The Pacific Coast population of the Western Snowy Plover (Anarhynchus nivosus nivosus) is a federally Threatened species and is a California Species of Special Concern. Knowledge about their nest site selection requirements is important to this species’ management in the San Francisco Bay area. We measured the percentage of sand, percentage of crushed oyster shells, number of shells, and total shell surface area for 56 nests of Western Snowy Plover at the California Least Tern colony at Hayward, California, over a 14-year period (2008-2021). Using pairwise t-tests, we compared these measurements to those obtained from 56 randomly chosen non-nest sites. Results indicate that plovers select nest sites with greater oyster shell metrics than paired random sites. The contrast in the shell-related nest metrics (i.e., the difference between the nest site and the paired site) showed significant, positive relationships with both hatching success and number of fledglings using linear regression analysis. These results, supporting federal species recovery, suggest that the Western Snowy Plover may improve their nesting success through oyster shell enhancement, which may provide more camouflage for the eggs and chicks than locations with fewer shells.

The Purple Martin has been a California Species of Special Concern since 1978. Its population and geographic range have declined substantially due to competition with the European Starling, snag nesting habitat loss, bridge construction disturbance, utility pole replacement, and insecticide-induced loss of prey. A statewide status assessment has not been conducted since 2008 and is needed to identify conservation priorities and management needs. We are leading a statewide survey in 2025 using specialized methods developed for a 2009 Northern California survey. Our review of 2011-2024 eBird records identified 234 known breeding sites. We will re-survey 28 of the 2009 survey sites to determine changes in occupancy and populations and 58 additional eBird sites to establish baseline populations. We will develop a habitat-population model to estimate the numbers of potential breeders in areas not accessible to eBirders. Results from the survey, eBird, and habitat-population model will be combined to estimate the statewide population. Our review of eBird records indicates that martins are concentrated in coastal conifer forests, and they nest mainly in snags (43% of sites), utility structures (32%), and bridges (17%), each of which has unique management threats that should be addressed to conserve the species.

Urbanization is a rapidly expanding global phenomenon that significantly changes natural habitats, often forming novel ecosystems characterized by various urban features. These changes affect bird species, including Cardueline finches, like lesser goldfinches (Spinus psaltria), house finches (Haemorhous mexicanus), and pine siskins (Spinus pinus), which are common in California and frequently found near bird feeders. In urban environments, these birds face stressors such as elevated noise, light pollution, chemical exposure, and increased human activity. These stressors can also affect vectors of bloodborne parasites, such as avian haemosporidians, impacting parasite-host dynamics. To test whether urban stressors result in an elevated stress response, we quantified the heterophil/lymphocyte (H/L) ratios in finches, across an urban-rural gradient. H/L ratios are a well-known indicator of stress and immune function in birds. We hypothesize that urban-dwelling Cardueline finches will show higher H/L ratios, reflecting a stress response due to the prevalence of stressors in urban environments. To control for H/L ratios, we are quantifying parasitemia of haemospordians and other bloodborne parasites, as acute infections are expected to elevate these ratios. Our findings will provide insights into how urbanization influences the stress and immune response of finches, contributing to a broader understanding of how urbanization impacts wildlife health.

The once common, endangered Southwestern Willow Flycatcher (Empidonax traillii extimus, SWFL) is rare throughout California, with scattered populations occurring at a few sites in the state. Most of these populations have declined over the past 10 to 20 years, however, the Owens River population has increased during this time. We monitored SWFLs on the Owens River during the 2021-2023 breeding seasons to determine abundance, nesting success, and nest parasitism rates. We estimated that there were at least 57 to 101 SWFL territories in our study area from 2021 to 2023, a significantly higher population than reported in 2016 (27). We recorded an average apparent nest success rate of 52% (n=38, 2021 and 2022); and an average parasitism rate of 34% (n=61, 2021-2023). Willow Flycatcher breeding success on the Owens River over the past years has apparently been good enough to support a healthy flycatcher population despite Brown-headed Cowbird parasitism. Nonetheless, reducing cowbird parasitism would be a good management strategy given the decline of Willow Flycatcher populations in California. Increasing the number of young birds produced in this population could also increase the dispersal of flycatchers to other areas along the Owens River as well as beyond the Owens River watershed.

Purple martin (Progne subis arboricola), a California Species of Special Concern, historically nested in existing wood power poles at Mt Thayer in Santa Clara County. By 2008, only 100-220 pairs were estimated in the southern coastal area of California from Marin to Santa Barbara Counties and are suggested to be close to local extirpation, making the small Mt Thayer population regionally significant. For several years the Midpeninsula Regional Open Space District has monitored martin breeding activity at Mt Thayer and provided a variety of nesting structures ranging from traditional nest boxes to experimental hollow cavity structures that emulate natural habitat, to support state goals of reaching >2,000 martin pairs and to support nesting in cavities rather than bird boxes or houses. In 2024, martins selected experimental cavity structures over traditional boxes, and successfully fledged a record high of 13 young at the site. Cavity structures can be used as an effective tool to support nesting martins and to facilitate preference for natural cavities. The District has formalized the cavity structure design to share with others for implementation at suitable sites within their western range.

Preferred Session: Natural History of Birds

Abstract:

Sex-based habitat segregation in passerines is most commonly documented on wintering grounds with growing evidence of its importance during migration. However, sex-based habitat segregation in passerines on their breeding grounds is poorly represented in the literature. An examination of perch height, perch exposure, and prey-capture site exposure as part of a 20-year study of leg-banded Southwestern Willow Flycatchers (Empidonax traillii extimus) in San Diego County demonstrates significant differences between sexes. Possible explanations for sexual segregation include differential forage selection, reduction of depredation risk in relation to activity budgets, higher reproductive success, and possible enhancement of genetic health. The latter two factors may may be of specific relevance to semi-colonial species given the high level of “stolen copulations” reported (and in this study, observed) and which result in extra‐pair paternity. My findings recommend specific habitat management, restoration, and habitat creation strategies that address the needs and preferences of both sexes, a missing element in habitat management and recovery planning for this endangered species.