Carryover Effects of Winter and Pre-breeding Conditions on Reproduction in Northern Common Eiders Somateria mollissima borealis Nesting in Arctic Canada
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Life-history stages are conceptually and temporally separated, yet interactions between these stages still occur. The overwintering phase of the annual cycle has become increasingly recognized for its selective pressures which can generate carryover effects. Migratory animals like the northern common eider provide a good model system to explore carryover effects because individual experiences can vary throughout different stages of their life history such as during overwintering, breeding, and migration. The overarching goal of my thesis is to investigate the underlying mechanisms that influence breeding including the potential for carryover effects of environmental factors across multiple seasons. To achieve this goal, my work has four objectives: 1) establish a minimally invasive method to assign common eiders to overwintering sites following arrival to breeding grounds, 2) use this method to gain insight into when common eiders form breeding pairs, 3) characterize changes in male physiology and condition throughout the pre-breeding period and how male physiological state influences female reproduction, and 4) link winter and spring environmental conditions to breeding decisions. In Chapter Two, I use stable isotopes of carbon to assign an individual to their overwintering site. In Chapter Three, using the stable isotopes of carbon in claws and blood to represent locations in winter and spring, respectively, I show some pairs form on the wintering grounds, but the majority of pairs form during spring, which may have fitness benefits in some years. In Chapter Four, using a suite of physiological traits, I demonstrate male mate guarding behaviours and energetic costs are primarily aimed towards securing paternity, rather than benefiting female reproductive traits. In Chapter Five, I use a path analysis to demonstrate both spring and winter conditions can impact breeding, but eiders are likely able to buffer for poor winter conditions during spring to invest in reproduction. Collectively, my thesis shows, although there are apparent benefits to the timing of pairing, and winter conditions impact arrival body mass, ultimately female reproductive decisions are influenced by spring breeding conditions. Truly, conditions during the pre-breeding period are the most important factor impacting reproductive investment via the ability for females to accrue fat stores.