Intra-tropical bird migration research in the 21st century: new insights about a complex system

Recent studies show that intra-tropical bird migration is much more common than previously thought. However, we still know very little about the details of such intra-tropical movements, including how many species migrate in the tropics, when and to where different populations migrate, the underlying mechanisms responsible for such movements, or the risks to survival during migration. Differences in the seasonality of tropical vs. temperate ecosystems and in the life history strategies of intra-tropical migrants vs. migrants breeding at temperate regions suggest that intra-tropical migrants are under a different set of selective pressures than their temperate counterparts. Proposed drivers of intra-tropical migration are spatial and temporal variation in the availability of food resources, risk of predation, limitation of nesting sites, competition for food, and weather. Understanding which populations migrate within the tropics, as well as what determines individual migratory strategies is imperative to a full appreciation of how and why birds move across the planet, as well as how to best conserve and manage their populations. We provide a synthetic review of the geographic distribution of migrants in South America and discuss potential mechanisms underlying different patterns of movement (e.g., altitudinal migration). We also highlight some recent discoveries from ongoing research using a diversity of novel tracking technologies and discuss important future directions for research on intra-tropical migration.