Avian Taxonomy

All birds are related. Over evolutionar[y time, different groups of birds arose while others went extinct. Over the years early naturalists tried to tie the relationships of birds by their behavior, their color, their songs, and their anatomy. All of these methodologies had some merit, but today ornithologists have DNA as a tool and have refined their classification to a high degree of certainty.

Lets examine the taxonomic scheme. Birds are vertebrates, part of the subphylum vertebrata which has five classes: pisces, the fish; amphibia, the amphibians; reptilia, the reptiles; mammalia, the mammals; and aves, the birds. Class Aves then breaks down into Orders, Families, Genera, and Species.

Let's use the American Robin as an example; its taxonomy is as follows:

Class Aves

     Order Passeriformes

          Family Turdidae

               Genus Turdus

                     Species migratorius

This scheme tells us that the American Robin, belonging to the Order Passeriformes, is a passerine or songbird, along with about half of the birds in the world. Some of these birds, along with the robin, belong to the Turdidae, the thrush family - containing not only thrushes, but solitaires, chats, bluebirds, and the Redwing, Fieldfare. and Eurasian Blackbird. Further narrowing the classification, the robin has the genus name of Turdus, as do the true thrushes and various other species such as solitaires, cochoas, ant-thrushes, and bluebirds. Now we are down to migratorious, which tells us the specific bird, the American Robin, and no other. Turdus migratorious is the binomial scientific name, belonging to no other creature. This taxonomic scheme puts every species in its place relative to other species. The official definition of a species is that it is reproductively isolated from all other species. That's a pretty good definition, but it doesn't always hold. We'll discuss that in another blog.

For many hundreds of years, the similar appearances of birds made naturalists think they were related. Sometimes they were but often they were not. The below share many similarities - size, bright colors, behavior, diet, etc. but the sunbird belongs to the songbird order, Passeriformes, while the hummingbirds are in the Caprimulgiformes, the nightjar order. They are no more closely related to each other than they are to ostriches!

Notice that family names (Turdidae, Sylviidae, Paridae, Gaviidae) are capitalized and end in -idae while order names (Passeriformes, Anseriformes, Podicipiformes, Apodiformes) are capitalized and end in -iformes. Scientific and common names are further explained in Names.

There are such things as subspecies, versions of species that differ in color, size, song or some other aspect but that can interbreed. Their scientific name is a trinomial. One example is  the Tundra Swan which as two subspecies: theTundra Swan (Whistling), Cygnus columbianus columbianus and the Tundra Swan (Bewick’s), Cygnus columbianus bewickii. 




Vigor's Sunbird and Rufous Hummingbird

Click photos to enlarge.


Without getting into too much detail - we'll do that in another blog - here's how ornithologists use DNA. DNA is extracted from the blood of two bird species. The double-stranded DNA is split into single strands. The the two different DNA strands are mixed so that some of the individual strands of one species pairs up with another, forming a hybrid double-stranded DNA molecule. Then heat is applied to this double strand which will cause the strands to separate at some point as the heat rises. The more heat it takes to separate the strands, the more similar the strands are , reflecting the relationship between the birds. DIstantly related hybrid DNA separates at relatively low temperatures, indicating only distant relationships between the birds.

There is also DNA sequence analysis, but I won't get into that. You can read a comprehensive and fascinating history of ornithological classification and taxonomy and a detailed explanation of DNA analyses in a book chapter entitled DNA Analyses Have Revolutionized Studies on the Taxonomy and Evolution in Birds.