Author: Whitehorne, Ivy Bronwen Jean
In some areas, populations of American dipper consist of altitudinal migrants and sedentary individuals that remain at low elevation year-round. Migrants have lower reproductive success due to later initiation of breeding, but also have higher annual survival than sedentary individuals (residents). I first evaluated whether age effects explain this difference in reproductive success. I then studied this apparent trade-off by evaluating if it is an expression of the cost of reproduction, and investigating if use of a multipurpose territory year-round restricted the foraging or site fidelity of residents during winter. Age effects do not explain the differences in reproductive success between migrants and residents. Additionally, there was no evidence that the greater reproductive effort of residents results in poorer physiological state that could negatively affect survival. However, migrants and residents differ in their fidelity to their wintering area, which may contribute to higher overwinter survival for migrants.
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