As indicated above, dominance and other factors could influence movements through direct effects on individuals at the time of migration, or through evolution, by providing a consistent selection pressure for the subordinate sex to migrate furthest, at a different time, or to occupy a different type of habitat, so that such features become innate and under genetic influence. Hence, dominance could drive differential migration at either a mechanistic (proximate) or evolutionary (ultimate) level, or both. The one could be a precursor to the other. Research on captive birds has provided evidence for genetic control of sex differences in migration behaviour in at least three species, namely the Dark-eyed Junco Junco hyemalis, White-throated Sparrow Zonotrichia albicollis and Blackcap Sylvia atricapilla (Chapter 20). This matched the situation in the wild, and suggested an inherent difference between the sexes, possibly arising from different selection pressures on each sex.
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