Same Name, Different Bird
When Europeans arrived in North America, they noticed a plump, upright bird with a warm red-orange breast, pulling worms out of the grass. It tugged at their homesick heart strings, and they named this bird after a familiar bird from home — the robin.
Today we know that other than their coloration, the European Robin and the American Robin have little in common. It's a roughly 400-year-old case of mistaken identity.
The European Robin is a tiny round-bodied sprite weighing about as much as four sheets of paper. The American Robin is a thrush nearly twice its length, and its closest relative isn't the European Robin at all — it's the Common Blackbird of European lawns and nursery rhymes.
"New World" and "Old World"
When talking about Old World birds, the focus is on Europe, Asia, and Africa, while New World birds are found in the Americas. For a few centuries, explorers and settlers carried their bird names across oceans and pinned them onto whatever looked vaguely familiar. Professional ornithology — and the family trees eventually mapped by ornithologists – arrived much later and have been trying to correct the record ever since.
Birds may share a name without sharing a lineage for a few reasons. One is due to convergent evolution: unrelated birds that take up the same job in similar habitats tend to evolve into similar shapes, because nature only has so many good solutions to a given problem (all “seedeaters” might eat seeds, but they are not necessarily from the same bird family). The other is what you might call convergent naming —humans reaching for similarities or comforting words to describe what they see.
Common Examples
There are many examples of these similarly named but completely unrelated birds.
Robins. The European Robin belongs to the Old World flycatcher family while the American Robin is a thrush. And the various "robins" of Australia — scarlet, flame, yellow — belong to yet a third, entirely separate bird grouping. Three continents, three unrelated birds, one red breast and one borrowed name.
Blackbirds and Orioles. The "four-and-twenty blackbirds" of the old rhyme were Eurasian Blackbirds (also known as the Common Blackbird), part of the thrush family. In North America, blackbirds are part of the Icterid family along with orioles, grackles, meadowlarks, and cowbirds, with no counterpart in Europe. That means that birds which look similar and share a name may not be closely related. The Eurasian Golden Oriole in Europe may look like California’s Hooded Oriole, but they sit on distant branches of the avian family tree.
Warblers. Old World warblers are mostly small, restless, drably colored insect-hunters that birders identify more by song than by sight. On the other hand, New World warblers are often brightly colored, displaying a dazzling array of yellow, blue, orange, and black as they flit among the trees. Like the Old World warblers, these birds are so named because they pour out beautiful songs, making them a joy to hear.
Flycatchers. Both hemispheres have birds that perch, dart out to snatch an insect from midair, and return to the same twig. In the Old World they're the flycatchers of the family Muscicapidae, while in the New World they're the tyrant flycatchers: phoebes, pewees, kingbirds, and others. Same hunting trick, same name, unrelated birds.
Vultures. Old World vultures — the ones wheeling over the African savanna — are members of the hawk and eagle family. New World vultures, including the Turkey Vulture, Black Vulture, and the condors, belong to a separate family found only in the Americas. All are bald-headed, broad-winged, soaring scavengers, but their similarities are a prime example of convergent evolution. The differences are telling: many New World vultures locate carcasses partly by smell, a rarity among birds, while Old World vultures lack a good sense of smell.
Sparrows. There are some Old World birds which are also found in the New World. One example is the House Sparrow, that chirping bird found across the Americas which was first released in Brooklyn in 1851. The native sparrows of North America — Song Sparrow, White-throated Sparrow, Fox Sparrow, and the rest — are often small brown birds, with a similar look to the House Sparrow, but from an entirely different evolutionary family. The pattern by now is predictable: early birders used "sparrow" for just about any small brown bird, and the label stuck.