For Christmas this year, my younger brother gifted me with a copy of The Sibley Birder’s Life List & Field Diary. It’s a book that you can use to keep track of what birds you’ve seen. It’s common for avid birders to have a checklist of birds they have yet to see. If you ever look through a bird guide like this (or insect, mammal, or any other kind of guide), you may notice a rather peculiar addition. When scrolling through the list of birds, you might see some that stand out, such as the passenger pigeon or ivory-billed woodpecker. Those of you who know your birds know that these species are extinct. So why would a bird guide include them?
Well, as it turns out, there is an important difference between “probably extinct” and “definitely extinct”. While it has been many decades since the last confirmed sighting of these birds, there is a chance, however miniscule, that groups of them are still around somewhere. They would likely be extremely small populations, extremely localized to a small area. That is why nobody comes across them. The problem with declaring a species extinct is that, in order to definitively prove it, you would have to somehow find proof that there were no individuals of that species left. And finding evidence of absence is much more easily said than done. (This is also why the US legal system puts the burden of proof on the prosecution).
Thankfully, although we might not be able to find evidence of extinction, we can use mathematics and statistics to help determine whether or not a species is extinct. By correlating factors like number of sightings, frequency between sightings, habitat area, and human surveys, we can create a confidence interval telling us how likely it is that there are no individuals of a species left. In fact, we can use similar statistics to determine how many species of animals we have yet to discover.
For instance, let’s look at perhaps the most well known extinct bird – the dodo. The dodo, Raphus cucullatus, inhabited the island of Mauritius, a tiny island in the Indian Ocean. Dodos were flightless, birds related to pigeons. Now, contrary to popular myth, the dodo was not hunted to extinction because it was stupid. In all likelihood, the dodo was actually already at risk before humans ever arrived. They seem to have been rare by the time European settlers arrived in the 1600s, and there is no evidence of overhunting. But the pressure added by all the animals brought by Europeans combined with the destruction of their habitat eventually became to much for them. The last accepted sighting of a dodo was in the year 1662. Biologists have used statistical analysis to determine that the last dodo likely died some time between 1688 and 1715.
However, although these species may have been declared extinct, there is still a chance that there are survivors out there. For this reason, animal and plant guides will often still include these species. That way, if someone happens to spot one, they will be able to identify it. The odds are long, but it would be a great loss to science if someone did see an ivory-billed woodpecker, but misidentified it as a pileated woodpecker, which looks extremely similar. The YouTube channel Atlas Pro did a fantastic video on extinct bird species of North America. I highly recommend watching it. So although these extinct birds are checkboxes I’ll never get to check, I am glad they are listed nonetheless.
As a quick recommendation before I end, I use an app called Merlin to help me identify birds. It is published by the Cornell Lab of Ornithology. It can identify birds from a picture, by their calls, or by their features.
If you know anyone who would like to receive these, please have them send an email to [email protected]. And if you think birding is too nerdy for you, let me know and I can take you off the list.