Definitions and nuances:
A shared language for discussing altitudinal migration has not been used consistently in the literature. Both the term used and the definition of the phenomenon itself lack a standardized definition (Barçante et al., 2017), which makes it difficult to find and compare studies across disciplines and taxonomic groups. In fact, the patterns and processes that constitute altitudinal migration and the taxa considered altitudinal migrants have differed widely among studies, with most definitions focused heavily on birds (Burgess & Mlingwa, 2000, Boyle, 2017; Hsiung et al., 2018; Tsai et al., 2021; Williamson & Witt, 2021). A conceptual framework for studying altitudinal migrations needs to start with a standardized definition and understanding of the use of the term. Here, we suggest a revised definition for altitudinal migration asa seasonal round trip between breeding and non-breeding ranges for part or all of a population along an elevational or bathymetric gradient that results in a biologically relevant shift in distribution . But then what do we mean by “biologically relevant”?
We believe biological relevance depends on the taxon in question and whether the vertical movement directly impacts an organism’s fitness—either by ensuring successful migration and subsequent reproduction, or by harming their chances for survival. Challenges to an organism’s fitness can fall under two, non-mutually exclusive categories: seasonal elevational movements that impart (1) physiological and/or (2) ecological changes on migrants. Physiological challenges can occur when individuals move across extreme elevational gradients (i. e. Bar-headed Geese; Scott et al., 2015), which requires adaptations for the extreme differences in partial oxygen pressure (Williamson & Witt, 2021). Vertical physical barriers, such as waterfalls or mountains, or dams, can also impose physiological challenges to survival by blocking migratory routes or requiring physical exertion to cross (Cosgrove et al., 2018). Physical barriers may present different strengths of physiological or ecological change; in some instances, certain taxa may move across a given physical barrier daily, while other taxa may only do so once a season or only once in their lifetime. Ecological challenges may include changes in habitat, intraspecific competition, predation risk, climatic changes, trophic interactions and/or resource availability (Alerstam & Bäckman, 2018). The distance covered by altitudinal migrants varies among taxa, but the migration event typically involves movement between breeding and non-breeding ranges that extend far beyond the organism’s home range in either site (Teitelbaum et al., 2015). On one extreme, more vagile species may move more than 2000 m in elevation and thousands of kilometers in latitude, which presents clear physiological challenges for survival in dramatically different environments across their annual cycle (Williamson & Witt, 2021). In contrast, some snails undertake amphidromous migrations from saltwater to freshwater habitats that cover less than a hundred meters of elevational change (Villeneuve et al., 2019), while some terrestrial species only move a few hundred meters in elevation (i.e. some Drosophila; Mitsui et al. 2010 or Trochilidae; Tinoco et al., 2009). These may seem like trivial elevational changes for more vagile taxa, yet represent dramatic shifts in ecology and physiological conditions between breeding and non-breeding sites. Thus, although physiological and ecological changes are difficult to quantify and observe directly, they are nevertheless more important than changes in elevation that meet a numerical or statistical threshold between breeding and non-breeding territories. Imposing a strictly numerical definition to identify altitudinal migrants without considering the biological relevance of the migration event may erroneously include or omit altitudinal migrants and bias the inferences drawn from such studies.
Even with a revised definition grounded in biological relevance and scale, there are still many nuances and potential points of confusion when discussing and classifying altitudinal migrants. For instance, animals can move both latitudinally and vertically during seasonal movements, as exhibited by monarch butterflies (Danaus plexippus ), which migrate over the course of multiple generations from high-elevation mountains in the Transvolcanic Belt of central Mexico to breed at low-elevation sites in the eastern United States and Canada during warmer spring and summer months (Kimura, 2021). There is not widespread agreement in the literature if altitudinal migration is strictly a short-distance migration phenomenon or if it encompasses such long-distance migration events as well. Some comparative studies or syntheses either do not address this explicitly, or allow for some but not all latitudinal migrants that also move in elevation to be classified as altitudinal migrants (Boyle, 2017; McGuire & Boyle, 2013; Pageau et al., 2020). Other studies state it is strictly a short-distance phenomenon (Hsiung et al., 2018), or may encompass long-distance latitudinal migration as well (Barçante et al., 2017a; Rappole, 2013; Williamson & Witt, 2021). Migrants that move across large latitudinal expanses while also changing elevations experience similar or greater physiological and ecological changes as short-distance altitudinal migrants. As such, we feel that work on altitudinal migration should encompass both long- and short-distance migrants, and that authors should explicitly state their criteria and rationale for classifying altitudinal migration.
The complex migratory patterns of monarch butterflies illustrate another nuance of altitudinal migration: partial altitudinal migration. Not all monarch populations are migratory (Chowdhury et al., 2021), as opposed to obligate migrants where all populations within a species are migratory. Here, we follow the broader definition of partial migration (Terrill & Able, 1988), where partial altitudinal migration (including differential migration; Dingle & Drake, 2007) not only includes distinct migratory and non-migratory populations, but also encompasses within-population variation in migration strategies, including taxa wherein only certain ages, sexes, or other demographic subunits of a population migrate (e.g. White-ruffed manakin Corapipo altera, Boyle et al., 2011). The majority of altitudinal migrants are also partial migrants (Hsiung et al., 2018), such that some subpopulation or demographic group does not participate in each annual migration event. Conversely, many long-distance latitudinal migrants undertake ‘obligate’ or complete migration, in which each demographic class of each population participates in seasonal movements (Newton, 2012).
Although altitudinal migration is a specific type of and part of the broader study of animal movement, ‘movement’ is not synonymous with ‘migration’. Rather, migration is a specific form of animal movement. Importantly, altitudinal migration is seasonal and cyclic with individuals and populations moving between distinct breeding and non-breeding areas at different elevations, which distinguishes it from other forms of movement where populations may shift their elevational distribution, but do so non-annually (i.e., dispersal, irruptions). Some cyclic daily movements may occur across vertical gradients, such as “hilltopping” in some insects, in which they form daily leks on top of hills or mountains (Kimura, 2021), and aquatic animals undergoing “diel vertical migration”, in which individuals change water depth between night and day (Chapman et al., 2012). However, these are daily rather than seasonal movements at different elevations or depths, and therefore do not fall under our working definition of altitudinal migration.
Migration and movement exist along a behavioral and ecological continuum, and distinguishing migration from the broader study of movement is not always straightforward (e.g. some flies in the family Calliphoridae; Kimura 2021). For example, two polar breeding birds—Ivory Gull Pagophila eburnea in the Arctic, and Snow Petrel Pagodroma nivea in the Antarctic—both breed on high remote rocky outcrops (up to 2000 m and 2500 m, respectively) and make regular foraging trips to the ocean (Carboneras et al., 2020; Mallory et al., 2020). Under our definition, these daily behaviors of adults are best categorized as ‘movements’. However, the young that are born at high-elevation nesting sites and return to breed after spending the non-breeding season at sea level could be considered altitudinal migrants. Despite the similar movement ecology of these two species, only the Ivory Gull is considered an altitudinal migrant, whereas no Antarctic birds—including the Snow Petrel—are considered altitudinal migrants (Barçante et al., 2017a; Hsiung et al., 2018). The study of migration is embedded within the broader study of animal movement, but determining where the narrower study begins is sometimes difficult due to complex life histories involving seasonal and/or ontological changes in elevation. A unified conceptual framework for altitudinal migration will help unpackage these complexities and alleviate this confusion. One can envision a multivariate space to describe the how organisms vary in their migratory behavior; for example, it may be helpful to place organisms along continua that describe variation whether organisms exhibit latitudinal migration, altitudinal migration, or both in conjunction with whether or not those taxa are complete or partial migrants (Figure 2).
Finally, establishing a consistent language for the field is important to help improve the visibility and searchability of the primary literature involved. Though “vertical” or “elevational” migration may be seen as more accurate descriptors of this type of seasonal movement across biomes and taxonomic groups, the term “altitudinal migration” has historical precedence (i. e. Todd and Carriker 1922; Presnall, 1935) and is more frequently used in the literature (2840 Google Scholar search hits for “altitudinal migration” compared to 464 for “elevational migration”; searches done on 15 August 2023). We used the program Publish or Perish (Harzing 2007) to quantify temporal trends in Google Scholar hits for publications that use the terms “altitudinal migration” and “elevational migration”, finding that altitudinal migration was established as a term far earlier and is used far more often than “elevational migration” (Figure 3). Thus, although these terms have often been used interchangeably,we recommend using the more commonly used term altitudinal migration. We do not just advocate for its use because it is the more commonly used and historical term, but also because it better encompasses terrestrial and aquatic migrations. In that regard, “vertical migration” is perhaps a more accurate term, but is used predominantly in aquatic systems (over 141,000 Google Scholar search hits on 15 August 2023) to describe a different phenomenon of daily movement across multiple trophic levels up and down the water column rather than the seasonal movements we focus on here (Chapman et al., 2012). Use of a single term not commonly used elsewhere in the literature will aid researchers and search engines in identifying information on this phenomenon rather than often-conflated terms (Leeming 2023). ’Altitudinal migration’ may seem a misnomer due to the definition of altitude in aviation as the distance above the Earth’s surface as opposed to above sea level, yet we nonetheless recommend its use to promote a consistent vocabulary to facilitate dialogue across disciplines and taxonomic groups.