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In this study, we tested four hypotheses—the body size, arrival time, dominance, and behavioral flexibility hypotheses—regarding phenotypic contributions to the loss or gain of migration in the Taiwan population of Asian house martins (<i>Delichon</i> <i>dasypus</i>). This population exhibits partial altitudinal migration with some martins remaining at mountain breeding grounds year-round and some migrating to lower elevations during winter.In this study, we tested four hypotheses—the body size, arrival time, dominance, and behavioral flexibility hypotheses—regarding phenotypic contributions to the loss or gain of migration in the Taiwan population of Asian house martins (<em>Delichon dasypus</em>). This population exhibits partial altitudinal migration. Our results most supported the behavioral flexibility hypothesis, which predicts that resident individuals tend to have larger brains than migratory ones, potentially associated with higher levels of foraging innovation. We argue that surviving in the harsh winter condition at mountain areas requires large brains, an energetically expensive trait that may further inhibit migration in resident Asian house martins. We also found that residents tended to have relatively smaller beaks, which likely help reduce heat loss in mountains during winter.