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UBC Theses and Dissertations

Effects of climate on timing of breeding, reproductive output and population growth of song sparrows (Melospiza melodia) in the Southern Gulf Islands, British Columbia Wilson, Scott


In recent years, there has been a growing interest in the effects of global climate changes on avian population dynamics. Several studies have now shown that climatic oscillations such as the El Nino Southern Oscillation and the North Atlantic Oscillation affect a number of avian species through their impacts on local weather and ocean temperatures. In this thesis, I first reviewed the effects of these climatic oscillations on birds with a focus on identifying the underlying mechanisms by which they influence population change. I then examined the effects of climate on reproduction and population dynamics of song sparrows (Melospiza melodia) in the Southern Gulf Islands of British Columbia, Canada. Here, I first considered the influence of the El Nino Southern Oscillation on timing of breeding, fledgling production and population growth on Mandarte Island. I found that over the past 28 years, annual timing of breeding has not advanced in response to global warming, but has varied considerably among years in relation to variation in the El Nino Southern Oscillation. Females bred earlier in warmer El Nino years and later in colder La Nina years. Early breeding increased reproductive output, primarily because it increased the overall length of the breeding season and allowed females to make more nesting attempts. Despite this, timing of breeding had little effect on population growth, primarily because density-dependent effects on juvenile recruitment had an over-riding influence on population change. I then examined the regional influence of climate on reproduction in six adjacent song sparrow populations. Here, I predicted that if climate was the dominant factor affecting reproduction, we should observe synchrony in annual reproductive rates across all populations. In contrast, if local factors that vary among populations have the greatest influence on reproduction, we should observe asynchrony in reproductive rates. I found that populations displayed synchrony in the onset of egg laying, suggesting that climate affects timing of breeding similarly across all populations. However, populations displayed considerable variation in reproductive output, which was largely driven by differences in the extent of nest predation and brood parasitism. Rates of nest predation and brood parasitism were high in populations close to Vancouver Island, and lower in the more isolated populations. Overall, the results of this thesis suggest that populations will vary in their response to climate change depending on the influence of climate on demographic parameters that contribute most to population change and the relative influence of other ecological factors on reproduction and survival.

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