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UBC Theses and Dissertations

Biomechanics and behavior of hummingbird molt Christensen, Beth A.

Abstract

This thesis explores how hummingbirds cope with molt, a costly component of their annual cycle, on two very different time scales. My first experiment explores aerodynamic explanations for the unique, non-sequential primary molt pattern followed by all hummingbirds, with the molt sequence of the two outermost primaries reversed in order compared to other birds. I simulated both the ancestral, sequential molt pattern and the non-sequential pattern in rufous hummingbirds (Selasphorus rufus) by removing primary 9 or 10, respectively. I used high speed video (500 fps) to film hovering events before wings were altered, immediately after I removed either feather, and approximately one week later. From the video, I estimated wingbeat kinematics of birds and used oscillator and aerodynamic theories to predict and interpret results. Results suggest possible aerodynamic benefits of the pattern followed by hummingbirds compared to the basic pattern they have evolved away from. My second study investigates how captive molting hummingbirds alter their behavior during molt. I used focal animal sampling to record frequencies and durations of flights, feeding bouts and aggressive encounters before, during and after natural molt. Molting birds flew less, fed less frequently, and engaged more often in aggressive encounters during molt than non-molting periods. These behavioral changes may be a mechanism to partially or entirely offset costs of the molting process. Natural selection has resulted in diverse ways to reduce the costs of molt, including how birds molt. Hummingbirds strictly adhere to a unique primary molt pattern, and results of this study show evidence of possible benefits. Changing behavior allows individual birds a means to compensate for the costs of molt on a daily basis. Overall, these are only two ways that hummingbirds cope with a necessary component of their annual cycle.

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