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Abstract Giant hummingbirds (Patagona spp.) are extraordinarily large hummingbirds whose taxonomy has been muddled for two centuries. Patagona systematics were recently redefined in a study of migration, physiology, and genomics, revealing two species: the Northern Giant Hummingbird and Southern Giant Hummingbird. Here, we re-evaluate taxonomy and nomenclature of the genus in light of its newly clarified biology and species limits, analysing data from 608 specimens and wild-caught individuals spanning 1864–2023. The forms gigas and peruviana were both described based on multiple syntypes. The possible syntypes for Patagona gigas are dubious, so we designate a neotype for this taxon. The genetic identity of the peruviana lectotype remains untested, but its plumage appears to match the northern species. We critically considered the identity and usage of gigas and peruviana, respectively, and examined identification challenges that fostered taxonomic uncertainty. We endorse the name Patagona gigas for the Southern Giant Hummingbird and Patagona peruviana for the Northern Giant Hummingbird. We found that ~33% of specimens (74 of 226) in major museum collections that are labeled peruviana are actually misidentified gigas and we include this full list to correct the historical record. Finally, to facilitate identification and future study of these two cryptic species, we provide comprehensive information on plumage, measurements, and seasonal ranges. 

The ecoevolutionary drivers of species niche expansion or contraction are critical for biodiversity but challenging to infer. Niche expansion may be promoted by local adaptation or constrained by physiological performance trade-offs. For birds, evolutionary shifts in migratory behavior permit the broadening of the climatic niche by expansion into varied, seasonal environments. Broader niches can be short-lived if diversifying selection and geography promote speciation and niche subdivision across climatic gradients. To illuminate niche breadth dynamics, we can ask how “outlier” species defy constraints. Of the 363 hummingbird species, the giant hummingbird (Patagona gigas) has the broadest climatic niche by a large margin. To test the roles of migratory behavior, performance trade-offs, and genetic structure in maintaining its exceptional niche breadth, we studied its movements, respiratory traits, and population genomics. Satellite and light-level geolocator tracks revealed an >8,300-km loop migration over the Central Andean Plateau. This migration included a 3-wk, ~4,100-m ascent punctuated by upward bursts and pauses, resembling the acclimatization routines of human mountain climbers, and accompanied by surging blood-hemoglobin concentrations. Extreme migration was accompanied by deep genomic divergence from high-elevation resident populations, with decisive postzygotic barriers to gene flow. The two forms occur side-by-side but differ almost imperceptibly in size, plumage, and respiratory traits. The high-elevation resident taxon is the world’s largest hummingbird, a previously undiscovered species that we describe and name here. The giant hummingbirds demonstrate evolutionary limits on niche breadth: when the ancestral niche expanded due to evolution (or loss) of an extreme migratory behavior, speciation followed.