Climate change implications for distribution, phenology and conservation of Olive-sided Flycatchers (Contopus cooperi) and Western Wood-Pewees (C. sordidulus) in northwestern North America

  • Author / Creator
    Stehelin, Tara
  • Northwestern North America is predicted to experience some of the world’s greatest human-caused climate change. Understanding the impacts of associated changes will be imperative to the conservation and management of northern-breeding birds. In particular, long distance migrants and aerial insectivores, such as the Olive-sided Flycatcher (Contopus cooperi, OSFL) and the Western Wood-Pewee (C. sordidulus, WEWP), may be impacted disproportionately, in interaction of climate change with additional ex situ population stressors, such as habitat loss and changes to insect populations. Climate-mediated effects on distribution and abundance of these species, both of which have experienced dramatic population declines over the past half century, might be mediated independently of habitat loss by identification of areas of potential climate macrorefugia. To describe contributions of habitat elements on distribution and abundance of these species I generated boosted regression tree models, using data from point counts conducted between 1992 and 2014 at 1049 unique locations in the boreal and hemiboreal zones of northwestern North America. Bootstrap runs of models randomly selected abundances and absences from each location, stratified by number of observations, then built stagewise models from a suite of climate, landcover, topographical and disturbance covariates. I included offsets for unequal detectability of birds by observers in model development. Covariates describing vegetation and landcover were most important in describing abundance, followed by those for climate and topography. Influences of many covariates were non-linear and specific to species, but overall they described habitat that was forested, mid-elevation, topographically complex, and moderate in temperature, precipitation and length of summer season. Relative habitat associations revealed that open forest types, tundra, wet areas, riparian habitat, and old burns were positively associated with predicted abundances of both species. To predict the influence of climate change on distribution, I generated a second set of models with seven climatic covariates from a baseline time period of 1981 – 2010, and two future time periods: 2041-2070 and 2071-2100, under comparatively high (RCP8.5) and mid-low (RCP4.5) greenhouse gas emission trajectories. I included a small number of baseline landcover and topographical spatial covariates in these models to constrain prediction outside of the plausible range. Mean outputs projected to grids revealed high predicted relative abundances in the northwestern terrestrial regions of the study area, especially in riparian areas and mid-high elevation forest. Applying a spatiotemporal gradient approach, I found areas of low climatic suitability (negative bioclimatic velocity) in central and northwestern BC, and this velocity became more pronounced with time. Areas of positive bioclimatic velocity occurred in small, isolated regions in far western Alaska and southern/interior BC, although at a micro-refugium level details were specific to species. Predicted population size was initially high, and either remained the same, or declined between the baseline and future scenarios for OSFL, but increased for WEWP. Similarly, areas of predicted abundance (higher than the median abundance from the baseline period) declined between the baseline and future scenarios for OSFL (as indicated by more areas of loss than gain), but increased for WEWP, underscoring the importance of planning proactively for future species-specific needs. Most predicted gains in distribution were in the far northeastern and northwestern portions of the study area, calling into question the utility of these potential refugia given size and accessibility. Most predicted losses in abundance were inland regions in the northern part of the study area (central Alaska and Yukon). Climate change may alter phenology of community members, affecting timing of availability and abundance of essential insect prey for breeding birds. Data to support this general prediction remain scant, especially for North American birds. Phenological investigation into the abundance and diversity of insect prey, as well as breeding events and success of breeding birds in southern Yukon, revealed patterns of potentially declining insect abundance with year between 2013 and 2017, advancing laying dates with year, but not advancing arrival dates, highly variable and possibly declining nesting success with year. An index of daily abundance of insect prey was compared with phenology of these two flycatchers, but evidence of phenological asynchrony using was limited, in part because insect abundance did not reveal obvious or predictable annual peaks. Nonetheless, my results suggest that climate change will influence distribution, abundance, and breeding biology of these flycatcher species.

  • Subjects / Keywords
  • Graduation date
    Fall 2020
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
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