Tanner and Population Density

I recently gave a talk to the Rockland County Audubon Society, and a member raised what I think is the strongest question about our evidence and about the persistence of the Ivory-billed Woodpecker in general. “How could the species have survived in such low numbers and at such low densities?”

In other posts, we’ve pointed to evidence that Tanner missed a population in Mississippi and was somewhat cavalier in his approach to evaluating potential habitat, disregarding advice Herbert Stoddard gave him in 1936, “The area where they (Ivory-billed Woodpeckers) may occur at present is simply tremendous, not restricted as many believe.”

A recent study on Magellanic Woodpeckers points to another factor that raises even more doubt about Tanner’s estimated population of 22 in 1939.  The study was conducted in an old growth Lenga forest in Patagonia. According to this study: “Our results show that Magellanic Woodpecker family groups require a minimum of 100 ha in old-growth forest habitat; thus, forest patches in less favourable forest conditions (e.g., younger, managed, fragmented, mixed forests) should probably be much larger to support a resident pair or family.”

The specific criteria that Tanner used for estimating the 1939 population at approximately 22 are unclear, but he assumed a maximum carrying capacity of six square miles per pair. The Wikipedia entry on the IBWO is even worse and is generally rife with bad information; the editors there expand Tanner’s six square miles to “9.7“.  Elsewhere, Tanner suggested a minimum home range of two and a half to three square miles. These numbers are somewhere between six and 16 times the minimum for a large southern congener that, like the ivorybill, lives in more temperate habitat than others in the genus.

Thus, there is a strong possibility that Tanner severely underestimated ivorybill populations in Florida. If he was so badly wrong about home range, he’s more likely to have missed populations in areas that he rejected for being suboptimal and not expansive enough.

Double the minimum acreage required by the magellanic in optimal habitat and apply that number to the ivorybill, and Sherburne, a large Louisiana WMA, could theoretically be home to just under 90 family groups. (We don’t think this is the case.) Even at 450 hectares per family group, the carrying capacity in Sherburne would be nearly 40. Such numbers are improbable in the extreme, but 9 or 10 family groups in an area that size would be very hard to detect.

With significantly smaller home range requirements, a substantially larger population in 1939, and a recognition that Singer Tract-like conditions are not a requirement (as Tanner himself made clear), various survival scenarios become considerably more plausible, assumptions about low densities become more questionable, and the quantity of potential habitat is far greater than anyone has imagined.

Edited to add: Although the study referenced above is more recent, Noel Snyder made the same basic argument about magellanics and other congeners in The Travails of Two Woodpeckers (2009). Snyder posits that hunting pressure, not habitat loss, was the primary cause of the ivorybill’s decline.

Snyder (who to the best of my knowledge has little hope for the ivorybill) does not fully address how Tanner’s assumptions might have affected his population estimates and habitat evaluations. He also doesn’t consider how taking Tanner at face value has influenced both search protocols and the “credibility” of post-Singer Tract reports. Nevertheless, he does hint at what I suspect is the key to the species’ survival: “With food supplies degraded, not eliminated, a reasonable possibility appears to exist that many ivory-bill populations in logged regions might still have found enough food to persist and might have endured at modest densities, had they been free of shooting pressure. The long persistence of the ivory-bill in one quite thoroughly logged region in Cuba supports this possibility . . .”

In my view, Snyder goes a little too far in downplaying specialization as a factor, even if Tanner overplayed it. It’s pretty clear – from range, habitat, and morphology – that ivorybills are more specialized than pileateds. But if the IBWO did persist after World War II and Snyder is right that hunting was a major factor in the species’  decline (even in the Singer Tract), there may be even more room for optimism, since hunting practices changed considerably in the post-war era.



10 thoughts on “Tanner and Population Density

  1. In my opinion, and I am far from an expert, there may be too many things that are extrapolated from Tanner’s research. Of course the reason this is done is because his research is almost all people have to work with when trying to figure things out so they painstakingly pore over every word he wrote and said. I’m not sure how one person can walk through areas as immense as some of the ones he did and even begin to take a guess at numbers. When looking at areas like the Pearl River it seems almost silly to me that one person could accurately make estimations from an area so large in a relatively short period of time. I go back and forth between being a believer and a skeptic, but I’d say I’m a little more on the side of being a believer. I look at some of your scaling pictures and think to myself, what else could be doing that?

  2. I’m glad you get the point about the scaling; it seems easier to convey in talks than in writing. Showing some of the bark chips helps.

    Regarding Tanner, I’m working on a follow-up that will supplement other posts on the history of the Singer Tract and further debunk the idea that it was a “virgin” forest or a primeval wilderness.

    To be clear, I have no beef with Tanner as an observer; he was meticulous, and I think very accurate (though I wish he’d gone into more and explicit detail about feeding sign). I do have problems with many of his assumptions (some of which he held before doing the bulk of his fieldwork), with inferences he drew from the data, and with how dogmatic he became about these assumptions and inferences.

  3. So after searching for how many years, there’s still no incontrovertible proof that the IBWp lives…

    But hey, at least it gives you a reason to trundle around the woods. God knows not enough Americans even appreciate “Nature”, let alone engage with it.

    We’ve got a pair of pileated wps over here. They’re pretty much stunning, and it never ceases to amaze to see them in suburbia. Can’t imagine what it would’ve been like to see an IBWp in the field, or an Imperial…

  4. Under most circumstances, we wouldn’t approve this comment let alone dignify it with a reply, not because it expresses disagreement but because of its tone. I’ve talked to Frank, who may also weigh in, and we agree that we should make an exception in this case, given the poster’s recent blog on the difficulty of getting decent photographs of Pileated Woodpeckers in a suburban setting.

    We go into great detail in multiple posts about the difficulties involved in searching for ivorybills let alone documenting them . . . . scarcity wariness, challenging terrain, and on and on. Add to that the very limited and often speculative information about the species’ natural history, and you’ve got a challenge that is many orders of magnitude greater than that of snapping a few pictures of PIWOs in one’s neighborhood.

    As for “years” of searching, well, it depends what you mean by years. There have in relatively few sustained and focused searches, and in many cases they really didn’t know what to look for. (We’ve tried to piece that knowledge together; it died with Kuhn, not Tanner.) I’m going to pull a lengthy quote from another post that addresses this issue:

    “There are huge swathes of potential habitat in the southeast that get little human traffic, especially outside of hunting season, and many of these have not even been considered, let alone visited. It’s not uncommon for foreign (and some domestic) ornithologists to assume that conclusive imagery should have been obtained by now, just because it’s the US and birding is popular here and that extinction is likely because several organized searches have failed to come up with something definitive. Many American birders with little knowledge of or experience in the rural South jump to similar conclusions.

    The mere fact that there have been formal, funded searches matters very little. The difficulties in obtaining documentation of an extremely rare, wary bird species that requires a large home range in secluded, difficult habitat are monumental.

  5. Sorry for wording my comment such that the tone seemed anything other than Internet-conversational. While it would probably be stupefying to rediscovery the Ivory-billed Woodpecker, I feel pretty fortunate to have been able to observe this pair up-close three times now without having to walk farther than into my backyard. They’re really amazing.

    Is there any similar hope that Campephilus principalis bairdii might still be alive somewhere? I’ve logged a couple dozen trips to Cuba to compete – including 5 or 6 times all the way out to Baracoa on the northeastern coast, and throughout Parque Nacional Alejandro de Humboldt – but I wasn’t into birding then (my loss) and had never even heard of the Ivory-billed Woodpecker (Cuban or otherwise).


  6. Thanks for the apologies . . . no worries. I love PIWOs myself and have spent a good deal of time studying them too, not just for comparison’s sake. Seeing my first one ca. 1970 was a huge thrill. A lot of international ornithologists have more hope for the Cuban species. (Based on DNA, it’s distinct, although that’s not universally accepted.) I’m somewhat less optimistic, as I suspect that subsistence hunting remained considerably more common there than in the U.S. I’m least optimistic about the Imperial.

    Edited to add: I’d love to go to Cuba (not only to look for ivorybills), but first things first.

  7. I hope you make it there at least once!

    Hey not to chatter on endlessly, but do you recommend “Ghost Birds: Jim Tanner and the Quest for the Ivory-billed Woodpecker, 1935–1941”? I happened across its entry on GoogleBooks, read some of the pages they presented, and requested it via interlibrary loan from my local public library yesterday. Should have it in a few days but hadn’t seen any comprehensive reviews or other feedback on the text.

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