Abstract
Three hypotheses have been proposed to explain sexual differences in wintering latitude for different bird species: (1) intersexual behavioral dominance leads the subordinate sex to migrate farther to avoid competition; (2) intrasexual selection favors those individuals of one sex that arrive earlier and thus selects for wintering closer to the breeding ground; and (3) sexual differences in physiological tolerance allow the larger sex to survive harsher climates. Using sex, age, date, and location data from specimens collected south of the breeding range in the western hemisphere, I tested predictions of these hypotheses for two scolopacid shorebirds showing reverse sexual size dimorphism, the red phalarope, Phalaropus fulicarius, and the sanderling, Calidris alba.Neither red phalaropes nor adult sanderlings showed any sexual difference in wintering latitude. First-winter male sanderlings tended to winter farther south than first-winter females. Combined with comparative data from other species of shorebirds and passerines, these results are consistent only with hypothesis 2.
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