Portland State University. Department of Biology
Michael T. Murphy
Term of Graduation
Date of Publication
Master of Science (M.S.) in Biology
1 online resource (xi, 121 pages)
Reproductive success, in any species, relies both on the habitat utilized to reproduce and on the successful reproductive output of individual animals within a population. Populations in turn rely on the reproductive fitness of the individual. Whether populations remain stable, grow, or decline ultimately relies on the combined reproductive output of the individuals within that population.
We studied the Willow Flycatcher (Empidonax traillii; henceforth WIFL), a well-studied Neartic-Neotropical migrant passerine whose breeding range extends from the east to west coast of North America. Across much of their range, and in particular in the western regions, breeding population are in decline, with habitat loss likely being the main factor responsible for their declining numbers. Habitat preferences, nesting biology, and documentation of reproductive success of WIFLs have been described in semi-natural (i.e. nonurban) habitats over much of their range, but to date, no work has been done to document the same statistics in urban settings. The work described herein was initiated with the goal being to begin to rectify this deficiency. I conducted research on the nesting biology and habitat use of WIFLs in two parks in Portland, OR. My research was conducted from June to August in 2019 and 2020 at Powell Butte Nature Park and Foster Floodplain Natural Area. At both sites I located nests of breeding pairs, documented nest success, and measured habitat use and nest placement in trees/shrubs and compared habitat structure of sites that were used for nesting by WIFLs to nearby sites that were not used for nesting to evaluate habitat preference.
I found that habitat structure of used sites was similar between parks, and that at both parks WIFLs preferred nest habitats with greater canopy cover, lower ground cover, and greater foliage height density 1-3 m above ground than what was available. We found no link between habitat use and nest success. However, nest success was dependent on the location in the nest tree/shrub that was used for nesting. Successful nests had more cover surrounding the nest, were placed farther from the center of the nest tree/shrub, and were placed relatively closer to the edge of the nest plant's foliage.
I also incorporated vital rates (birth and death rates) into a population model to simulate future trajectory, projected over 10 years, of WIFLs in Portland's urban environment. After correction for exposure time using Mayfield methods, nest success of WIFLs was 47%, a value similar to that recorded at multiple sites across North America. Failed breeders typically replaced their initial nesting attempt. To estimate seasonal reproductive output we combined nest success, number of offspring fledged/successful nest, and a simulated number of renesting attempts. To fully parameterize the model to estimate population growth rate, I also "borrowed" estimates of both adult and first-year survival from studies of WIFLs conducted elsewhere. Over a range of rate of nest replacement rates (0.6, 0.7, and 0.8), and number of replacement nests allowed (0, 1, or 2), I found that population decline was inevitable using our observed rates of nesting and fledging success. Raising fledging success to 3.0 young/nest generated stable populations if 2 replacement nests were allowed at the highest rate of nest replacement. Additionally, we found that nest success was significantly higher when nests were placed in two specific plant species, Pacific Ninebark (Physocarpus capitatus) and Himalayan blackberry (Rubus armeniacus). Simulations that assumed that all nesting attempts were made in those 2 plant species indicated that positive population growth was achievable with all combinations of vital rates. My results demonstrate the necessity of renesting after failure for Portland's WIFL populations to persist. Moreover, given the high nesting success of WIFLs in Himalayan blackberry, my results suggest that we need to rethink our attitude towards this invasive plant species that may offer respite to declining WIFL populations.
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Doorly, Stephanie Alexandra Byrne, "Urban Nesting Biology, Population Viability and Ecology of the Willow Flycatcher, Empidonax traillii" (2022). Dissertations and Theses. Paper 6245.