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Open Access Publications from the University of California

Scholarly Works (4 results)

  • Article
  • Peer Reviewed
UC Davis Previously Published Works (2014)
In metapopulations in which habitat patches vary in quality and occupancy it can be complicated to calculate the net time-averaged contribution to reproduction of particular populations. Surprisingly, few indices have been proposed for this purpose. We combined occupancy, abundance, frequency of occurrence, and reproductive success to determine the net value of different sites through time and applied this method to a bird of conservation concern. The Tricolored Blackbird (Agelaius tricolor) has experienced large population declines, is the most colonial songbird in North America, is largely confined to California, and breeds itinerantly in multiple habitat types. It has had chronically low reproductive success in recent years. Although young produced per nest have previously been compared across habitats, no study has simultaneously considered site occupancy and reproductive success. Combining occupancy, abundance, frequency of occurrence, reproductive success and nest failure rate we found that that large colonies in grain fields fail frequently because of nest destruction due to harvest prior to fledging. Consequently, net time-averaged reproductive output is low compared to colonies in non-native Himalayan blackberry or thistles, and native stinging nettles. Cattail marshes have intermediate reproductive output, but their reproductive output might be improved by active management. Harvest of grain-field colonies necessitates either promoting delay of harvest or creating alternative, more secure nesting habitats. Stinging nettle and marsh colonies offer the main potential sources for restoration or native habitat creation. From 2005-2011 breeding site occupancy declined 3x faster than new breeding colonies were formed, indicating a rapid decline in occupancy. Total abundance showed a similar decline. Causes of variation in the value for reproduction of nesting substrates and factors behind continuing population declines merit urgent investigation. The method we employ should be useful in other metapopulation studies for calculating time-averaged reproductive output for different sites.
    Cover page: Combining Site Occupancy, Breeding Population Sizes and Reproductive Success to Calculate Time-Averaged Reproductive Output of Different Habitat Types: An Application to Tricolored Blackbirds
    • Article
    • Peer Reviewed
    UC Davis Previously Published Works (2023)
    There is increasing awareness of the negative ecological and environmental effects of widespread use of pesticides on the landscape. Spillover or drift of pesticides from agricultural areas has been shown to impact species health, reproduction, and trophic dynamics through both direct and indirect mechanisms. Neonicotinoid insecticides are associated with observed declines of insectivorous and grassland birds, and these environmental pollutants are a significant conservation concern for many species that have experienced past or current population declines. Due to the high efficacy of these modern insecticides in depressing local insect populations, insectivorous birds can be negatively impacted by a pesticide-mediated reduction in food supply. Neonicotinoids may act synergistically with other stressors, such as habitat loss, to exacerbate threats to species or population viability. The Tricolored Blackbird is an insectivorous grassland bird of conservation concern in California, USA. Due to the high association of this species with agricultural habitats, we sought to quantify the amount of neonicotinoid residues in Tricolored Blackbird carcasses as a first step in assessing how this species may be impacted by pesticides. Out of 85 salvaged carcasses sampled (N = 24 adults, N = 3 fledglings, and N = 58 nestlings), only two contained detectable levels of target compounds. These were an adult and one nestling that contained clothianidin residue (40 ppb and 7 ppb, respectively); both of these birds were salvaged from breeding colonies associated with dairy farms in Kern County, California. We suggest that further work is needed to assess neonicotinoid exposure of Tricolored Blackbirds in dairy-associated breeding colonies.
      Cover page: Neonicotinoid exposure in Tricolored Blackbirds (Agelaius tricolor)
      • Article
      Recent Work (2007)

      This abstract reports on the results of a California joint DOT-university project to develop database, modeling, and GIS tools and to publish an electronic manual and digital library to address animal-vehicle collision reduction and connectivity issues in the state.

      Despite the potentially large impacts of roads on wildlife movements and mortality, California has historically lacked standardized information tools to enable wildlife and transportation planners and managers to best identify and mitigate those impacts. These issues are being addressed by several California Department of Transportation research and development initiatives, including Animal Vehicle Collision Reduction and Connectivity Issues, Fish Passage, and Advanced Mitigation, much in collaboration with the University of California. Goals include addressing:

      • Developing Useful Metrics, Standardizing Data Collection Techniques and Data Analysis Models/Tools for Assessment

      • Improving Safety and Delivery in the Caltrans Planning and Project Development Process

      • Strategic Outreach and Engagement

      • Development of Guidance Documents and Support Tools for Analysis based on the developed Metrics, Standards, and models

      • Working at a program level to optimize and leverage funding opportunities for collision reduction and connectivity issues

      Elements of a new analytic framework to address this need include integrative GIS tools to identify, on both local and regional scales, locations of core wildlife populations, mapping of least-cost-path movement corridors to identify locations posing high risks of crossing mortality, a library of species-specific information on movement patterns and models, tools to develop more systematic documentation of road-related wildlife mortality, and a clearinghouse for evaluating structures, technologies and networking approaches to remotely detecting the presence of wildlife around transportation facilities, evaluating impacts, and mitigating the effects. Improved data from remote sensing and GIS clearinghouses, new methods and sensors for detecting animals and movement, emerging technologies for networking distributed and heterogeneous data, new data standards and models, and better integration with other information sources can all contribute to decreasing road impacts on animal populations and movements. Results of this project include a newly published California manual for managing wildlife crossing issues and GIS, database, and supporting digital library tools on-line at the Information Center for the Environment (http://ice.ucdavis.edu).

        Cover page: An Analytical Framework for Wildlife Crossing Policy in California
        • Article
        • Peer Reviewed
        UC Davis Previously Published Works (2013)
        Population trends represent a minimum amount of information required to assess the conservation status of a species. However, understanding and detecting trends can be complicated by variation among habitats and regions, and by dispersal connecting habitats through source-sink dynamics. We analyzed trends in breeding populations between habitats and regions to better understand the overall dynamics of a species' decline. Specifically, we analyzed historical trends in breeding populations of tricolored blackbirds (Agelaius tricolor) using breeding records from 1907 to 2009. The species breeds itinerantly and ephemerally uses multiple habitat types and breeding areas, which make interpretation of trends complex. We found overall abundance declines of 63% between 1935 and 1975. Since 1980 overall declines became nonsignificant and obscure despite large amounts of data from 1980 to 2009. Temporal trends differed between breeding habitat types and were associated with regional differences in population declines. A new habitat, triticale crops (a wheat-rye hybrid grain) produced colonies 40× larger, on average, than other breeding habitats, and contributed to a change in regional distribution since it primarily occurred in a single region. The mechanism for such an effect is not clear, but could represent the local availability of foodstuffs in the landscape rather than something specific to triticale crops. While variation in trends among habitats clearly occurred, they could not easily be ascribed to source-sink dynamics, ecological traps, habitat selection or other detailed ecological mechanisms. Nonetheless, such exchanges provide valuable information to guide management of dynamic systems.
          Cover page: Understanding the contribution of habitats and regional variation to long‐term population trends in tricolored blackbirds
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