Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Master of Science

Program

Biology

Supervisor

MacDougall-Shackleton, Elizabeth

Abstract

Many animals display differential migration, where members of a population breed at the same site but overwinter different distances from the breeding grounds. Effects of overwintering latitude on apparent survival are difficult to study because only returning individuals can be sampled to assess overwinter latitude. I used stable isotope analysis of deuterium from 558 winter-grown claw tissue samples (δ2Hc) to examine the relationship between annual apparent survival and overwinter latitude in a differentially migrating population of Song Sparrows (Melospiza melodia). I integrated breeding-site capture histories of 383 song sparrows into a modified Cormack-Jolly-Seber model incorporating a hierarchical model of δ2Hc, then performed data analysis using a Bayesian framework via MCMC sampling to assess how apparent survival varies with overwinter latitude. Males had higher apparent survival and recapture probability than females, but overwinter latitude lacked association with apparent survival. These results help explain how differential migration is maintained in this population.

Summary for Lay Audience

Many birds migrate, defined as seasonal movement between two or more distinct locations (usually, breeding and overwinter sites). Migration provides both costs and benefits, for example, allowing birds to escape harsh winter conditions on the breeding grounds but carries great risks during the journey. In some bird species, individuals in the same breeding population vary in their migratory tendency or their overwintering destinations. For example, individuals in a population may all breed in the same location, but overwinter in different sites (known as differential migration). Such variation raises the questions of what the best overwintering locations are and the survival consequences of overwintering in different environments. Deuterium, an isotope of hydrogen (1H), can be used to infer overwinter latitude in migrating species since the ratio of deuterium varies with latitude in both the environment and in animal tissues However, sampling deuterium to assess overwinter latitude can only be done for individuals that survive and return to the breeding grounds. I combined capture records from 383 individual Song Sparrows (Melospiza melodia) in a differentially migrating population, together with deuterium values derived from 558 claw samples (some individuals sampled in multiple years) to assess the relationship between overwinter latitude and apparent survival. I used a statistical model that estimates how apparent survival and the likelihood an individual is caught vary with overwinter latitude, I found apparent survival did not vary with overwinter latitude, given that death cannot be distinguished from leaving the population. Overwintering latitude was overall similar between the sexes, but varied greatly among individuals. However, both apparent survival and the likelihood of being captured were higher in males than females. My results help to explain the maintenance of differential migration in this population. Song Sparrows may adapt well to most overwintering environments, the costs and benefits of migration distance balance out between individuals, and/or survival during the breeding season may be paramount. Lower apparent survival in females could reflect sex differences in reproductive investment and competitive ability, and/or females being more likely to leave the population.

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