The willow flycatcher (Empidonax~traillii),a small passerine that nests in riparian deciduous shrub assemblages in the Sierra Nevada, is impacted by livestock grazing through alteration in shrub structure and upset of nests. From 1983 through 1986, four of 20 studied nests were destroyed by livestock prior to the young fledging. During 1987, no nests were upset by cattle. This may have been the result of a new livestock management program that reduced stocking levels by 4 0 percent and held 75 percent of the remaining livestock in a fenced pasture until 15 July. Attributes of nests that make them susceptible to cattle induced upset include their low height within the shrubs, the small diameter and therefore weakness of their supporting limbs, their proximity to water, the low branch density near the nest, and their proximity to shrub edges, including trails created by cows where they travel through shrubs. During 1987, nonviability was high as seven of 14 eggs f ailed to hatch. After the 1987 breeding season, a landowner in Dinkey Meadow bulldozed, as range improvement, an area where two of the three territories were held each year of our studies.
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