Systematics is the reconstruction of evolutionary relationships of species.  In other words, it involves creating a "family tree" of modern and historical species.  Systematics deals with the comparison of...

  • fossils,
  • preserved specimens,
  • behavior, and
  • DNA

There are three main processes that cause the diversity of life on Earth...

  1. Phyletic evolution - the gradual change of a single lineage
    (one species or group of species into another).
  2. Speciation - the splitting of one lineage (species) into two or more new ones.
  3. Extinction - the end of a lineage (species).

The basic unit of biological classification is the species.  A species is a group of natural populations that interbreed to produce fertile offspring.  These groups are reproductively isolated from other such groups - meaning they do not mate with them.


Geographical isolation results when populations are physically separated and unable to interbreed.  This is the main way in which speciation occurs.  Bird populations typically become isolated in one of two ways...

  1. A bird population colonizes an island and becomes isolated from the main population on the mainland.  The finches of the Galapagos Islands and the honeycreepers of Hawaii are two examples of this.
  2. Fragmentation of habitats that had previously been unbroken isolates the populations in the fragmented areas from each other.  This happened to toucans when the Amazon rain forest was fragmented during the Pleistocene Epoch.

Remnant  populations are created when habitat is fragmented and the populations in some of the fragments die out.  The remaining populations may be separated by thousands of miles - such as the African River Martin (Congo River Basin) and the White-eyed River Martin (Thailand).


Scientific Names are important for communication between scientists, because the same animal may have many different common names in any given language.  A bird's scientific name is comprised of two words: the first is its genus name, the second its species name.  The scientific name is always either italicized or underlined.

     Carduelis tristis - American Goldfinch

No other living thing is allowed to have the same scientific name.

Genus and species fit into a hierarchy as follows:








All birds fall under the following categories:

    Kingdom Animalia (all animals)

    Phylum Chordata (vertebrates)

    Class Aves (Birds)

From there, they are classified into 30 different orders.


Taxonomy is the science of naming and classifying organisms according to a set of standardized rules.  Each group of related organisms is called a taxon (pl. taxa).  Class Aves is a taxon.  So is the Order Piciformes (woodpeckers).  Even a single species can be a taxon, because it is a group of related individuals.

Note: The names of Orders end in "-formes", while the names of Families end in "-idae."

In theory, each taxon is monophyletic, which means that it is a group of birds that share a common ancestor.

Taxonomic characters are specific traits that are shared by birds with common ancestors.  These traits are used to construct bird "family trees".  Conservative characters are of the most value, because these are traits that do not change easily and, therefore, contain clues to bird ancestors.  Convergent evolution can make classification difficult, as unrelated species may develop similar traits.  Some important taxonomic characters are...

  • arrangement of bones in the bony palate (roof of mouth),
  • nostril structure
  • structures of leg muscles and foot tendons
  • arrangement of toes
  • structure of syrinx
  • behavior
  • vocalizations (songs/calls)
  • proteins
  • color patterns in the downy feathers of baby birds

When a group of species are found to share a set of unique traits, it is evidence that they evolved from a common ancestor.  For example, the songbirds of Order Passeriformes all share the following unique traits:

  • preen gland (makes oil for feather upkeep) with a unique structure,
  • specialized perching foot with a large hallux,
  • simplified foot tendons,
  • uniquely arranged deep tendons, and
  • unique sperm shape.

The fact that passerines share all of these unique (other bird groups don't have them) traits indicates that they are monophyletic (they have all evolved from a common ancestor).


Cladistic analysis (cladistics) is the study of evolutionary branching sequences.  In other words, the goal of clasistic analysis is to create evolutionary trees, based on taxonomic characters.  This uses primitive (ancestral) characters and derived (advanced or more recent) characters.

For example, penguins evolved from petrel ancestors.  The penguin's flipper-like wing is the derived character, while the petrel's flying wing is the primitive character.  Since all penguins share the derived flipper-wing, we can assume that they all share a common ancestor.

A cladogram is a diagram that shows possible evolutionary relationships, based on primitive and derived characters.  While there many be several ways to arrange a cladogram, the simplest is usually the most likely.  Learn to draw one here.


Biochemical Analysis involves the comparison of proteins, enzymes, and DNA between species.  These studies usually support traditional classifications of birds.  Sometimes, however, they overturn traditional groupings, expose incidents of convergent evolution, and suggest unexpected relationships between groups.

DNA-DNA hybridization is a process where the double strands of DNA are separated by heat.  The resulting single strands are then joined with single-strand DNA from a different species of bird.  This hybrid DNA is heated until the strands separate, and the temperature is noted.  The key to this is: the more similarities between strands, the higher the temperature needs to be to separate them.  Since a 1% increase in similarity between strands requires a 1o C increase in temperature to separate them, this technique is an accurate way to investigate the relatedness of different species.

DNA analysis typically looks at two types of DNA...

  • Mitochondrial DNA (mtDNA) is a small, circular DNA strand that is found in the cell's mitochondria.  This DNA tends to evolve faster than other types, and can be used to study relationships between closely related species.
  • Nuclear DNA (DNA found in the cell's nucleus) evolved much more slowly, and shows distant and ancient relationships.

Subpages (1): Files for Systematics