What 'species' actually means
The intuitive definition — 'a species is a group of animals that look the same' — fails on closer examination. Look-the-same animals can be reproductively isolated (different species in the biological-species sense). Look-different animals can interbreed freely (same species, just polymorphic). The dominant working definition in vertebrate taxonomy is the biological species concept (BSC): a species is a group of populations that can interbreed and produce viable, fertile offspring with each other but not with members of other species. This is workable for most cases but has edge cases (hybrid zones, ring species, allopatric populations that haven't been tested for interbreeding). Several alternative species concepts exist for the edge cases, and the field hasn't fully converged on one.
Pre-DNA, species identification depended on morphology (shape, plumage, anatomy) and behavior (especially mating displays).
How DNA changed things
Pre-DNA, species identification depended on morphology (shape, plumage, anatomy) and behavior (especially mating displays). DNA evidence allows direct measurement of genetic distance between populations, which often reveals patterns that morphology missed: populations that look similar but are deeply genetically diverged, populations that look different but are genetically very close, populations that intermediate genetically between two named species and reveal them as a single species with regional variation. The cumulative effect over the past three decades has been a series of taxonomic revisions across most bird families, with some species being split and others being merged based on the genetic evidence.
The carrion-hooded crow split
Carrion crow (Corvus corone) and hooded crow (Corvus cornix) were historically considered a single species with two color forms. The carrion form is all-black; the hooded form has grey body with black head and wings. The two forms hybridize freely in central Europe along a narrow hybrid zone. DNA evidence, combined with detailed analysis of the hybrid zone genetics, supported splitting the two as separate species despite the hybridization — the genetic distance is large enough, and the hybrid zone narrow enough, that the BSC supports separation. The split is now accepted in most authoritative taxonomies (IOC, Clements, eBird) since around 2008-2015.
The northwestern-American crow merger
Northwestern crow (Corvus caurinus) was historically considered a separate species occupying the Pacific Northwest coast from Alaska to northern Washington, distinguished from American crow by slightly smaller size and somewhat different vocalizations. DNA evidence and detailed analysis of populations along the contact zone in Washington and British Columbia supported merging northwestern crow into American crow as a subspecies (Corvus brachyrhynchos caurinus). The merger was formally accepted by the American Ornithological Society in 2020. From a research perspective, populations previously studied as 'northwestern crow' are now considered the same species as the broader American crow.
Other corvid revisions
Several other corvid taxa have had recent revisions or are under active debate. The Mariana crow on Guam has been split from related Pacific species. Several jay and magpie species in Eurasia and the Americas have had subspecies elevated to full species status. The yellow-billed and red-billed choughs are debated. The Cuban crow and other Caribbean Corvus species have had their species-level designations debated. The cumulative pattern is: the genus Corvus is being reorganized as DNA evidence accumulates, with the practical effect that species lists from 1980 don't match species lists from 2026. Reading older corvid literature requires translation between older and newer species designations in some cases.
What this means for CrowLingo
The atlas focuses on American crow (Corvus brachyrhynchos), which is taxonomically stable as a species though its boundaries with adjacent Corvus species are continually being refined. The merger of northwestern crow into American crow as a subspecies (2020) means Pacific Northwest crow recordings that older sources might have categorized as 'northwestern crow' are now appropriately categorized as 'American crow subspecies caurinus' — same species, different geographic and morphological subset. Cross-species comparisons with carrion crow, hooded crow, common raven, and other closely-related Corvus species are appropriate when the species relationships are understood; the species boundaries the literature uses now are what shape the comparison framing.