Survival and Movements of Fledgling Willow and Dusky Flycatchers

Authors: L. M. Vormwald, M. L. Morrison, H. A. Mathewson, M. C. Cocimano, B. A. Collier

Estimates of productivity usually ignore survival during the postfledging period, the time between a bird's leaving its nest and departure for migration or movement to an area for wintering. During the first 1–3 weeks after fledging, young birds are limited in mobility, making them vulnerable to fatality, especially from predation. To help fill this information gap, we examined the period of postfledging dependence of the Willow (Empidonax traillii) and Dusky (E. oberholseri) Flycatchers in the central Sierra Nevada, California. For fledglings we examined factors that influenced survival, movements, and habitat use and home-range size. We observed family groups daily during the postfledging dependence period and resighted individual band combinations. Fledglings' survival ranged from 46% to 76% and varied by year and species. Fledglings moved on average 45 m from the nest per day during the dependence period. We detected family groups in their natal meadows from 13 to 33 days. We detected Willow Flycatchers in riparian shrubs 94% of the time, along the upland forest edge for the remaining 6%. We detected Dusky Flycatchers in riparian shrubs 70% of the time, in upland forest for the remaining 30%. For both years of our study combined, mean 95% home-range sizes were 1.80 ± 1.44 (SD) ha for the Willow Flycatcher and 1.82 ± 1.70 ha for the Dusky Flycatcher. Mean 50% core areas were 0.33 ± 0.27 ha for the Willow Flycatcher and 0.38 ± 0.44 ha for the Dusky Flycatcher.

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