Digging Deeper into Tanner, Part 3 of 3 – Prey SpeciesPosted: March 24, 2016
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.