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UBC Theses and Dissertations

Elevation and the avian phenotype : field and experimental studies of breeding dark-eyed juncos Bears, Heather


Forty percent of the terrestrial planet is mountainous, yet little is known about how breeding elevation affects avian phenotypes. I studied dark-eyed juncos breeding at the extremes of their elevation range in Jasper. AB ('Low'; 1000 m and 'High'; 2000 m a.s.l.) from 2000-2005. I compared reproductive and morphological traits in free-living birds between elevations to establish patterns of change with breeding elevation. I subsequently investigated mechanisms underlying those patterns by collecting hatchlings and adults of both sexes from each elevation and raising them in a common lab environment. The common lab experiment allowed me to determine the amount of variation due to phenotypic plasticity in response to local conditions. Measurements in the field included indicators of reproductive stage, seasonal reproduction, philopatry/survival, age ratios, morphometrics, and local weather. Measurements in the lab included indicators of reproductive development over time (after birds were stimulated to breed with an increasing photoperiod), and morphometrics (after birds replaced feathers in captivity). I addressed the following questions: (1) How do life-histories vary with breeding elevation?, (2) What environmental factors correlate with reproductive timing between elevations?, (3) Are reproductive differences between elevations due to local genetic adaptation or phenotypic plasticity?, and (4) Does morphology vary with elevation, and are differences due to local genetic adaptation or phenotypic plasticity? High-elevation birds in the field became reproductively capable >6 weeks later than low-elevation birds and produced half the number of broods and offspring per season. High-elevation habitats were not occupied by younger, smaller (i.e. , less competitive) birds, and mark-recapture analysis suggested that high-elevation birds live longer. High-elevation males and females had longer tails and wings, respectively, than low-elevation birds. Both populations initiated more nests as rain and insect abundance increased. However, high-elevation birds initiated nests synchronously when the snow melted, while low-elevation birds initiated nests as growing degree days increased. In captive birds, the timing of breeding readiness was reversed in the lab relative to the field in both sexes and ages: i.e., high-elevation birds were able to breed earlier than low-elevation birds. Birds from both elevations increased morphological trait sizes in the lab relative to the field, suggesting they were released from growth constraints. However, after birds grew in the lab, relative differences between elevation groups remained (in the same direction as in the field), or were exacerbated. The reproductive and morphological responses observed in the lab relative to the field suggest that environmental constraints and countergradient forces can interact in complex, unexpected ways to shape avian phenotypes among elevations.

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