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UBC Theses and Dissertations

How and why a small songbird, the Oregon Junco (Juncus hyemalis oregonus), breeds over a steep elevation gradient : shifting life-histories, adaptations, and costs and benefits with elevation Bears, Heather


I used a comparative approach to examine phenotypic plasticity in morphology and life-history traits of a songbird, the Oregon Junco (Juncus hyemalis oregonus), at the lowest (1, 000 m asl; 4 sites) and highest (2, 000 m asl; 4 sites) elevational extremes of its breeding range in Jasper National Park, Alberta, Canada. Further, I tested if juncos differ in stress levels and sensitivity to stressors ("adrenocortical sensitivity") among elevations by measuring baseline corticosterone concentration, and increases in corticosterone 30 and 60 minutes after exposure to a standardized stressor. The extremes of the elevation range used encompassed very different environmental conditions, with high elevations experiencing colder temperatures, more precipitation, delayed snowmelt and frequent snow and hail storms throughout the summer. Biotic communities also changed considerably with elevation, with high elevation habitats having lower plant productivity, and hosting different animal communities. These abiotic and biotic variables were expected to cause measurable differences in selected morphological, life history and endocrinological traits in juncos among elevations. High and low elevation males and females did not differ in wing length, beak size, tarsus length, body mass, and intrafurcular fat. However, high elevation males had longer central retrix feathers, which could enhance balance and stabilization in windy conditions. Intrafurcular fat levels in females at high elevations also decreased seasonally. Nests at higher elevations were constructed with greater amounts of insulative materials and placed in better-buffered microclimates. High elevation habitats supported lower junco densities. Juncos at high elevations produced fewer broods per season compared to low elevation juncos. In addition, the number of fledglings per brood surviving to 20-30 days of age was lower at high compared to low elevations. Adults and offspring returned at similar rates to their sites of capture at low and high elevation sites between years. Overall, high elevation birds had a lower seasonal fecundity. Despite this seasonal deficit in reproduction, juncos at high elevations were not more likely to belong to an inferior competitive class (younger, smaller or later arrivers). Thus, the wide spread in elevation occupied by this bird is not caused by dominants excluding subordinants from optimal habitat, but by a trade-off in costs and benefits among elevations. Lower fecundity at high elevations may have been partially compensated by low levels of infection by the blood parasites Haemoproteus, Leucoctozoon and Trypanosoma spp, in adults and fledglings. Also, fledglings at high elevations were heavier and had more intrafurcular fat prior to fall migration. These 3 factors could increase adult and fledgling survival. Baseline corticosterone concentration in juncos did not differ among elevations. Stress-induced corticosterone increases in juncos did not differ markedly among elevations during the initial stages of the stress reaction, but the degree of increase was slightly greater between 30 and 60 minutes post-capture at low elevations. Females had a lower adrenocortical response to stress than males, and breeding individuals had a lower response to stress than non-breeders. The influence of gender and breeding stage may be explained by a previously established interaction between testosterone and indirect upregulation of corticosterone in juncos.

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