Towards a unified conceptual framework for studies of altitudinal migration: linguistic nuances, taxonomic prevalence, and emerging opportunities

The study and importance of altitudinal migration has attracted increasing interest among zoologists. Altitudinal migrants are taxonomically widespread and move across environmental gradients that transect altitudinal and latitudinal gradients as partial or complete migrants, subjecting them to a wide array of environments and ecological interactions. Here, we present a brief synthesis of recent development and suggest future directions towards a more taxonomically inclusive conceptual framework for the study of altitudinal migration. Our framework centers on a working definition of altitudinal migration that highlights the biological relevance and scale of movement for a given taxon and its life history. Even with a revised definition, however, the distinction between the narrower phenomenon of migration and wider study of movement often blurs. We discuss nuances of altitudinal movements and encourage comparisons among taxa from divergent lineages and biomes that encounter different ecological interactions and physiological challenges across their annual cycles. We also summarize new opportunities, methods, and challenges for the ongoing study of altitudinal migration. A persistent, primary challenge is characterizing the taxonomic extent of altitudinal migration within and among species. Fortunately, a host of new methods have been developed to help researchers assess the taxonomic prevalence of altitudinal migration---each with their own advantages and disadvantages. An improved conceptual framework of altitudinal migration will allow researchers that study disparate disciplines and taxonomic groups to better communicate and operate in a comparative framework to test hypotheses regarding the evolutionary and ecological drivers underlying variation in altitudinal migration among populations and species.