Partial migration, variation in the percentage of a population that completes a migration, can be influenced by the local environment and condition of an individual. We examined the direct and interacting effects of habitat quality and gender on migration decision by manipulating population density and sex ratio in a factorial field experiment using aquatic enclosures. In partially migrating red-spotted newts (Notophthalmus viridescens), we measured the percentage of newts migrating to the terrestrial habitat vs. overwintering as pond residents. Density significantly influenced migration, with 63% of newts migrating from high-density enclosures compared to 39% from low-density enclosures. Newts also migrated earlier from high-density enclosures, but no significant effects of the sex ratio treatment were found. Females migrated earlier than males, and 64%more females developed the migrant phenotype, suggesting important sex-based trade-offs of migration. No differences were found between migrants and residents in initial body size, counter to our prediction that larger individuals would be more likely to remain pond residents. This study demonstrates experimentally that migration can be a plastic response influenced by both local density and gender.
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