Imagine you are Alexander Von Humboldt exploring the wilds of South America at the start of the 19th Century. During your studies of the vegetation zones of the Andes mountains you would have noticed vultures, large soaring birds with dark plumage and featherless heads that often featured brightly colored skin. When Humboldt’s expedition came across a dead vicuna or other large mammal they might well have seen vultures feasting on the carrion. Humboldt probably noticed that Just as the vegetation zones in the Andes were similar in general characteristics but differed in specifics from this in European mountains, so to did the vultures of the Andes differ from those in Southern Europe. They were clearly different species but similar in many characteristics and Humboldt may well have thought them close relatives. But he would have been incorrect.
The vultures of North and South America belong to a different group of birds than the vultures of Europe, Asia, and Africa. The new world vultures are in their own family the Cathartidae, whereas the old world vultures belong to the same family as hawks and eagles. These two families are fairly closely related but the vulture appearance and way of life seems to evolved independently in both. Thus vultures are a classic case of convergent evolution.
Convergence was recognized from the earliest studies of evolution. It is one of the two major causes of similarity among different species. The other is similarity due to common ancestry. We see this all the time in birds as it is one of the things that makes bird identification challenging. The various species of Empidonax flycatcher, so similar except for their songs, all look the same because of their close relationship. Presumably their recent ancestors all looked the same way. In the same way the thrushes of our forests (Hermit, Swainson’s, Gray Cheeked, etc) all share a basic color pattern that they inherited from their common ancestor.
But now think of a brown thrasher. It has a similar color pattern to the thrushes but it is not a close relative, belonging to a different family (which, interestingly, is known as the mimic thrushes). The thrasher’s close relatives include mockingbirds and catbirds, which have very different coloration patterns. The similarity in color of the thrasher and the thrushes is due to natural selection favoring that pattern in birds foraging on the forest floor (note that it also occurs in ground-living warblers) and causing birds whose ancestors did not resemble one another to become more similar in appearance over time. The thrasher and the thrushes converged on the same basic color pattern.
Convergence is usually discussed in the context of spectacular examples such as the eyes of cephalopods (squid and octopus) and their striking similarity to the eyes of vertebrates. Or the vulture example I used to start. However, not all examples of convergence are so dramatic. I’m going to give a few avian examples below.
Sunbirds and Hummingbirds. The sunbirds are small nectar feeding songbirds from the old world tropics. They are fairly similar in appearance (brightly colored male, small size, frequently curved beaks) to the new world hummingbirds which are not songbirds.
Gulls and Tubenoses (Albatross and relatives). Gulls are largely coastal birds that are relatives of terns and sandpipers. The tubenoses (such as the fulmars shown below) are open ocean birds that seldom approach land except to breed. The closest relative of the tubenoses are penguins and loons. The flight patterns of gulls and tubenoses are completely different but their overall coloration and body shape are often quite similar.
Penguins and Alcids. This one is well known as puffins are often described as the penguins of the north. Other alcids such as razorbills and murres are also somewhat ‘pengiunesque’. The alcids are actually close relatives of gulls and terns while penguins are related to tuberoses and loons.
Warblers Old and New World. This is one of several examples where the same name has been used for different groups of birds in the old and new worlds because of their general similarities. Both types of warbler are small, active, insectivorous birds. The old world warblers (which are the subject of considerable taxonomic confusion) are generally less colorful than their new world counterparts (such as the reed warbler below). The two types of warbler come from completely different major groups of songbirds.
Flycatchers Old and New World. Similar situation to the warblers above. The new world flycatchers (also known as tyrant flycatchers) are suboscines, a huge group of passerine birds that is super common in the new world tropics. Old world flycatchers are oscines (true songbirds).
Swifts and Swallows. Swallows are songbirds, swifts more closely related to hummingbirds. Their similar body forms are due to both foraging for insects in flight.
Treecreepers and Woodcreepers. The treecreepers are a temperate old world (except for our brown creeper) group of songbirds and the wood creepers are suboscines (like flycatchers) from the new world tropics. Both forage on the trunks of trees.
Frogmouths and Potoos. Both of these types of birds are members of the nightjar group and thus their similarity in being nocturnal foragers with big mouths is due to common ancestry. Their large size and similar appearance is convergence. Potoos are new world birds while frogmouths are old world.
Toucans and Hornbills. I’m not sure if this is a good example are not. Both types of birds have greatly enlarged and colorful beaks and they are not closely related. However hornbills are a bit more generalist than toucans. Toucans are forest birds that mostly eat fruit although they will eat small animals as well. Hornbills are more omnivorous and occur in other habitats.
Falcons and Hawks/Eagles/Vultures/Kites. This is perhaps the most striking examples as until fairly recently falcons were thought to be close relatives of the other ‘raptors’. However it turns out that they are close relatives of parrots and that the similarity is due to convergence.