Digging Deeper into Tanner, Part 2 of 3 – Foraging Substrates

Part 1 is here.

The second observation from the Singer Tract I want to discuss took place in May 1932, when Audubon Society President, T. Gilbert Pearson and Audubon Sanctuary Director Ernest G. Holt were the first ornithologists to observe Ivory-billed Woodpeckers (a minimum of three separate individuals) on the Singer Tract. As discussed in an earlier post, a newspaper account describes Pearson’s observations as follows: “The birds were feeding on stumps of rotting trees, the tops of which had been broken off. A favorite place for feeding is also on dead limbs at or near the tops of the very tall sweet gum trees found abundantly in this region.” It seems likely that some of these “stumps” broke due to having been weakened by larval infestation, although other factors undoubtedly were in play.

Pearson described his visit to the Singer Tract in Bird Lore (not available online) and included a photograph of the tree on which an ivorybill was first sighted. The tree has been heavily worked on by woodpeckers, and notwithstanding the poor quality of the image, the excavations look similar to some discussed in this speculative post.

Holt:Pearson Tree

Pearson’s observations present an interesting comparison with Tanner’s from later in the 1930s. Tanner wrote that “Ivory-bills in Louisiana usually feed high in the tops of dying trees, but they are not averse to coming close to the ground.” If there was any question, Tanner’s Plate 11 shows digging similar to that pictured by Pearson above and also shows that foraging occurred close to the ground.

(There are a couple of additional points of disagreement with my contributing biologist here. I think Pearson’s feeding tree is in a more advanced state of decay than the one shown in Plate 11; at the very least it has been far more heavily excavated. I also think that Pearson’s reference to “rotting stumps” implies a more advanced state of decay than what Tanner documented for Ivory-billed Woodpecker foraging at John’s Bayou while feeding young.)

Since Tanner’s monograph was published, a misunderstanding has arisen – that the Singer Tract birds were canopy specialists that rarely or never foraged on boles or near the ground. Tanner contributed to this misunderstanding after the publication of the monograph by dismissing reports of birds being flushed from near the ground and using this as one of his arguments for designating the species as extinct. If there was any doubt that Ivory-billed Woodpeckers foraged on boles (all the way to base), at least two of the four plates in the monograph (7 and 9) show scaled trunks; another (11) shows a hackberry bole (or stump) that has been scaled and excavated. Allen and Kellogg (1935) reported watching a female ivorybill foraging on the ground “like a Flicker”, and their photographs of foraging show feeding to near the ground on multiple small, rotting pines in Florida and on a “gum” in the Singer Tract.

Tanner did not quantify the frequency with which the Singer Tract birds foraged at different levels beyond saying that they “usually” fed high, but these remnant populations were clearly not averse to feeding low when circumstances required it, and as will become clear in the next post, there’s good reason to suspect that foraging lower on trees might have happened more frequently outside of breeding season.

In the back and forth leading up to this post, my interlocutor had an important insight. He suggested that some misunderstanding has arisen regarding the species of wood boring beetles that would be most important as prey for Ivory-billed Woodpeckers, at least during breeding season. Anecdoctal reports from many familiar with ivorybills, including Alexander Wilson and John James Audubon, have led to an emphasis on large Cerambycid larvae as a major prey item. Of the three Ivory-billed Woodpecker stomachs examined (p. 50), Cerambycid and other large beetle larvae made up high percentages of the animal matter identified in each stomach. It’s clear that ivorybills do feed extensively on large Cerambycid larvae and may prefer them at times, but as with all things ivorybill, nothing is straightforward. There will be more on this in the next post.

Tanner was the first and only observer to attempt to quantify at the availability of wood boring larvae and foraging substrates used by both Ivory-billed and Pileated woodpeckers. As I read Tanner, ivorybills, but not Pileateds, fed under the tight bark of dead on high limbs; both species feed on “hard but partly punky” stumps, and Pileated Woodpeckers (but not ivorybills) feed on “punky, and punky and rotten” limbs, stumps and logs.

I think Tanner was categorical about this in both the table and the text on p. 52. My contributor strongly disagrees and reads Tanner as not dismissing the possibility that ivorybills foraged on more decayed wood, including “punky and rotten” limbs, stumps, and logs, only that he did not observe this for the successfully reproducing Ivory-bills at John’s Bayou during his three year study.

Either way, crucial to the hypothesis that food was limiting to Ivorybills during the breeding season was Tanner’s sampling of wood boring insects among these three different substrates in the Singer Tract.

In May of 1939, Tanner did a survey in a freshly cutover area near Horseshoe Lake. He sampled eight .25 acre plots. He expressed some doubts about the data because he had difficulty finding dead limbs among the smashed tops but only to the extent that the “amount of Ivory-bill food discovered was undoubtedly less than was actually there” and the “calculated proportion of Pileated to Ivory-bill food was quite a bit greater than normal.” (pp. 50-51.) While two young birds were seen in the vicinity of Horseshoe Lake in 1932, this area was not in any of the home ranges delineated by Tanner, and he did find Cerambycid larvae under the bark of dead limbs that he apparently collected, presumably in non-random fashion, from downed wood in the John’s Bayou area. I’ll discuss these findings in the next post.

These caveats aside, the findings are dramatic because they seem to conflict with the observations mentioned above and even some of Tanner’s observations and data, particularly with respect to Cerambycid larvae.

The more decayed type of wood was over 12 times more abundant than the hard but punky stumps, the scarcest substrate in the sampled area but also the one with the highest relative abundance of Cerambycid larvae. Thus, this class seems to have offered by far the highest return on foraging investment, 237 cubic centimeters of insect volume in a mere 30 cubic feet of wood.

The higher, freshly dead limbs contained no Cerambycid larvae at all. Tanner estimated the total volume of insects for this class was 27 cubic centimeters in 80 square feet of foraging surface, a truly minuscule quantity. (He measured the surface area rather than the volume in this class.) The volume of insects in the most decayed substrates was 1154 cubic centimeters in 386 cubic feet of wood, abundant but not nearly as concentrated as in the “hard but punky” stumps.

This led Tanner to estimate that Pileated Woodpeckers in the Singer Tract had access to 40 times more food than ivorybills (p. 51) and to infer that this explained the much higher density of Pileateds (about 36 for each ivory-bill) there. I suspect that there’s another major factor that accounts in part for this difference.

I’m puzzled by the fact that Tanner never mentioned ants and termites when comparing ivorybills and pileateds. Long before he conducted his study, ants and termites had been documented as a primary Pileated Woodpecker prey species.

“F. E. L. Beal (1911) gave the results of examination of the contents of 80 stomachs collected far and wide throughout the range of the species . . . Beetles made up 22.01 percent of the total, and ants 39.91 percent. As many as 2,600 ants were counted in a single stomach. The ants were “mostly of the larger species that live in decaying timber.”

In another study of 23 PIWO specimens, also cited by Bent, ants comprised 60% of the stomach contents.


Pileated Woodpecker Feeding on Ants, Tensas NWR, 2010

There’s no evidence to suggest that ivorybills ever preyed on ants. Pileateds can gather and regurgitate ants, termites, and their larvae in great numbers, whereas Ivory-billed Woodpeckers must bring live beetle larvae to their young. It does not seem farfetched to suggest that the Pileated Woodpecker’s ability to exploit this abundant resource is a major reason for its relative success and that the ivorybill’s inability to exploit it could have been a major limiting factor on population and fecundity.

Tanner may have proceeded from the assumption that if there was competition between these two species, it would be for available “borers” (meaning beetle larvae), but the omission seems problematic and hence puzzling to me because it leaves a distorted picture of the degree of competition between the species and makes them seem more similar than they are in fact. It also skews the data Tanner presented and discussed on p. 51, since the volume of food available to Pileated Woodpeckers, but not ivorybills, in more decayed substrates would be substantially greater if ants were included.

There’s no reason to doubt Tanner’s observations with respect to high branch scaling, but it still seems paradoxical that the John’s Bayou birds “usually” foraged on parts of trees that apparently contained a lot less food. We’ll explore this paradox in more detail in the next installment.


One Comment on “Digging Deeper into Tanner, Part 2 of 3 – Foraging Substrates”

  1. motiheal says:

    Sounds right to me.

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