Elevational niche-shift migration: Why the degree of elevational change matters for the ecology, evolution, and physiology of migratory birds

Abstract 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.