated with the nest success of either species and include short-tailed weasel, Douglas squirrel, brown-headed cowbird, Clark s nutcracker, and Cooper s hawk. The magnitude of the effects of the different predator species on nest success varied depending on the prey species and the stage in the nesting cycle. Nest location relative to the forest edge and the amount of the meadow covered with water also influenced nest success. Short-tailed weasel, Douglas squirrel, lodgepole chipmunk, deer mouse and unidentified chipmunk species were photographed depredating yellow warbler nests baited with zebra finch eggs. Management recommendations include the active flooding of portions of the meadows to restrict meadow access to forest edge associated nest predators and habitat restoration in areas of meadows that avoid the creation of ecological traps should be employed to insure the persistence of high quality breeding habitat for willow flycatchers and yellow warblers in the central Sierra Nevada.