Postfledging Survival and Movements of Willow and Dusky Flycatchers in the Central Sierra Nevada



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Understanding factors limiting population growth is critical for species exhibiting declining populations. Reproductive success has an important effect on population dynamics; however, our ability to accurately estimate productivity is limited. Studies on avian breeding biology have focused on nest survival; however, surviving to fledging does not ensure survival to the end of the breeding season. Furthermore, our understanding of habitat selection by birds based on the nesting cycle may not adequately represent the breeding habitat requirements because habitat use often changes after the young leave the nest. My goal was to examine the postfledging dependence period of two flycatcher species in the central Sierra Nevada: the California state endangered willow flycatcher (Empidonax traillii) and the dusky flycatcher (E. oberholseri). My focus was to estimate fledgling survival and examine factors that influence survival, evaluate postfledging movements and habitat use, and estimate post-breeding home range sizes of postfledging flycatchers. I monitored nests of both flycatcher species, individually color banded nestlings, and observed family groups daily once the young fledged. Flycatcher fledgling survival ranged from 46 percent to 76 percent and varied by year and species. Survival was lowest during the first week of the postfledging dependence period for both species. Fledgling flycatchers moved on average ~45m per day during the dependence period. I detected family groups in the natal meadows from 13 to 33 days. I detected willow flycatchers in riparian shrub vegetation 94 percent of the time, with the remaining detections being along the upland forest edge. Dusky flycatchers were more likely to use upland forest vegetation after leaving the nest, as I detected them in riparian shrub vegetation 70 percent of the time. For both years combined, mean 95 percwnt home range sizes were 1.80 ? 1.44 ha for willow flycatchers and 1.82 ? 1.70 ha for dusky flycatchers. Mean 50 percent core areas were 0.33 ? 0.27 ha for willow flycatchers and 0.38 ? 0.44 ha for dusky flycatchers. My results suggest that using fledgling survival throughout the dependence period to assess reproductive output is more accurate than using nesting data alone. Furthermore, postfledging family groups used a larger area of habitat than what is typically estimated from territory mapping singing males. Future research should continue to stress the importance of gaining knowledge about the postfledging period, especially for species with declining populations.