eWestern Birds

The Quarterly Journal of Western Field Ornithologists

Vol. 35, No. 1
March 2004
Western Field Ornithologists

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Warbling Vireo Nesting Ecology in the Northern Sierra Nevada
Julia I. Smith, Mark D. Reynolds, and Gretchen LeBuhn

ABSTRACT: In California, for unknown reasons, the Warbling Vireo (Vireo gilvus swainsonii) has poor reproductive success, and its numbers have declined over the past 20 years. From June through August 1998 we monitored 70 nests of the Warbling Vireo in a previously unstudied population on the eastern slope of the northern Sierra Nevada. Nests were generally placed 7 m or higher in mature trees, situated at over 50% of the nest-plant’s height, and most often in the periphery of the nest-plant’s foliage. The two most common nest-plant species were the Quaking Aspen (Populus tremuloides, n = 30) and Lodgepole Pine (Pinus contorta, n = 30). Such heavy reliance on a conifer by the Warbling Vireo has not been reported previously. Nest success was not significantly associated with habitat or nest-site characteristics; thus successful sites did not differ dramatically from unsuccessful sites. Most egg dates were concentrated within a single week (2–9 July), and most of the successful nests were in the egg-incubation phase during the first two weeks of July. Eighteen nests fledged at least one Warbling Vireo; one nest fledged a Brown-headed Cowbird (Molothrus ater). Seventy-four percent of nesting attempts failed; most failed nests showed signs of avian rather than mammalian predation. We recommend that vireo conservation efforts in the northern Sierra Nevada focus on increasing the availability of suitable nest sites by promoting mature stands of aspens and pines offering well-concealed nest sites in the periphery of the foliage, limiting forest disturbance in July during the critical nesting period, and minimizing environmental modifications that favor avian nest predators such as Steller’s Jay (Cyanocitta stelleri).

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