Chemistry of Birds

The Chemistry of Birds (11): Avian Species Definition

September 14, 2025 by Kai Pflug No Comments

While the link between avian species and chemistry might seem spurious – maybe something coming from a chemist who wants to demonstrate that “chemistry is everything” – it is actually quite clear-cut and strong.

Birds are of the same species if their genetic information is very similar, and not of the same species if it is not. And the genetic information of birds (and other animals) is encoded in their DNA, which is a chemical molecule.

Or to quote the National Human Genome Research Institute, “Deoxyribonucleic acid (DNA) is a molecule that contains the biological instructions that make each species unique”.

(Sadly, the website also has a warning stating “Due to reduction in workforce efforts, the information on this website may not be up to date, transactions submitted via the website may not be processed, and the agency may not be able to respond to inquiries.” In a few years, US Americans will wonder what evil tricks other countries played on them that made them pass the USA with regard to scientific progress.)

DNA consists of two strands (chains) of polynucleotides. The “poly” means that each strand is composed of a large number of nucleotides. And each nucleotide in itself has a similar (but not identical) structure – it contains a sugar, a phosphate group, and one of four nitrogen-containing nucleobases. The way the genetic information is stored in the DNA is in the sequence of the four possible nucleotides in the overall chain.

What information is it that is stored in the avian DNA? Information on

  • The shape of beaks, wings, and feathers
  • The color of feathers
  • Behavior, such as some calls and migratory behavior
  • Physiology, such as the metabolism or the immune system

But isn’t the DNA between two different humans – or birds of the same species – also different? True – but the differences are very small. So, only if the difference in DNA exceeds a certain threshold does it make sense to speak of different species.

As a rough rule, the difference in a specific region of the DNA (the mitochondrial COI gene, which is often used for DNA barcoding in birds) needs to be 2-3% or higher to separate different species. If the difference is smaller, hybridization may still be possible.

And what does that mean in terms of a timeline? How long does it take for two bird populations starting with identical DNA to develop a difference of 2-3%?

A rough estimate gives a value of 1 to 5 million years. This is based on the fact that the DNA of a species mutates at a relatively constant rate of 1% per million years. So, if our two populations are separated and mutate at this rate, after 1 million years, their DNA differs by about 2% (as each has moved away by 1% from the starting point). To reach a divergence of 3%, it will take about 1.5 million years. As some mutation rates are lower, even longer times up to the 5 million years mentioned above are possible.

So, if the chemicals that characterize two individual birds – their DNA – are different enough, they are of different species. And if there is a difference, but it is not very big – say, somewhere between 0.1 and 1% but well below 2% – this may indicate subspecies of the same full species.

Illustrations:

“DNA – Blue” by Spanish Flea is licensed under CC BY-NC-ND 2.0.

By File:Difference DNA RNA-DE.svg: Sponk / *translation: Sponk – Chemical structures of nucleobases by Roland1952, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=9810855

Written by Kai Pflug
Kai has lived in Shanghai for 21 years. He only started birding after moving to China, so he is far more familiar with Chinese birds than the ones back in his native Germany. As a birder, he considers himself strictly average and tries to make up for it with photography, which he shares on a separate website. Alas, most of the photos are pretty average as well.He hopes that few clients of his consulting firm—focused on China’s chemical industry—ever find this blog, as it might raise questions about his professional priorities. Much of his time is spent either editing posts for 10,000 Birds or cleaning the litter boxes of his numerous indoor cats. He occasionally considers writing a piece comparing the two activities.