DOI: 10.5061/dryad.t1g1jwt1b
title: Elevational niche-shift migration: Why the degree of elevational change matters for the ecology, evolution, and physiology of migratory birds

description: 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 &gt;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.
description: Data from the Global Biodiversity Information Facility (GBIF) and
 www.BirdTree.org. Source code for reproducing analyses can be found
 at https://github.com/jlwilliamson/ENSM. 
description: See README.txt file for description of all files included in this
 data publication. Note that DOI links are provided for GBIF datasets for
 reproducibility of analyses, though data in these tables are not original
 data taken directly from GBIF. See analysis code on GitHub for further
 details.