Great article, Ronnie! And amazing photos, too. Thanks for writing and sharing it.
I'll readily admit I have only a cursory grasp of the mechanics of imaging, but isn't it true that you can effectively omit focal length from the DoF discussion? That is, if your subject fills the frame in the same way, with a given aperture the DoF will be same for any focal length (with camera-to-subject distance obviously changing to achieve the same framing with the different focal lengths).
So, really, it's the way you're filling the frame with your subjects (which makes for the best wildlife photos) that leads to the tiny DoF. The sensor size is only relevant in that it determines what focal length you need to use to achieve a given framing.
Someone please correct me if any of these assertions is wrong.
You are correct, but you can't just omit focal length because it is part of the formula for DoF. This article was not directed for those that fill the frame with their subject since when you do fill the frame the DoF will never be enough to capture all of the subject. Which honestly is only a small percentage of people photographing wildlife. This article was directed at people that shoot images like this.
300/4 Wide-Open at 125 Feet by Phocal Art, on Flickr
The above image was taken with the 300/4 shot at ƒ4 from a distance of 125 feet with a jaw dropping 6'4" DoF. Despite this I will regularly see photos like this where they have stopped down to ƒ8 or more with the reasoning to get all of the bird in focus. Why do they stop down? Because they have the misconception about DoF being razor thin with telephoto lenses, which it is but only when you are filling the frame with your subject.