UBC Theses and Dissertations
Demographic shifts and the role of climate warming in a switch from migrant to resident life history Visty, Hannah
Identifying causes and consequences of variation in species life history should improve predictions about how climate and land use change will affect the demography and distribution of species in future. Sooty fox sparrows (Passerella iliaca unalaschcensis) were documented as obligate migrants, abundant in winter but with only three breeding records in coastal habitats of British Columbia and Washington prior to 1950. Because this subspecies has since established year-round resident populations in this region, I studied resident populations of sooty fox sparrows to test theory on how climate change and life history might affect the demography and distribution of a new partial migrant. I estimated demographic vital rates in one recently established resident population on Mandarte Is., BC, using color-banded birds. Annual fecundity (F) was higher than reported in migrant populations studied previously in Alaska and Newfoundland, supporting the hypothesis that residents invest more in reproduction on average than migrants within species. I also estimated high juvenile and adult overwinter survival (Sj = 0.32 ± 0.06, and Sa = 0.69 ± 0.05) and population growth (λexp = 1.61 ± 0.57), implying rapid population growth. I next tested the hypothesis that climate warming facilitated the establishment of resident populations by reducing the net benefit of migrating out of the wintering area to breed. Because resident sooty fox sparrows breed earlier and longer than migrants, I asked if climate warming during the pre-breeding period coincided with patterns of establishment by: 1) characterizing the pre-breeding climate niche of resident populations using species distribution models, occurrence data, and monthly climate records, and 2) testing if the emergence of the pre-breeding climate niche currently occupied by resident populations corresponds to the first reports of sooty fox sparrows breeding after 1920. Niche models suggest that the mild, near-shore climate niche now occupied by recently established resident populations expanded dramatically from 1920 to 2015 in a pattern matching early records of breeding by sooty fox sparrows within their historic wintering area from 1950. Whereas prior studies focus on climate warming affecting overwinter survival, my results suggest warming may also affect migration through improved fecundity and breeding niches.
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