eBird does this with “slashes”, for example Willow/Alder Flycatcher, Downy/Hairy Woodpecker. But the two most similar species aren’t always the two most closely related species (i.e. not monophyletic). More discussion here: https://forum.inaturalist.org/t/ability-to-id-to-a-group-of-visually-similar-species-within-a-genus/5655
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