Urban nesting biology of a threatened subspecies of Willow Flycatcher
Abstract
The Little Willow Flycatcher (Empidonax traillii brewsteri), a subspecies of Willow Flycatcher found in northwestern Oregon, is a declining population unit that is a Species of Concern for the US Fish and Wildlife Service (USFWS). Its breeding status in Portland’s greenspaces is unclear as previous surveys detected singing males without evidence of breeding. We thus conducted surveys and nest searches in 2019 (May through August) at Powell Butte Nature Park, Foster Floodplain Natural Area and Mason Flats to confirm breeding, and quantify nesting biology. The 17 nests found confirmed the presence of breeding birds. Clutch size averaged 3.3 eggs (2 to 4) and successful nests fledged an average of 2.4 young (2 to 3). After correcting for exposure time, 41% of nests fledged young. To evaluate whether the observed nesting productivity could sustain populations in Portland we simulated population growth using known values for adult (Sa = 0.610) and juvenile survival (Sj = 0.300) from the literature, with the additional assumption that females would make one renesting attempt if their first nest failed. Our preliminary analyses indicated that Portland’s Little Willow Flycatcher population is likely in decline as the estimated growth rate (λ = 0.810) was well below that needed for replacement (λ = 1.0). To achieve replacement levels, nestling production must increase. Additional field work is needed to confirm the low nesting success, but to also determine renesting propensity of females that fail, identify causes of failure, and assess the impact of invasive vegetation on nesting success.
Subjects
Animal ecology, Conservation biology, Wildlife biology, Habitat assessment
Persistent Identifier
https://archives.pdx.edu/ds/psu/33835
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Urban nesting biology of a threatened subspecies of Willow Flycatcher
The Little Willow Flycatcher (Empidonax traillii brewsteri), a subspecies of Willow Flycatcher found in northwestern Oregon, is a declining population unit that is a Species of Concern for the US Fish and Wildlife Service (USFWS). Its breeding status in Portland’s greenspaces is unclear as previous surveys detected singing males without evidence of breeding. We thus conducted surveys and nest searches in 2019 (May through August) at Powell Butte Nature Park, Foster Floodplain Natural Area and Mason Flats to confirm breeding, and quantify nesting biology. The 17 nests found confirmed the presence of breeding birds. Clutch size averaged 3.3 eggs (2 to 4) and successful nests fledged an average of 2.4 young (2 to 3). After correcting for exposure time, 41% of nests fledged young. To evaluate whether the observed nesting productivity could sustain populations in Portland we simulated population growth using known values for adult (Sa = 0.610) and juvenile survival (Sj = 0.300) from the literature, with the additional assumption that females would make one renesting attempt if their first nest failed. Our preliminary analyses indicated that Portland’s Little Willow Flycatcher population is likely in decline as the estimated growth rate (λ = 0.810) was well below that needed for replacement (λ = 1.0). To achieve replacement levels, nestling production must increase. Additional field work is needed to confirm the low nesting success, but to also determine renesting propensity of females that fail, identify causes of failure, and assess the impact of invasive vegetation on nesting success.