Dichotomous keys

I have always liked the sound of the term “dichotomous keys”. I have also long been fascinated by how these keys allow for the identification of living organisms via a series of paired choices. Dichotomous keys are forerunners of the digital age, where it is all about yes or no, 1 or 0.

While bird identification guides have largely abandoned dichotomous keys, they are certainly of historical interest and they continue to be essential for scientists. So here is a look at some dichotomous keys that include the California condor or its nest.

As many are not familiar with dichotomous keys, I’ll introduce them with the key in Florence Merriam Bailey’s Handbook of Birds of the Western United States (Houghton Mifflin 1902).

01 Bailey 1902 - Cover

This book was the guide for western bird enthusiasts in the early 1900s.

Now imagine we have a California condor in front of us but don’t know what kind of bird it is.

After choosing whether the bird in question is a water bird or a land bird, Bailey’s key asks whether or not the bird’s bill is strongly hooked. If we correctly decide that condors have a strongly hooked bill, Bailey then asks about the bird’s toes. Here’s how this looks in the original:

02a Bailey 1902 - Key a.jpg

As a result of 3 choices from 3 pairs of possibilities, Bailey has guided us to know that we’re looking at a bird of prey.

Next we’re asked about the head and feet, and with just one more choice, Bailey tells us we have a vulture.

02b Bailey 1902 - Key b

Finally, we have to consider wing length.

02c Bailey 1902 - Key c

A condor’s wing is more than 30 inches in length so we now know that we are looking at a bird in the genus Gymnogyps. And there’s only one of those, Gymnogyps californianus, the California condor.

As a result of making just 5 choices, our bird has been identified!

Creating dichotomous keys that do their job, especially for as many birds as are found in the western USA, is a significant task. Only someone with deep and wide knowledge of birds can create such a key.

03a Ridgway 1887 - Title

The key in Robert Ridgway’s A Manual of North American Birds (Lippincott 1887) is more complex than Merriam’s because Ridgway is working with even more species. I have to make 8 choices just to determine that I have a bird of prey. Ridgway’s intended audience was scientists so the choices he offers are more comprehensive and rigorous. For example, Ridgway doesn’t rely on a “fuzzy” choice such as water bird or land bird. To be clear, this is not about one key being better than another. This is about the key’s intended purpose and audience.

03b Ridgway 1887 - Key.jpg

The above is just a portion of Ridgway’s key along the path to the California condor.

04a Stevenson 1942 - Cover

Elmo Stevenson’s Key to the Nests of Pacific Coast Birds (Oregon State College 1942) is as the title indicates. Here are the last choices leading to the identification of a California condor’s nest:

04b Stevenson 1942 - Key

The key in “Status of Rare and Endangered Species: Management and Special Survey Investigations, July 1, 1967 to May 30, 1968”, a report produced by the California Department of Fish and Game, is notable because it treats mature and immature birds as if they were different species. So one choice leads to either an immature condor or an immature bald eagle. Another choice leads to an adult condor or a turkey vulture. (In both cases, further choices are needed to arrive at the California condor.)

Not all self-proclaimed keys are dichotomous keys. For example, Frank Chapman’s Color Key to North American Birds (Doubleday, 1903) is really a checklist. But the inclusion of color images of birds in this book was definitely an innovation.

Dichotomous keys that include the California condor may also be found in:

Bryant, Harold C. “California hawks: how to identify them”. California Fish and Game. July 1921.

Blake, Emmet Reid. Birds of Mexico: a guide for field identification. University of Chicago Press. 1953.

Booth, Ernest Sheldon. Birds of the West. Stanford University Press. 1950.

Coues, Elliott. Key to North American birds. Naturalists’. 1872.

Another dichotomous key with the California  condor is noted in the post Teaching resources from the early 20th century.

For more on Florence Merriam Bailey, see Women of the saga.

The color image of the California condor in Chapman’s key is shown in More color illustrations from books: 1912-2010.