Over the past 20 yr Warbling Vireo (Vireo gilvus) populations have declined in California. We monitored Warbling Vireo nests in the high elevations of the northern Sierra Nevada in the Tahoe National Forest near Truckee, California. Nest survivorship was low (29.9% Mayfield estimate) compared to Warbling Vireo populations outside of California, but similar to levels reported for other California populations. Brown-headed Cowbird (Molothrus ater) parasitism in the study population was low (7%). Warbling Vireo nest fate was related to nest-site location; successful nests in lodgepole pine (Pinus contorta) were situated on the west or “warmer” side of the tree, in the outer periphery of the foliage, and in areas with slightly less canopy cover compared to unsuccessful nests. These features of the nest site may help reduce the threat of predation and combat cold stress. Our results argue that Warbling Vireos in the northern Sierra Nevada have low nest survivorship (similar to populations in other areas of the state), and that successful nests are often found on thin branches well removed from the main stem.
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Vol. 76 • No. 4