Elevational migration can be defined as roundtrip seasonal movement that involves upward and downward shifts in elevation. These shifts incur physiological challenges that are proportional to the degree of elevational change. Larger shifts in elevation correspond to larger shifts in partial pressure of oxygen, air density, temperature, and ultraviolet (UV) exposure. Although most avian examples of elevational migration involve subtle shifts that would have minimal impacts on physiology, shifts of any magnitude have previously been considered under the broad umbrella of “elevational migration”. Here, we consider extreme seasonal elevational movements (≥2,000 m), sufficient to shift the elevational dimension of the eco-climatic niche. Migratory bird populations typically maintain inter-seasonal stability in the temperature, precipitation, and elevational aspects of their climatic niches, a tendency that likely reflects genetic physiological specialization on environmental conditions such as atmospheric pressure. A shift of ≥2,000 m involves a ≥20% change in air density and oxygen partial pressure, sufficient to incur functionally impactful declines in arterial blood-oxygen saturation and require compensatory shifts in respiratory physiology. We refer to this phenomenon as elevational niche-shift migration (ENSM). In this review, we analyzed >4 million occurrence records to identify 105 populations, representing 92 bird species, that undergo complete or partial ENSM. We identified key ecological and evolutionary questions regarding the causes and consequences of ENSM. Our synthesis reveals that ENSM has evolved independently in at least 29 avian families spanning 10 orders. Nonetheless, ENSM is rare relative to other forms of seasonal migration, consistent with the general tendency of seasonal niche conservatism by migratory species and evolutionarily conserved elevational range limits. For many migratory species and populations, within-species patterns of migratory connectivity are not sufficiently understood to determine ENSM status. ENSM is distinguished by its scale within the broader phenomenon of elevational migration. Critical examination of ENSM illustrates fundamental constraints on the ecology and evolution of migration systems, topographical influences on geographic patterns of migratory connectivity, and the remarkable metabolic flexibility of certain bird species that allows them to occupy disparate elevations across different seasons.
LAY SUMMARY
Migratory birds that make extreme elevational movements between breeding and nonbreeding seasons cope with many challenges, including fluctuating oxygen availability.
We call this phenomenon “elevational niche-shift migration” (ENSM), because large seasonal elevational shift (≥2,000 m) are accompanied by large shifts in ecological and climatic conditions that define a species' niche.
We analyzed >4 million occurrence records and searched the literature to identify the subset of migratory birds that undergo ENSM: 105 populations, representing 92 species, 29 families, and 10 orders.
ENSM has evolved >100 times across the avian tree, with most instances in the Himalayas and the Andes. Songbirds comprise >75% of ENSM populations. More than 90% of ENSM populations breed high, migrating to nonbreeding grounds at lower elevations.
ENSM exemplifies a striking seasonal niche shift, whereby birds use phenotypic flexibility to maintain performance under vastly different atmospheric pressures, while exhibiting traits typically associated with high-altitude species.
