Species with partial migration, where a portion of a population migrates and the other remains residential, provide the opportunity to evaluate conditions for migration and test mechanisms influencing migratory decisions. We conducted a five-year study of two populations of red-spotted newts (Notophthalmus viridescens) at Mountain Lake Biological Station in southwestern Virginia. At the study site, newts either migrate to the terrestrial habitat overwinter or stay as residents in the breeding pond. We used multistate mark-recapture methods to 1) test for differences in survival probability between migrants and residents, 2) determine if migrants breed every year or skip opportunities for reproduction, and 3) estimate the frequency of individuals switching migratory tactic. We used estimates of life history parameters and migratory patterns from our natural populations in combination with previous experimental work to evaluate processes maintaining partial migration at the population level and to assess mechanisms influencing the decision to migrate.

Results/Conclusions

Based on capture-recapture information on over 3,000 individuals, we found that newts can switch migratory tactics over their lifetime. Pond residents have higher reproductive success and overwinter survival compared to forest migrants. We conclude that migrants and residents coexist through conditional asymmetries, with residents having higher fitness and migrants ‘making the best of a bad job.’ Environmental conditions can also play an important role in the maintenance of partial migration within a population; we found support for density-dependent processes in the pond strongly influencing the probability of migrating. On the individual level, we found that newts are more likely to switch from residency to migrating than the reverse and males are more likely to remain as residents. Differences between the sexes are likely driven by reproduction benefits of residency for males and high energetic costs of breeding for females, which result in lower breeding frequencies and reduced ability to remain as a resident. Our work illustrates how migration can be influenced by a complex range of individual and environmental factors and enhances our understanding of the conditions necessary for the evolution and maintenance of migration.