The authors of Wildlife and Plants, 3rd edition (Marshall Cavendish Reference) write that the nest tunnels are "3-7 feet long." In Water Birds of California, author Howard L. Cogswell says "3 to 6 feet (even 10)," while World Book Encyclopedia reports "4-15 feet." So which is it?
Those who read non-fiction often seem to have the impression that facts are facts. Period. The writer of non-fiction must merely learn them and report them, and that's that. I've gotten this attitude often from children, egged on by the adults in their lives. "You wrote X, but we read Y in another book. Who is right and who is wrong?" Cheers for the author of truth. Jeers for the liars. But in reality, the depth of kingfisher tunnels (and many other facts) are unknowable. It might be instructive to consider how the figures above have come to light.
Over 200 or so years of American ornithological research, what percentage of kingfisher nests have been measured? I'd guess a sliver of a fraction of one percent. That alone tells us something: we don't have a very strong sample here. And of those that have been measured, what fraction of the locations of kingfisher nests across North America have been measured? Another small fraction. Is there a wide variation in tunnel depths? It's unlikely we know. Would different average lengths be found from Florida to Washington, from southern California to northern Maine? Possibly, but who knows? Are the tunnel lengths longer where the birds are digging into a softer material, shorter in a harder material? We don't know. Or longer in one climate than another. It's unlikely anyone would ever fund a study to find out, and probably for good reason.
So that's a point: does it really matter? I can't think of a good reason to study such a subject exhaustively. But if a question has not been studied in a statistically responsible way, we can't really know the answer, can we? We can take a stab at it to get a general idea. So any of the above reported lengths is probably as good as any other. If someone found a nest burrow 15 feet long, it's worth marveling at such a feat on the part of these birds. But overall it probably doesn't make much difference whether the tunnels usually range from 3-6 feet or 4-10 feet, and whether the maximum is 10 or 15. So no one is out there doing studies to find out... and we may never find out.
My point is that facts are a much squirmier subject than many people realize, and it would be instructive for readers and thinkers to keep that in mind. One manifestation of this uncertainty is found in the common frustration at diverse and changing health recommendations. People blame science. I think they would do better to modify their expectations.
In an article called "The Fisher King," in the August-September 2009 issue of National Wildlife, respected naturalist and author Les Line describes kingfishers and their nesting habits but does not mention any measurements for how deep their nest tunnels can be. Perhaps Mr. Line is onto something. But I do find myself wanting to hang some kind of measurement on the subject. So, I am willing to accept the varied figures I do find, knowing that no definitive numbers can be stated, and savoring a certain tantalizing deliciousness in the uncertainty.