As usual, much of this report will be focused on bark scaling. I found an unprecedented amount of fresh work this trip, a total of 29 trees, all sweet gums. I only counted live and freshly dead trees that appeared to have been scaled within the last year, and probably more recently than that, in most cases. As will be discussed, I was able to ascertain that 11 of these trees had been worked on no earlier than March 15th. I was selective about what I included in the count, relying on my years of experience looking at scaling and how its appearance changes over time and this passage from Tanner for the criteria:
Ivory bill sign shows as bare places on recently dead limbs and trees, where bark has been scaled off clean and to a considerable extent. Pileateds do some scaling too, but it is usually confined to smaller limbs and those longer dead. Freshness of the sign can be judged by any appearance of weathering, which will soon turn bare wood a grayish color. Extensive scaling of the bark from a tree which has died so recently that the bark is still tight, with a brownish or reddish color to the exposed wood showing that the work is fresh, is one good indication of the presence of ivorybills.
We had a number of visitors during my stay. Tom Foti joined me again on Tuesday and Wednesday, and Phil and Eric Vanbergen came along on Friday. I appreciate the Vanbergens’ help in collecting the data I’ll be discussing. It’s great to have such enthusiastic young people involved. Meanwhile, John Williams (Motiheal from ibwo.net) visited and spent four days in the field with Frank. Both John and Frank are planning to provide their own accounts, and those will be posted in the weeks ahead.
A general note about the week, leaf out progressed rapidly, and the change between Sunday the 20th and Saturday the 26th was dramatic. Nonetheless, I was able to find a good deal of feeding sign later in the week.
I arrived on the evening of Saturday, March 19th, and Frank and I spent the 20th in the northern sector. There had been severe flooding in the area earlier in the week; the waters had receded – we suspect by the 15th or 16th and certainly no earlier than the 15th. One of our trail cams, placed about 4’ above the ground, was completely overtopped, ruining the card and probably the camera as well. Such floods are exceedingly rare, perhaps a once in 500 year occurrence in the area. Fortunately, flooding tends to recede rapidly, but crossing both permanent and seasonal water bodies remained a much bigger challenge due to deep water and slick banks. The most stunning aspect of the flooding was the near total scouring of leaf litter in many parts of the search area, leaving bare soil and deposited silt visible. The landscape was transformed, and familiar spots looked radically different.
Frank often describes walking through the forest on dry leaf litter as “walking on cornflakes.” The absence of leaf litter limits the noise made by walking. This may be advantageous between now and late fall. Unfortunately, I anticipate being able to visit the area only once more before summer, probably in June.
The flooding had another benefit this trip. The absence of leaf litter makes it much easier to find fresh bark chips on the ground and to determine with some degree of certainty when scaling has taken place. The flood waters receded no earlier than March 15, so all fresh chips found below trees where the leaf litter had been scoured were no more than a week old.
When we reached the vicinity of the downed top, first discussed here, we heard a loud single knock. Frank’s initial reaction was that it might have been a gunshot, but we both agreed that the sound seemed to have come from a nearby source; we heard no other shots that day and saw only one vehicle, almost 2 miles away. Later in the morning we heard a couple of weak possible double knocks and later a very good sounding one.
We also found a little bit of scaling just north of the northern concentration discussed below. While some of it looked to be quite fresh, we did not find any bark chips.
The scaling in the first of the above photographs is somewhat marginal, as only a single smaller upper limb is involved. While I’m unsure, I don’t think I counted either of these trees, as I only started keeping track later in the week; both examples came from very close to the northern cluster discussed below.
I was on alone on the 20th, and I returned to the same area. I found a good deal more scaling.
In many cases, the scaling shows sign of progressing from treetops down, as Tanner described.
The detail of the small tree, scaled down to where small branches are still in leaf, is at the edge of a small pond around or in which I found five other trees with recent scaling on them, as well as two more with older work (not counted).
There was new work on one of the trees I found last month, the larger one in the background, below. I found several other scaled trees in the immediate vicinity, including the one in the foreground, on which we’ve now deployed a camera, and much of that work was fresh too. I chose a spot for a stakeout and spent about an hour watching the treetops in this area of concentrated work. This location is 140 yards south of the small pond described above and is at the southern boundary of the cluster. During the stakeout, I heard a loud single knock that seemed to have come from the vicinity of the small pond.
As I was leaving, I passed the pond again and found what appeared to be new scaling on one of the trees at its edge. There were fresh chips in floating in the water at the base.
Tom Foti arrived on the morning of the 22nd. We spent the day in the one of the southern areas where we’ve found concentrations of bark scaling in past years and where there have been both possible visual and auditory encounters. We found several scaled trees in this area but did not see or hear anything.
I met up with Tom on the morning of the 23rd; I had decided overnight to be more methodical in my approach to documenting scaling. I’ve been so focused on what might be diagnostic that I haven’t attempted to quantify what I’ve found thus far and haven’t kept detailed location information. Thus, it seems like a good idea to start keeping better track. This should prove useful if we can document that ivorybills are present and that they are responsible for the bark removal.
Tom and I heard 6-8 likely kents at ~9:00 am, this at the downed top where we had the camera, the same location where Frank had his sighting last spring. The calls came from three directions, south, east, and west.
We headed south and met up with Frank and John in the core of the northern concentration, south of the pond. We did an extended playback series; John will have more to say about the specifics in his post. We all heard a nearby double knock during the playback; Tom, John, and I were sitting close together near the speaker and thought it was a single, but Frank, who was positioned closer to the source of the natural sound, called it as a double.
We found some very fresh bark chips (moist with sap) at the base of a 12” DBH dying sweet gum that has areas of scaling high on the bole. The tree (which is shown above) is only a few meters from the one found last month. We’ve deployed a camera aimed at this bole. Given the quantity of activity in the area and the evidence of return visits to feeding trees, we hope to get some hits before long.
We removed a piece of bark from a looser spot on a nearby downed tree (which had been fed on by woodpeckers both before and after it fell). Beneath the bark were Cerambycid larvae, pupae that I also suspect are also Cerambycids, and what I think may be a very young Elaterid larva. We placed some of these larvae and pupae on the piece of bark to illustrate. We suspect that Allen did the same for what became Plate 10 in Tanner.
On the way out and not far from the cluster, I spotted what appears to be the start of a large, irregularly shaped cavity. We’ll monitor this and see whether there’s any further excavation.
It rained heavily on the morning of the 24th. I spent part of the afternoon trying to take measurements but didn’t have much success, since I was using an ordinary tape measure.
On the 25th, Phil and Eric Vanbergen joined me and we took measurements in the two areas where there are concentrations of scaling, finding several more trees in the process. When I got back to Frank’s, the forester’s DBH measuring tape I ordered had arrived, making it possible for me to take measurements on my own.
I spent the 26th measuring suspected feeding trees in the southern area and found several more with recent work on them.
Except for feeding sign, I did not see or hear anything suggestive of ivorybills during my last three days in the area.
Now I’ll turn to some of the data I collected this week.
I counted 29 suspected recent feeding trees in the two areas, 13 in the northern sector and 16 in the southern. I did not count work that appeared to be more than a year old or work that was limited to very small branches.
The areas are 2.05 miles apart. The northern area was logged (probably partially) in 1905, although there may have been some later selective cutting. The southern area was logged in 1935. Forest composition is somewhat different between the two areas, with sweet gums seeming to be less predominant in the southern one. In the southern area, the scaled trees are in a narrow, almost linear strip with an area of .13 square miles/83.2 acres/33.67 hectares. The northern cluster is more compact and polygonal, with a total area of .03 square miles/19.2 acres/7.7 hectares. Within both areas, scaled trees were often found in groups of 2-6 – 11 out of 13 trees in the northern area and 11 out 16 in the southern. (This includes the cluster in and around the pond, which is perhaps 30 meters in diameter, but otherwise applies to trees that I estimated to be 20 meters apart or less.)
Scaled trees ranged from 6.5” dbh to over 5’ (estimated) for a gum with a split trunk, one stem live and the other dead. All but 3 inaccessible trees were measured.
76% of the trees were alive, sometimes just barely, with scaling on dead or nearly dead limbs or boles. There was scaling on live parts of one or possibly two of the trees.
Though we have found scaling on boles of larger trees in the past, all trees scaled on boles were 12” DBH or less. While these measurements may not be meaningful absent a random sampling of trees in each sector for comparison, I thought the numbers might be of some interest even now, especially in light of the recent discussion of Tanner:
This is obviously a very small sample, but I think it’s interesting nonetheless. The three smallest trees in the northern sector were all in or near a pond that appears to have had its outflow blocked in recent years. They probably died due to the change in hydrology. But for that difference, there seems to be an even greater favoring of 25-36” DBH trees than found by Tanner, and this is so even in the less mature southern sector (again without data on overall composition). This year, feeding sign has been found exclusively on sweet gums. We’ve found a few scaled oaks over the years and more bitternut hickories; I suspect the latter are being fed upon at a high rate relative to abundance. We’ve discussed doing some random sampling for tree size and species, but given our limited resources, this may not be worthwhile or feasible at present.
Of course, none of this proves that ivorybills are in our area, but I think it’s another indication that they are. The best-case scenario is that the dramatic increase in scaling this year and in this season is related to there being young in a nest or nests.
At an IBWO Recovery Team meeting during 2007, a report was shared by Dr. Nathan Schiff and his colleagues at the USDA Forest Service’s Southern Hardwoods Laboratory that more formally described many of the paradoxes that have been discussed in this series of posts. It provides more information on what is known today about the ecology of the wood boring species documented as having been fed upon by Ivory-billed Woodpeckers. Schiff and his colleagues point out that the larvae Tanner collected from a John’s Bayou nest cavity and those described from stomach contents don’t prefer sweetgums, don’t live in high branches, and spend the bulk of their lives in the heartwood, often in the lower parts of trees.
While it’s not mentioned in the Schiff et al. paper, Mallodon dasytomus or what Tanner called Stenodontes (by far the largest single food source in his sample of remains from the nest cavity described above) is commonly known as the “hardwood stump borer”, and Neandra brunnea, a close relative of another known prey species, Parandra (or Hesperandra polita), is called the “pole borer”. These beetles have a life cycle of 3-4 years.
The authors point to direct evidence that of the six species of insect identified in Tanner’s monograph, none would use wood consistent with the high branch/sweet gum focused foraging model. In addition to Mallodon, and P. polita, these species are: Neoclytus caprea (banded ash borer), Dynastes tityus (Eastern Hercules beetle), Alaus ocualtus (eyed click beetle) or a close relative, and an unidentified Scolytid or bark beetle (not found by Tanner.) We have found both P. polita and A. oculatus adults on suspected feeding trees.
Scolytids are tiny. Neoclytus begins its one year life cycle in early spring; the larvae start feeding under the bark and then burrow into the sapwood, where they pupate and spend the winter before emerging as adults. The species prefers ash but may also occur in hickory, oak, and elm. It is found in downed logs, as well as standing trunks and limbs of stressed to dead trees. Dynastes tityus or Hercules beetle larvae live and feed in the “rotting heartwood of logs and stumps.” Alaus larvae are predatory on Cerambycid larvae and live in decaying stumps and logs; eggs are laid in the ground. In addition, the authors point out that at least some of the larvae Tanner found under bark on higher branches (p. 42) require wood that’s in an advanced state of decay, when bark would be loose.
The insect larvae identified for Tanner (Mallodon, Alaus, Neoclytus, and Dynastes) came from remains he found in nest debris. I think this suggests he may have failed to observe or have unduly downplayed one or more foraging behaviors related to obtaining food for nestlings – excavation of very decayed stumps and logs and extensive scaling on boles in particular – since these are lower dwelling species and two of the four inhabit wood that’s in an advanced state of decay. My anonymous correspondent disagrees with my reading of Tanner but makes a very interesting observation that sheds important new light on the data.
Schiff et al. point to an apparent contradiction; none of the food items found in the nest reflect the preference for high branch foraging that Tanner described. The importance of large Cerambycid larvae (especially Mallodon or Stenodontes dasytomus) in the feeding of young ivorybills at Singer Tract remains unclear. Tanner’s observations indicate that most foraging events involved a substrate (recently dead or dying branches) that doesn’t support these large wood-boring larvae. Tanner reported that Mallodon and other large larvae that were “frequently carried in the bills of adult Ivory-bills”. Some fragments of larvae that were found in the remains of at least one nest cavity had to have come from boles or large, lower branches and were likely to have been obtained from longer dead wood, at least in some cases.
It appears that attention today on the Cerambycid larvae “paradox” may have been founded in part on a misreading of Tanner. The Cerambycid and other large larvae found in the three stomachs reported above were from birds collected in August and November, well after the breeding season. Because Mallodon is so large, was the most abundant prey species found in the nest, was identified in one of the stomachs, and was quite likely the species found by Wilson and others, many have interpreted Tanner as saying that it was the primary prey species. It was undoubtedly an important and calorically rich one, but Tanner’s observations suggest that smaller larvae played a more important role, at least in the case of the John’s Bayou birds.
While he frequently saw adult ivorybills carrying large larvae in their beaks, he observed the birds carrying large numbers of “small” larvae even more frequently. He noted the apparent conflict between his observations and what was found in the nest debris and resolved it by hypothesizing that the smaller insect parts probably remained “imbedded in the feces” and were “removed when the adults cleaned the nest” (pp.40-41). Thus, while there is direct evidence that large wood borer larvae were part of the ivorybill prey base, Tanner’s overall interpretation was that smaller larvae were more important during the breeding season. (pp. 40-41, 51-52).
Tanner admitted that he did not fully understand why ivorybills did not forage more frequently on substrates supporting larger larvae when they were fully capable of doing so. He speculated that the smaller larval woodborers when abundant “are very abundant” for short periods of time, beneath the bark of recently dead or dying wood. In sum, Tanner concluded “The Ivory-bill’s insect food supply is smaller, more variable and erratic, and more unevenly distributed than that of the Pileated.”
To reiterate, Tanner stated specifically that while most of his observations involved scaling of high branches, presumably for smaller larvae, he also observed scaling on boles where larger larvae dwell. Tanner suggested that foraging on trunks took place when trees were “longer dead” and that ivorybills “move downward with the progression of shallow borers” (p. 41) The balance of his observations (27.8%, a not inconsequential number) involved digging for “deeper-living” larvae that spend most of their lives in the heartwood, between the ground and the large lower branches.
In their unpublished manuscript, Schiff et al. concluded that: the “. . . Ivory-billed Woodpecker is an opportunistic feeder with catholic tastes that eats beetle larvae where it can find them and that it probably digs for them with its powerful bill. ” This conclusion was intended to challenge Tanner’s finding that food supply imposed a limitation on ivorybill nesting success, but a close reading of Tanner suggests this conclusion actually is not at variance. It is clear now that Ivory-billed Woodpeckers could and did forage by digging into older boles like Pileateds, though less frequently. It is also clear that they scaled bark at all levels. The new revelation for many of those interested in ivorybills today is that this species showed a preference for stripping bark in pursuit of large numbers of smaller larvae in recently dead and dying trees and carried these smaller larvae en masse to their young.
I think the foregoing observations make a lot of sense. As discussed, perhaps ad nauseaum, I have some doubts about what I take to be Tanner’s conclusions about decay class. I have questions about the way he characterized his data on tree size and wish he had quantified scaling on branches relative to scaling on boles. I also question his suggestion that scaling on boles was done on longer dead trees (and the rationale that trees die from the top down) because it doesn’t account for the fact that the larger bole dwelling Cerambycids can attack injured live trees and hasten or bring about mortality, as was the case with the suspected feeding tree shown on the homepage. I have little doubt about his observations at the nest. For one thing, the number of is considerably greater, 159 as opposed to 101.
There is some reason to think Tanner was at least partly correct with respect to variability and scarcity of this food supply, especially in the higher branches. As noted in the previous post, Tanner found no Cerambycid larvae at all in a random sampling of cutover plots near Horseshoe Lake. The location of these surveys was likely between the Bayou Despair and Greenlea Bend home ranges and not far from where two young birds were seen in 1932. Ivorybills were disappearing from these two home ranges, as well as from the nearby Little Bear Lake range, and the three ranges only produced one successful nest (Greenlea Bend, 1937) between 1934 and 1939. (p. 39), and it seems possible that scarcity of this food supply was a contributing factor.
Tanner specifically looked for insect larvae “from several situations similar to places where ivorybills fed”. This was presumably not a random sample. While details about these “situations” were not provided, they included: under bark of dead sweet gum and willow oak limbs (presumably downed), under the bark of a Nuttall’s oak (condition and part of tree unspecified but presumably a downed limb or limbs based on the species found), and the trunk of a dead hackberry. As might be expected, he found Mallodon, P. brunnea, and A. oculatus in the hackberry bole.
He found Urographis (now Graphisurus) fasciatus and Leptostylus sp. in both species of oak and in the sweet gum. These are small Cerambycids (adults up to 15 mm). He found small Cerambycids, Aegomorphus decipiens (now modestus) under sweet gum bark and Xylotrechus colonus under the willow oak bark. In addition, he found Pyrochroidae (torch beetle) larvae, possibly Dendroides canadensis, in sweet gum and Nuttall’s oak and unidentified Elaterids and Buprestids in the Nuttall’s oak.
Questions remain. Some of these larvae, the Elaterids and Pyrochroids in particular, are found under loose bark in decayed wood, suggesting that at least some of the infestation took place after the limbs Tanner examined had fallen. Aegomorphus also feeds in “soft, decaying hardwoods.” Graphisurus fasciatus is a common species that prefers trunks and large branches. Xylotrechus colonus, prefers “recently killed trees” and is described as “one of the commonest eastern Cerambycids”. At the same time, Tanner’s very limited random sample suggested that high branches had considerably less available substrate and food than other tree parts. This may suggest that sporadic, localized outbreaks of larval infestation in high branches are crucial for breeding.
There are a couple of added twists to this story. To restate and expand on the foundation of my hypothesis about diagnostic feeding sign: Campephilus anatomy, and especially that of the northern triad (Imperial, Cuban ivorybill, and U.S. ivorybill), is highly specialized. Members of this genus are built to scale bark with greater speed and efficiency than any other woodpecker species, but they are also certainly capable of digging. When they dig, they may be powerful, but I suspect their morphology makes excavation a less efficient foraging strategy.
In contrast to Pileated Woodpeckers, which have evolved to make perpendicular blows, ivorybills have pamprodactylous feet (an evolutionary adaptation that rivals the opposable thumb in terms of how radically it differs from other picids), longer necks, longer, stiffer tails, and larger, broader bills. All these adaptations enable them to deliver strong lateral blows but probably impact their ability to excavate. This may explain why many of the foraging pits shown in the Pearson photograph and in Plate 11 are skewed and why ivorybill nest cavities are asymmetrical. It might also explain why ivorybills dig relatively infrequently during breeding season and instead undertake long daily circuits to strip bark and gather larvae, both large and small, for their young.
Ivory-billed Woodpeckers don’t eat ants or termites and don’t regurgitate. They must obtain live, and when possible large, beetle larvae or large quantities of smaller ones. It’s beyond dispute that they do this most often by scaling bark and finding these insects at or near the exposed wood. Based on the presence of Neoclytus in the nest, it’s reasonable to infer that some prey species are taken early in the life cycle, before they burrow into the heartwood, while others simply live under bark. In addition, several species (Mallodon and H. polita at least) may be exposed when bark is stripped from the bole and their larvae are digging exit tunnels but have not yet sealed their pupation chambers. This is the time when the larvae are largest and most nutritious. This is the substrate in which Tanner found the highest concentration of food, and ivorybills are uniquely adapted for exploiting this opportunity. I believe we have seen evidence of this behavior on some hickories, sweet gums, and oaks in our search area.
One or both of these foraging strategies may be keystones. Fluctuations in the availability of these particular food sources might have a significant impact on nesting success.
Whether or not I’m exactly right about all this, I think there are several important points that deserve to be reiterated.
- The Singer Tract ivorybills “usually” or frequently fed on high, freshly dead sweet gum and Nuttall oak branches; what they were feeding on remains unclear; however, there is no doubt about the importance of the prey collected (whatever it was) at the treetops for raising young. Specifically, on April 23, 1939, Tanner observed both adults feeding “Baby Bunting” from prey captured from the top of a dead pecan (hickory), and also the long flights these three, along with “Sonny Boy” (the previous year’s young, still with adults), made from one foraging tree to another. There is also no reason to doubt that prey from treetops made up a substantial part of what was fed to the young before fledging.
- Ivory-billed Woodpeckers in the Singer Tract could and did feed at all levels and on wood in all stages of decay, but during breeding season, at least, they took most advantage of more recently dead and dying trees.
- Despite the habitat and tree species preferences documented by Tanner during the 1930s, the last few ivorybills could and did feed in areas and on tree species that Tanner did not document as being heavily used during his study. This was mostly in the 1940s, after massive cutting was under way. Subsequent interpreters of Tanner have inferred that these tree species and areas were unimportant or unsuitable, and some of Tanner’s later statements may have abetted this misunderstanding. The takeaway is that ivorybills will feed on a variety of tree species, provided the trees are stressed and infested with wood boring larvae that can be quickly collected by scaling bark.
- Prey species were most heavily concentrated in what Tanner called “hard but partly punky” stumps. Though it’s not explicitly stated, this class is likely to include the boles and large lower branches of standing trees, including Cerambycid infested trees that have not yet succumbed.
- Despite popular perceptions, large trees are not a requirement. Notwithstanding our disagreement about how to characterize foraging frequencies and size class explored at length in the first post in this series, my collaborator and I agree that insect abundance, not tree size per se, is the most significant factor. The foraging behavior documented by Allen and Kellogg and the nesting successes in mostly second growth but fire damaged forest (Mack’s Bayou) earlier in the 1930s support this interpretation.
I hope this series of posts will prove useful to other searches and that it provides greater clarity about ivorybill foraging behavior.
Addendum, March 26: A biologist wrote to point out that I may have been regurgitating conventional wisdom on the subject of Campephilus regurgitation. Some of the literature states that they do feed their young in this manner, and there is language in Tanner to suggest this may be so for ivorybills (pp. 74-75). “Often it seemed to be jerking as if working food from the back of its mouth.” As I read Tanner, the number of larvae that may have been regurgitated seems small, a single grub in at least one instance. The passage in Allen and Kellogg (mentioned in the comments) involved termites, and it is highly speculative. And I recall reading that ivorybills were hunted for food specifically because they didn’t taste of formic acid, unlike pileateds.
At present, I don’t think this information calls for a major revision of the hypotheses presented here, but I plan to do some additional research and may have more to say on these subjects in future. I’ll be completing a week in the field today and expect to post a trip report before too long. As a preview, I’ve found an unprecedented quantity of recent high branch and upper bole scaling this week, all of it on sweet gums.
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.
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.
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.
Although it is thematically quite different, this series of posts is rooted in my recent reexamination of my feeding sign hypothesis that culminates here. It was also inspired by my recent and much closer look at Tanner and the Singer Tract, new insights gleaned from old material, and the input of others that shaped the previous post. My original plan was to make this entry the last in the previous series, but since it has grown to over 5,000 words and addresses different issues, I decided to break it in three and will post the next two installments soon.
I’ve been engaged in an extended dialog with a biologist who is familiar with all the Ivory-billed Woodpecker literature and knows Tanner’s writings specifically. Our back and forth is the primary reason for the long interval between the previous post and this one. This person provided some very important insights that will be included in these posts. At the same time, we have a few points of disagreement. In the interests of transparency and allowing readers to draw their own conclusions, these points of disagreement will be disclosed in the text.
Ivory-billed Woodpecker foraging behavior and diet and what separates this species from Pileated Woodpecker and other North American woodpeckers are issues that have been hotly contested for years. In my view, ivorybills could (and presumably do) forage on any species of tree in any decay condition. However, Campephilus anatomy is specialized, and the only quantitative, observational data that exist on what this species does while feeding young (Tanner 1942) suggest some specialization was in fact occurring at least at the Singer Tract from 1937 to 1939. The problem is that many of the prey items (specifically identified in Tanner and emphasized by others since Tanner’s study), even during breeding, do not seem to match up well with the foraging substrates documented by Tanner as most used by ivory-bills feeding young.
In my view, there are some discrepancies between what Tanner observed and reported and the physical evidence he collected during his study related to ivorybill feeding. I also think there may be discrepancies between Tanner’s observations and those of others from the Singer Tract. At least one thing is clear, Tanner’s observations and the photographic record differ markedly from some of his later recollections. In addition, the monograph itself is sometimes ambiguous, as is evidenced by the disagreements mentioned above. It should become clear that the ambiguity and occasional lack of clarity in Tanner’s monograph have led many, myself and my collaborator included, into misinterpretations. We hope that this series of posts will shed more light and clear up some of the ambiguities.
As most readers already know, Tanner’s observations were restricted to one (and the same) family group each of the three breeding seasons during his study. While a sample size of essentially one family group would normally be a serious constraint for comparing with other information, it is important to point out this information represents the only detailed information we have on prey, foraging behavior, and breeding success for ivory-bills, keeping in mind this family successfully fledged young each of these three years. So the data and information Tanner reported on is directly relevant for understanding what was important for successfully fledging young under the conditions found at the Singer Tract during the late 1930s, but as Tanner himself pointed out in his monograph “…the conclusions drawn from them will not necessarily apply to the species as it once was nor to individuals living in other areas.”
Regarding the observations of others on the Singer Tract, I’ll begin with what may have been the last sighting of the John’s Bayou male. In August 1941, George Bick saw three ivorybills feeding in an ash flat near Sharkey Road, quite likely between the bridges over John’s and Methiglum Bayous, south and west of the John’s Bayou home range as delineated by Tanner. This is the only area along Sharkey Road that Tanner listed as “Ash Flat” on his 1941 map.
According to Bick, “I immediately stopped the car and noticed two Ivory-billed Woodpeckers perched in two small ash trees about eight inches in diameter, having recently killed tops. Only one of the birds was carefully observed. A bright, white bill, flaming red crest, and large white wing patch were all clearly noted as the bird remained at the tree. The second bird in a similar ash tree was observed less carefully . . . [A third bird] flew from a dying water-oak tree ten inches in diameter which had only a few curled brown leaves. A stripped spot about six by eight inches and about seventy feet from the ground was present on the trunk of this tree. This is thought to be a spot where the birds had been feeding and to represent the characteristic Ivory-bill ‘sign.’ In the immediate area were many ash trees with dead tops. Much of the bark was stripped in patches of varying size. This may possibly be old Ivory-bill feeding grounds.”
Logging had taken a significant toll in the Singer Tract by the time of Bick’s sighting. It’s thus possible that the birds were foraging in a suboptimal area due to logging pressure. Nonetheless, it’s still worth pointing out that Bick’s observations were in habitat and on tree species where Tanner observed virtually no foraging activity during his study (which ended two years prior, in 1939; he had no feeding observations on water oaks and only one on an ash). It’s also worth pointing out that Bick made specific reference to sweet gums (what he called “red gums”) as being abundant elsewhere but absent from this location.
My collaborator suggested that Bick’s inference that this ash flat was an “old Ivory-bill feeding ground[s]” is questionable. He suggested that changes in hydrology due to logging may have led to an ash die-off. He also noted that this was Bick’s only observation during his six month stay in the Tract, indicating that he was either not looking hard for ivorybills and/or that ivorybills were not using the ash flat on a regular basis. He added another caveat: it is important to remember that Bick’s observation was in August, well after the breeding season when even Tanner assumed foraging behavior for Ivory-billed Woodpecker likely expanded to different habitats and tree species than used during the time they were feeding young at John’s Bayou.
It’s interesting to note that the last known roost, where Don Eckelberry and young Billy and Bobby Fought famously said goodnight to a lone female ivorybill in April 1944, was apparently located in the ash flat where Bick saw his birds (W. Barrow pers. comm.). Just a few months earlier, in December-January 1943-’44, Richard Pough found a lone female roosting in the heart of the John’s Bayou range, about a mile north and east. According to Pough, who was convinced she was the last ivorybill in the Tract, this bird only crossed the Bayou once “for a brief visit to some trees a few hundred feet west of it . . . confining its activities to an area of hardly more than one quarter of a square mile, within which there were an unusually large number of dying trees.”
In our most recent conversation, my contributor and I touched on the question of whether Bick’s birds (and presumably the one seen by Eckelberry and the Foughts) were from the John’s Bayou family group. Either way, it’s a potentially interesting wrinkle. If the birds did come from John’s Bayou, this points to a heavier use of the ash flat for a period of years than is suggested by the limited information about the family group after 1939. All other observations – Pough, Peterson, Tanner, and Baker – were in the heart of the John’s Bayou home range, and at least one of those birds was reliably present there until shortly before Eckelberry and the Foughts said goodnight. On the other hand, if Bick’s birds were a different family group, it suggests that more ivorybills were in the Singer Tract in 1941 than is commonly assumed. (It’s worth repeating that Peterson wrote that one ivorybill was seen in December 1946, and the last letter to Tanner directly related to the Singer Tract birds says that game warden Gus Willett saw a pair in November 1948 and mentions other reports from around that time.)
To return to the Bick report: all of the trees seem to be in the smallest of Tanner’s size classes, 3-12″ in diameter. This class comprised 75.1% of the forest but was the source of only 12.7% of Tanner’s feeding observations. Tanner believed that ivorybills prefer larger trees because they “have more dead and dying wood” but his own data on this are ambiguous, and what he characterized as large seems problematic. The assumption about older trees having more dead and dying wood may have been true around John’s Bayou during Tanner’s study, but this is by no means always the case – the pine forests of Florida, for example, where Allen and Kellogg found abundant feeding sign on young dead pines, which are more vulnerable to fire than mature trees. And as pointed out in the previous post, even in the Singer Tract, the Mack’s Bayou home range was mostly second growth, so forest composition there must have been quite different.
There are a couple of ways to interpret this data. It’s true that 87% of the feeding was “on trees that are over a foot in diameter”, but this is somewhat misleading. 13-24″ diameter trees are the second smallest size class. They hardly qualify as large and approaching senescence, yet they account for 49% of Tanner’s feeding observations. It’s also true that, relative to abundance, the Singer Tract ivorybills showed a strong preference for trees in the 25-36″ class, but the abundance/observation ratios for 13-24″ trees and over 36″ trees are nearly equal, with a slight preference for the smaller size class not the largest. Thus, I think it’s equally accurate to characterize the data as showing that over 60% of observed ivorybill foraging was on smaller trees, under 24″ diameter at breast height and to reiterate that the most often used feeding trees were in the second largest size category, not the largest. (Tanner pp. 43-45).
On the other hand, there’s a good argument that the data show Ivory-billed Woodpeckers foraged on trees in the 13-24” class at 2.6 times the availability, in the 25-36” class at 6.7 times the availability, and in the 36” plus class 2.57 times the availability; there were very few trees in this size class, most of them sweet gums and a few Nuttall’s oaks (Tanner pp. 43-45). Contrast this with the 3-12” class, when the trees were 5.9 times more available than used.
A few additional points should be added to the mix. The numbers discussed above are aggregates, and size preferences were not at all evenly distributed among tree species. Fully 20% of Tanner’s total observations involved sweet gums in the 13-24” class, the most fed upon type. On sweet gums, frequency and abundance ratios are similar for the 13-24” and 25”-36” classes (the latter is the second most fed upon type, comprising around 18% of Tanner’s total observations). For Nuttall’s oak, 13-24” and 25-36” trees were approximately equal in abundance, but Tanner observed considerably more frequent feeding on the larger class.
My collaborator argues that it is more important is to recognize that when combining the data on sweet gums and Nuttall’s oaks, they collectively comprised 31.4% of the total forest and 79.3% of the foraging observations. Trees within the 25-36” class made up 31% and trees within the 13-24” class made up 29% of all foraging observations. Almost all of the trees in the 25”-36” class (5.2%) were in fact sweet gum or Nuttall’s oak, but for trees in the 13-24” class (18.3%) only about 5% (or about a third) were of these two species. This further highlights what Tanner described as heaviest use on sweet gum and Nuttall’s oak for the John’s Bayou family group over all other available trees, and a disproportionately high use of the second largest size class relative to abundance. However, this documented use pattern was not to the total exclusion of other tree species or even the smallest size class available.
This last was a point of contention. I took issue with aggregating sweet gums and Nuttall’s oaks, since they grow and mature at different rates. In addition, I think it’s important to highlight the fact that 13-24″ sweet gums were the single most fed upon type both in terms of frequency of observations and ratio of observations to abundance (albeit it by a small margin). As I see it, this undercuts the misinterpretation of Tanner that ivorybills are ‘large tree specialists’, a misinterpretation I think Tanner invited when he wrote, “The reason for Ivory-bills feeding on the bigger trees is that large, old trees have more dead and dying wood. Young trees grow rapidly and are resistant to the attacks of insects and disease.” As trees ‘mature’ their growth slows and becomes less vigorous, decay begins, insects attack them, and woodpeckers come after the insects.” (p. 43).
In light of this misconception, I also think it’s important to reiterate that in the aggregate, the over 36″ size did not show anything near the disproportionately high use of the 25-36″class. In fact, the rate was very slightly higher on the 13-24″ trees.
Regardless of how one interprets this very limited data set, the idea that Ivory-billed Woodpeckers required ‘large trees’ for foraging has become a truism. The reality is considerably more complex.
The next installment will focus primarily on decay class, and the final one will look at prey species. Stay tuned.