I have reviewed the entire late August-late October card and some of the June-August card for what we’ve designated as deployment 5 – a three-years dead Sweetgum stub discussed last summer. Based on approximately six months of data from this deployment, I think squirrels can be excluded as the source of extensive bark removal from mature, thick-barked hardwood boles, just as the data suggest that Pileated Woodpecker can be excluded as the source of scaling on hickories.
The only potential sources of the extensive bark removal under discussion are gray or fox squirrel, Pileated Woodpecker, and Ivory-billed Woodpecker. Pileated Woodpeckers appear to be unable to remove large quantities of bark from hickories in large pieces, and squirrels appear to be unable to do so on the weaker, thinner-barked sweetgums. Based on trail cam captures obtained thus far, Ivory-billed Woodpecker is the likeliest source for the extensive bark-scaling on hickories that we’ve found infrequently in our search area and that I’ve hypothesized is diagnostic for that species.
There were no woodpecker hits on this target tree, but there are multiple sequences involving squirrels. There was minimal little bark removal, and only from previously scaled areas. In fact, I have only detected one visible change to the bark. A small quantity was removed on June 9, between 11:44:13 and 11:44:33. This is shown in the details below.
Squirrels were active on this scaled patch over the course of the deployment, but whatever removed the small strip of bark on the lower right did so during that 20-second interval and was not captured on the trail camera. I think a woodpecker of some sort is probable, since a squirrel would likely have been visible on the trunk in preceding or subsequent frames.
More importantly, squirrels were captured on or around the scaled areas on multiple occasions, and the captures shed light the way they interact with bark on standing boles and what may limit their capacity to remove it.
This deployment ran from August 19-October 21. Squirrels were detected on 17 days and on or near the scaled surfaces on at least 6 of those days. As previously documented, squirrels displayed interest in the edges of the scaling and frequently appeared to be gnawing; however, they removed little or no bark. We now have numerous captures of squirrels on target boles, both scaled and unscaled, and no captures showing them removing bark in quantity or in anything other than small strips.
Squirrels are clearly capable of rapidly and efficiently removing bark from limbs, downed trees, and thinner barked boles. However, I think there are physical limits – body structure and incisor length – on their capacity to remove thick bark from standing boles.
The following images and time lapse clips show what squirrels do when confronted with thicker bark and suggest that when hanging onto a standing trunk, they lack the leverage to remove bark quickly and leave large pieces behind. This should apparent in the selection of stills and video clips shown below as well as in the sequences posted previously. (A brief discussion of squirrels on hickories follows the images.)
Up to now, I have not been differentiating among squirrel hits on targeted trees, squirrel hits on or near scaled surfaces, and squirrel hits in other parts of the frame. Suffice it to say there many, far more than woodpecker hits on both sweet gums and hickories. Squirrels frequently show an interest in the scaled surfaces and also in other damaged areas (like the fracture in the hickory bark shown below). To date we have no examples of squirrels removing any bark from hickories, regardless of condition. It stands to reason that the limits of their capacity on hickories would far exceed what limits their capacity on sweet gums.
I suggest reading Part 1 for background and context, if you haven’t already.
The target of this deployment (5/3-6/3/2018) was the sweet gum stub discussed here. The tree was killed when its top was blown off in spring 2015. A patch of recent scaling was found this season. I suspect the initial scaling is woodpecker work, but squirrel is also possible. The extent is modest in terms of what I hypothesize is diagnostic for Ivory-billed Woodpecker:
A particular and distinctive looking type of extensive scaling (large contiguous areas with bark removed) with associated insect tunnels on bitternut and pignut hickory boles – live trees, snags, and stubs – may be diagnostic for ivorybill. For recent work, the presence of large bark chips at the base of such trees is a related potential diagnostic.
Insect tunnels are present on this stub. Species is/are unknown, and tunnels are small compared to those found in the hickories.
In contrast to the hickory discussed in the previous post, there were no woodpecker captures over the course of this deployment and squirrels were very active on the scaled area, appearing on May 4, 5, 8, 10, 12, 13, 15, 17, and 22. There were multiple visits on some days, and the total time spent on the scaled area was significant, upwards of an hour, with at least one visit lasting nearly 25 minutes. It was surprising that squirrel activity ended on the scaled area ended on May 22nd, and there was none over the next 11 days.
Over the course of this deployment, squirrels removed a modest quantity of bark, apparently in strips, from part of the scaled area. They did this inefficiently – with some difficulty and with the grain. The bark, already softer and weaker than hickory, has weakened in death and is at best moderately tight (relatively easy to peel off by hand). Captures from the first and last full days (note the Hooded Warbler on the branch to the left) of the deployment reveal how little bark was removed, all or almost all from the right side of the scaled area. (Click on the images to enlarge them.)
This suggests that squirrels are unlikely candidates for removing bark from hickory boles in quantity, leaving large chips behind, or initiating extensive scaling on hickories. In my view, it’s probably impossible for them to do so. The results for Pileated Woodpecker from the hickory deployment and squirrel from this one support my hypothesis that Ivory-billed Woodpeckers are the source of the initial hickory scaling. But more data are needed.
Before turning to the trail cam captures and accompanying images of the scaled surfaces, I’ll provide some background information on the impetus for this post and on squirrel behavior.
An email discussion of squirrels and bark scaling was ongoing prior to my starting to review the images from this deployment. Wylie Barrow suggested an alternative explanation: that squirrels might be the source of much of the scaling (including the work on hickories) that’s taking place in the search area. He pointed out that . . . “Squirrels have removed bark from 1/4 to 1/2 of the trunk and several large branches from large oaks in my yard… and they work with great speed. They often leave large bark chips on the ground beneath the trees. Trees are living and bark is tight and fairly thick.” (W. Barrow, pers. comm.)
At first, I took some umbrage at this suggestion, thinking that I had thoroughly examined and considered what squirrels might be doing on the hardwoods in our search area and what the upper limits of their capacities might be. While my basic views on this are unchanged, and the trail cam images tend to support those views, I’m grateful to Wylie for keeping me on my toes.
It’s certainly true that in the past I have failed to consider squirrels and the role they might play in bark scaling, and this has led me down some blind alleys, as was discussed in a series of posts in early 2016. I have also been too confident in those conclusions, even though I think this material supports them. Wylie’s suggestion led me to conduct additional online research on squirrels (and he provided additional references).
I had a number of off-the-cuff theoretical and observation-based objections to Wylie’s suggestion.
One evolutionary objection is reflected in a comment I made early in our exchange: “the predator in question would have to have evolved to take advantage of this very narrow window of opportunity when the insects are near the surface . . .” I thought and still think this points toward a woodpecker as the source, and toward a Campephilus woodpecker in particular, since this foraging strategy is characteristic of the genus.
The hickory scaling is associated with sapwood dwelling Cerambycid infestation, and signs of woodpecker activity (targeted digging around exit tunnels) are present in all cases. The homepage tree was very recently scaled when found, and woodpecker evidence was present. Wylie replied that squirrels are opportunistic and might be feeding on larvae; he went on to suggest that woodpeckers following the squirrels and doing targeted digs around the exit tunnels was a possibility.
In one paper on a tropical species of squirrel, it was observed that they prefer palm nuts infested with beetle larvae. The authors also note that squirrels have a strong preference for obtaining food in the most efficient manner, and that Eastern gray and fox squirrels will choose nuts lacking an endocarp (the hard inner shell) over those that are harder to open. When confronted with an endocarp, the tropical squirrels would attack it at its weakest and thinnest point, as do Eastern gray and fox squirrels :
Two of these pores have dead ends (with 1-mm depth), and the third is the germinal pore, which is deeper but is closed by a soft and easily penetrable tissue, located on the side opposite the fruit’s internal gibbosity. The internal gibbosity is a projection of the endocarp that inhibits the squirrel’s access to the endosperm when the fruit is opened from the side containing the dead-end pores. The squirrel must determine the position of the internal gibbosity to avoid it and thus save energy and time in obtaining the endosperm. These rodents are known to identify the side without the internal gibbosity even before beginning to open the fruit, with >90 percent success (Bordignon et al. 1996, Mendes & Candido-Jr 2014). However, how the squirrel identifies the side without the internal gibbosity remains unknown. As the gibbosity is always on the side opposite the germinal pore (Bordignon et al. 1996), this pore is an important access point that the squirrel can use to open the fruit efficiently. It is believed that the squirrel manipulates the fruit by pressing the three pores with its upper incisors, using the pore without a dead end for support so that the lower incisors can open the endocarp (Bordignon et al. 1996).
Efficiency is one of the main factors that determine the foraging strategy of Sciuridae. A laboratory study conducted with the squirrels S. carolinensis and S. niger found that individuals preferred various species of nuts with low energetic value that lacked an endocarp or shell over high energy nuts with an endocarp (Smith & Follmer 1972). These results suggest that there is a high cost in energy expenditure for processing seeds with endocarps for these species.
(Alves et al. “Queen palm fruit selection and foraging techniques of squirrels in the Atlantic Forest,” Biotropica 50(2): 274–281 2018). Efficiency is an important consideration in this context, especially with respect to hickories.
The reasons squirrels strip bark are poorly understood. Pine (or red) squirrels attack a number of tree species, “[d]uring winter, spring, and early summer, bark stripping and tree girdling for consumption of phloem and cambial tissues is common (Hosley, 1928; Linzey and Linzey, 1971; Pike, 1934). Pine squirrels also eat the bark of rust galls (Salt and Roth, 1980) as well as sap from sugar maple trees (Acer saccharum) in the northeast (Hamilton, 1939; Hatt, 1929; Heinrich, 1992; Kilham, 1958; Klugh, 1927; Layne, 1954) and yellow birch (Betula alleghaniensis) in the Great Smoky Mountains (Linzey and Linzey, 1971). Widespread, systematic sugar tapping by pine squirrels occurs in New England (Heinrich, 1992).” (Steele, M. A. 1998. “Pine squirrel (Tamiasciurus hudsonicus),” Mammalian Species 586:1–9).
Red squirrels have also been observed feeding on spruce bark beetles. (Pretzlaw, et al. “Red Squirrels (Tamiascurius hudsonicus) Feeding on Spruce Bark Beetles (Dendroctonus Ruffipennis): Energetic and Ecological Implications”, Journal of Mammalogy, 87(5):909–914, 2006). This was a novel observation at the time, and the behavior appears to have been a sudden and opportunistic response to a climate change-related bark beetle outbreak that lowered cone production. Spruce bark is soft, flaky, and fairly loosely adhering, and the bark beetles spend approximately a year, the entirety of their larval life cycle, in the phloem and hence are a readily available food source for a prolonged period. Moreover, “[f]oraging for larval spruce bark beetles by red squirrels is an obvious and stereotyped behavior; squirrels situate themselves on the trunk of the tree near ground level and peel off the bark to reveal and ingest larvae.”
There seems to be less agreement as to why Eastern gray and fox squirrels strip bark. It has been suggested that a calcium deficiency might be primary driver. C.P. Nichols et al., “A novel causal mechanism for grey squirrel bark stripping: The Calcium Hypothesis,” Forest Ecology and Management 367 (2016) 12–20. Bark stripping by Eastern gray and fox squirrels seems to be more prevalent in areas where the species have been introduced, “[b]ark-stripping behaviour, reported so often in Europe (Shuttleworth et al. 2015), is extremely rare in their native range (Kenward 1989).” (Koprowski et al. “Gray not grey: The ecology of Sciurus carolinensis in their native range in North America”, posted on Researchgate.com, 2016).
While “extremely rare” is an overstatement, it does appear that bark stripping occurs more frequently in areas where gray and fox squirrels have been introduced. It is a major problem in the U.K and Europe but mostly an annoyance in the United States. It seems reasonable to infer that it is more common in suburban and residential areas than in mature bottomland hardwood forests, though Wylie points out that the discrepancy in the reporting may be due to demographic factors and that squirrel behavior in bottomland hardwood forests has been poorly studied.
Gray and fox squirrel bark stripping seems to occur most frequently on branches, and I found no images in which insect infestation of the scaled areas was apparent. In addition, the examples of extensive squirrel scaling found online in no way resemble what we’re finding on hickories. Thus far, we have found only two references to squirrels stripping bark from trees in the genus Carya, one from pecans in Georgia and one from limbs in West Texas pecan orchards, where fox squirrels have been introduced. It’s not clear what parts of the trees were involved in Georgia and whether this report also came from an orchard, but regardless, pecan bark is flaky and not criss-crossed, making it easier to scale.
While neither Wylie nor I conducted an exhaustive literature review, we found no records of gray or fox squirrels scaling bark from any bitternut or pignut hickories (Carya cordiformis and Carya glabra), be it on limbs or boles, in several Google searches. Given the extensive range of these species – most of the Eastern United States and into Canada – and the association between squirrels and oak-hickory habitats, if squirrel scaling of hickories occurred with any regularity within the natural ranges, one would expect references to be abundant in both the popular and scientific literature.
As mentioned in the previous post and implied above, I suspect that the criss-cross pattern that characterizes pignut and bitternut hickory bark is one factor that deters squirrels from removing it and may prevent them from removing it in large pieces. This relates more generally to the question of efficiency. The characteristics of hickory bark make it extremely difficult for any creature to remove. In addition to the pattern of the grain, it is literally the hardest, strongest, thickest bark in the forest. On mature boles it can be 3/4″ thick (compared to around 1/16″ for a hickory endocarp). It is tight (though less so when sap is flowing), and it retains these characteristics long after death. Bitternut hickory bark does not flake, and pignut does so infrequently and superficially.
Thus, both species are exceedingly poor candidates for stripping by squirrels, especially when sweet gums and an array of other much easier targets are available. In contrast to the hickories, the target tree in this deployment was a sweet gum, three years dead, with thinner, considerably softer, loosening bark
As I see it, all of this militates against squirrels as the original source of the hickory scaling. While this is inferential and we have yet to document whatever initiates the scaling, the data obtained thus far support the inference. Only recently have we been able to deploy enough trail cameras for a meaningful and sustained effort. Nevertheless, we have had many hours of captures since 2009, in both search areas. To my knowledge, the only prior unambiguous capture of squirrel scaling is the one from 2015; it involved a downed, immature sweet gum with thin bark, which was easy for squirrels to scale. A second clip may show a squirrel removing a very modest quantity of thin bark from a sweet gum limb that was already being scaled by Pileated Woodpeckers (second video clip at end of post), and Wylie observed a squirrel scaling a sweet gum branch (on a roadside just outside the main search area) in December 2015.
I no longer think scaling on sweet gum limbs (so heavily emphasized in Tanner) is a strong indicator of ivorybill presence, at least not on its own, although what we’ve found in the search area seems to be unusual. Abundance, lack of correlation with low mast years, bark chips, absence of incisor marks, and indications of woodpecker activity, especially targeted digging, may all be suggestive. Sweet gums, which are very attractive to beavers, are likely one of the most desirable targets for squirrels as well, for reasons of flavor and efficiency.
But we have documented no squirrel scaling on hickories, live or dead, on limbs or on boles, partially scaled or with bark intact.
I think the results from this deployment shed considerable light on the issue of squirrels and bark scaling, especially what they do (or can do?) on a mature bole with thick bark. So let’s go to the videotape, as a New York sportscaster used to yell.
Squirrels on a Sweet Gum Bole
As with the previous post, our Plotwatcher Pro trail cam is programmed to capture one image every twenty seconds, and these time-lapse sequences have been converted into QuickTime movie format. If you want to get a clearer sense of how the squirrels are behaving, you can step through the films frame-by-frame. If you elect to watch just one of the clips, the one from May 8 that starts at Frame 1500 (the squirrel spent 24 minutes on the scaled surface) or the one from May 12 that starts at Frame 1574 might be your best bets. Discussion and close-ups of the scaled surface follows the bonus imagery.
While we had no woodpecker detections on the stub and bird captures were few, we did catch some hogs (piglets?) and a beaver. Also captured but not shown were a Northern Cardinal and an Eastern Phoebe.
Discussion and Details
As best I can tell, the only expansion of the scaled area involved a narrow strip at the upper right, probably no more than 12″ x 2″, and a little widening at the very top, although this was an area where the squirrels spent a considerable amount of time.
Let’s look at some details from that scaled area.
While there appears to have been some woodpecker excavation at the middle left of the larger scaled patch, there’s no readily apparent sign that woodpeckers have been after the insects that are feeding in the sapwood. Nor is there any strong indication that squirrels were feeding on insects over the course of this deployment, though it’s possible they took advantage of snails and beetles, like the ones in the photo, or slugs, which I also saw on the scaled patch.
The edges of the bark shown in the close-ups, especially the one at the top, show signs of having been gnawed, although this is subtle, and sometimes impractical as an identifier, since such close examination is not always possible in the field. I presume that the abundant squiggly abrasions to the surface of the underlying wood are incisor marks, something we have not observed with other scaling we’ve found.
With regard to what was left behind, the first three photos show what I found at the base of the snag when I discovered the scaling on May 1, 2018.
The large, though narrow, strip of bark was the biggest one I found at the base and is one of the main reasons I suspect that woodpeckers initiated the scaling with squirrels following, although I would not rule squirrel out completely. In any event, the bark was so soft and weak that it broke in my hand when I picked it up on June 11. The other thin strips are more consistent with what I’d expect for squirrel, and the tiny orange pieces of cambium are a giveaway.
The situation had changed little during this most recent trip. The picture with my boot shows the larger pieces of bark I found at the base, including the one shown above after it broke. They may be consistent with woodpecker (possibly including Red-bellied or Hairy), but I suspect that both squirrels and woodpeckers were involved in the bark removal.
Edited to add: For any extensive squirrel work on mature boles, especially hickories, I would expect to find many small pieces of bark on the ground, similar to those shown above, as in this dramatic example.
My main objective in targeting this stub was to observe it over time, more for what might happen as the decay advanced and whether it might become a target for ivorybills; it’s the type of “stump” that Pearson described as being favored by ivorybills after his visit to the Singer Tract in 1932, though Pearson’s “stump” (scroll down in the linked article) was much longer dead. The bark scaling, while interesting, was in the “could have been anything” category. Getting this data on squirrels was a pleasant surprise, one that I should have anticipated based on the small bits of cambium on the ground. My bias came into play, as I ascribed them to a smaller woodpecker. Between Wylie and the trail cam results, I’ve learned a lesson. In terms of the bigger picture, however, the results so far suggest that squirrels are not the source of the putative Ivory-billed Woodpecker scaling on hickories.
Summary/Bottom Line Up Front
5 of our 8 trail camera deployments (a 9th is planned) are targeting unscaled hickories, stubs and live trees that show signs of damage or decline. The main goal is to capture images of Ivory-billed Woodpeckers by identifying potential foraging trees and targeting them in advance of their being scaled (a long shot but the best strategy I can think of). A secondary purpose, especially with targets that already show some scaling, is to test my feeding sign hypothesis by documenting work by Pileated Woodpeckers and Eastern gray or fox squirrels (a much easier task). These are the only other local species that have the capacity to remove large quantities of tight bark from the boles of mature living and recently dead trees, though I suspect this capacity is limited. Results from two deployments, one on a still living but compromised and extensively scaled hickory (4/29-6/6/2018) and the other on a sweet gum stub (5/3-6/4/2018), suggest to me that neither Pileated Woodpeckers nor squirrels are the source of the initial, extensive hickory scaling.
Part 1 will address Pileated Woodpecker activity on the hickory, followed by a brief discussion of my visit to the search area from June 6-11. Part 2, which should be up within a week or so, will focus on squirrels, especially their activity on the sweet gum stub, which I think is analogous to the Pileated Woodpecker activity on the hickory. These are preliminary findings that may be contradicted by data obtained in future, but thus far they support the suggestion that Ivory-billed Woodpeckers are the source of the initial scaling.
Our PlotWatcher Pro trail cameras are dedicated time-lapse (as opposed to motion- activated) units. We set them to capture images at 20 second intervals, approximately between sunrise and sunset. The proprietary software makes it possible to create QuickTime movie versions of the time-lapse stills. The QuickTime versions are worth watching and are illustrative of behavior, but it’s important to remember that they are compilations of still images, not actual videos. Thus, it’s also worth stepping through them frame by frame. This will be an image heavy post but with a lot of analysis. I hope you’ll stay with it.
I am not planning to do posts on every review of every trail cam deployment but will post results that I think are a relevant or interesting to readers – PIWOs and squirrels spending time on scaled areas or removing bark, most or all woodpecker hits, mammals that are infrequently seen, identifiable birds, but not squirrels running up and down tree trunks.
Before turning to a discussion of the trail cam imagery, I’ll take this opportunity to restate and elaborate on my hypothesis. The imagery and discussion will be familiar to some readers, but I hope the new data will shed additional light, even for those who know the material.
Initially, I hypothesized that certain types of bark scaling might be diagnostic for ivorybill. Bear in mind, my focus has been on identifying a diagnostic, not covering the full range of what Ivory-billed Woodpeckers might do, so my criteria, especially this specific restatement thereof, may be overly narrow. Over time, and based on trail cam captures of Pileated Woodpeckers and a squirrel doing some scaling that seemed to match Tanner’s descriptions, I have refined and limited the hypothesis as follows:
A particular and distinctive looking type of extensive scaling (large contiguous areas with bark removed) with associated insect tunnels on bitternut and pignut hickory boles – live trees, snags, and stubs – may be diagnostic for ivorybill. For recent work, the presence of large bark chips at the base of such trees is a related potential diagnostic.
The tree on the home page (shown below) is one example. Numerous insect tunnels, some with signs of having been expanded by woodpeckers, must be visible. The appearance of the scaled areas is such that I can recognize older work even in the absence of chips. Because standing hickories (in drier areas at least) are slow to decay and the bark can remain tight for years, older examples persist, and I’ve found many of those over the years.
The hypothesis is founded on Tanner’s descriptions of scaling (although bitternut and pignut hickories were absent from the Singer Tract) and on the anatomical characteristics of Campephilus woodpeckers versus those of Dryocopus or Melanerpes, which appear to lack the physical capacity to do this type of work or do it extensively. The characteristics of hickory bark – chemical composition, strength, tightness, and thickness – are also central. I think the same limitation applies to squirrels, at least on hickories. This is not to exclude squirrels and PIWOs from coming along after the initial scaling and expanding it.
It recently struck me that grain may play a major role in limiting Pileated Woodpeckers and squirrels from scaling on hickories. Both species seem to follow the grain when stripping bark. Bitternut and pignut hickory bark differs from that of many bottomland hardwoods in having criss-crossed grain, making it considerably harder to remove, except in narrow strips like these, presumably removed from the target tree by a PIWO last year.
Based on the recent observations, PIWOs are more likely to dig through hickory bark to reach the sapwood than they are to scale it. I suspect it’s the case with the presumed 2015 PIWO “scaling” on hickory shown below, which is the only example of that type of work I’ve found. The differences may appear subtle in the photo, but they are more pronounced in situ; the work is patchy and discontinuous, and there are few or no exit tunnels. The chips at the base of this tree were small and included sapwood, indicating that this was actually shallow excavation with associated bark removal, something that is often and easily confused with true scaling.
Since 2013, we have found approximately 20 hickories that appear to have been recently or freshly scaled. Below are additional examples, plus some that were presumably several years old when photographed. In the case of the tree on the home page, the scaling was very fresh when found. I have not found any recent looking work of this type since March of 2015, though Phil found some last year that were probably scaled during 2016; one of these is the target tree in the sequences shown below. Steve Pagans has found several examples, but I’m not sure of any recent ones. Note the expansion of the exit tunnels, which makes it evident that woodpeckers have been active on the scaled surfaces.
A Cerambycid infesting the trees has been identified as Parandra or Hesperandra polita, a medium-sized, bole dwelling species with a 3-5 year life cycle.
H. polita remains, apparently of 2 adults, were found in one of the ivorybill stomachs examined by Cottam; it was collected in West Carroll Parish, August 19, 1903 and is referenced by Tanner. The adults of this species remain under the bark for some time after metamorphosis.
There may be additional subtle features, including chip characteristics and general appearance, that are relevant to other tree species and may be indicative if not diagnostic. A very nearly dead sweetgum, which had an intriguing cavity, from the old search area, is one example.
Trail Cam Captures – Hickory – April 29 -June 6, 2018
Over the course of this deployment, Pileated Woodpeckers were photographed on the target tree on 10 of 36 days, sometimes briefly and sometimes for periods of over 20 minutes. On some days in the latter part of the deployment, intervening foliage partially or fully blocked the lens, so it is possible there were additional visits. The camera is trained on the base of the scaled area, which extends to the broken top of the bole, and some of the sequences make it clear that pileateds are visiting the upper part of the scaled area as well. Phil Vanbergen found this tree last season; it is still living, although the sapwood is already somewhat punkier than what I’ve found on many longer dead hickories.
The first important point is that very little bark was removed over the course of this deployment, despite considerable time spent by Pileated Woodpeckers on the scaled area. The first two images are from the first and last days of the deployment for purposes of comparison. The new work is so minimal that it is only noticeable in the field upon close examination of the edges.
Pileated Woodpeckers were photographed on May 1, May 3, May 8, May 12, May 21, May 25, May 28, May 30, June 1, and June 4. A Hairy Woodpecker was captured on April 30 and on May 12. There were no squirrel captures on the scaled area. Individual frames and time-lapse films (when birds were present for more than a couple of captures) of the Pileated sequences follow. If you choose to watch/step through just one clip, I suggest the one from May 28, in which two birds are present, but viewing all of them will provide a better sense of what took place.
In addition to removing only a modest quantity of bark along the already scaled edge, despite being present on the scaled surface for over an hour altogether, it appears that the Pileateds are not actually stripping the bark but instead are excavating through it. This is also suggested by the quality of the bark chips on the ground. (The caption predates this imagery.)
The same appears to be the case for the presumed Pileated “scaling” on hickory in the preceding section. It’s unclear what the woodpeckers were seeking in terms of food, since there is no evidence of current or recent Cerambycid tunnels. In any event, the sequences give evidence of what Pileated Woodpeckers do on hickories, and just how inefficient they are at removing bark.
In the images we captured last year from a nearby deployment (on another scaled tree Phil found, approximately 100 yards away), the Pileated appeared to be scaling bark rather than digging through it, and this too was a very inefficient process. It involved removing long narrow strips, distinctly different from the chunks of bark shown above. In a sequence Frank and I captured on the homepage tree back in 2013, a Pileated appeared on that very recently scaled tree but removed no bark at all; a Hairy Woodpecker also appeared and removed a very small piece. (Scroll down to the bottom of that post for the relevant images.)
The trail cam captures validate my prediction that the source of this most recent work is Pileated Woodpecker.
Thus, we have documented multiple instances of Pileated Woodpeckers foraging on trees suspected to have been initially scaled by ivorybills. None of the imagery thus far shows pileateds removing bark rapidly, efficiently, or extensively. Such bark as they do remove is from areas that are already compromised, although I have found a single example of hickory bark presumably removed solely by a Pileated Woodpecker; it differs markedly from what I’ve hypothesized is Ivory-billed Woodpecker work. While it is difficult to prove a negative, the data gathered thus far suggests that Pileated Woodpeckers do not or cannot extensively scale tight bark from hickories. I suspect they lack the ability to do so.
Other Interesting Trail Cam Images
While these trail cams have limited usefulness, except at close range, all of our cameras are close to their main targets. They are positioned close enough for me to recognize lizards and even insects on the trunk but not identify species. Identification problems can exist for small birds too. I suspect the bird in the image below is a Brown Creeper, though I’m prepared to be corrected as it is so poorly resolved. If it is a Brown Creeper, it would be the latest record for the species in Louisiana.
And this series shows what appears to be a bobcat, at the lower right.
Some Brief Comments on the Trip
Nothing strongly suggestive of Ivory-billed Woodpecker was seen or heard – no ambient sounds or responses to ADKs. Tommy Michot joined me on Saturday, Sunday, and Monday morning. We deployed the 8th trail cam, aiming it at the bole of a sweet gum snag that has a scaled area; I suspect Pileated Woodpecker was the source, and squirrel is also possible, but the decay state is within the range Tanner associated with ivorybill, so this will be a long-term deployment.
Low water made it possible for us to explore some previously unvisited areas. Tommy did several stakeouts where Matt made the recordings this year and where I heard wingbeats and had a possible sighting. On Sunday evening, he observed a Pileated near the sycamore roost where I photographed one in April, and we watched it flying away from the roost on Monday morning. No wingbeats were audible in either encounter, as would be expected for Pileated Woodpecker as opposed to ivorybill.
Temperatures reached the 90s by noon every day, depressing avian activity and making fieldwork unpleasant. There were heavy rains on the afternoon of Sunday, June 10, so the 11th was particularly challenging in terms of heat, humidity, and suddenly high water levels. I had a close encounter with a cottonmouth that dropped from a tree limb within a foot of my head, landing in a slough that had been dry the day before.
One personal highlight was the abundance of chanterelles, especially after Sunday’s rain. I came home with several pounds, though transporting them was a challenge, and I couldn’t keep them clean. Despite a little grit, they’re delicious.
Here are some additional photos. There’s one new example of extensive bark scaling from the upland area shown in detail; it is interesting, but I would not consider it highly suggestive. There was also some recent scaling on a sweet gum that may be more suggestive. I was using a rented 4/3rds camera and a lens with an effective 200-800 mm reach. I had trouble adjusting to the electronic viewfinder, but it was good to get a closer look at one of the cavities found this season. I also found feathers from a Pileated Woodpecker that was presumably recently taken by a raptor; this was in an upland area within 50 yards of where I parked. This is the first time in all my years in the field the that I’ve found PIWO remains.
Stay tuned for Part 2. Like this one, it will initially be password protected so that colleagues can review it before publication.
According to Tanner, scaling bark was the Ivory-billed Woodpecker’s primary foraging strategy during breeding season in Louisiana. Tanner wrote that the ivorybill is “capable of easily scaling away heavy bark that other woodpeckers could not loosen.” (Tanner 1942). All woodpeckers in genus Campephilus have specific anatomical characteristics that enable them to forage in this specialized way (Bock and Miller 1959). Following Tanner, most post-Singer Tract search efforts have looked for feeding sign as an indicator of presence. Because Tanner’s descriptions are somewhat vague and many of the photographs showing feeding sign are poor, these efforts have tended to focus on decay state and bark adhesion without taking bark characteristics and tree species sufficiently into account. I posit that tree species and bark and wood characteristics are key factors that should be considered. I further posit that extensive bark scaling on live and recently dead hickories (genus Carya) may be beyond the physical capacity of the Pileated Woodpecker.
As all regular readers know, I’ve been somewhat obsessively focused on bark and bark scaling since my earliest years of ivorybill searching. The reason for this interest is simple: it’s how Tanner found ivorybills or inferred their presence when he couldn’t find them (Tanner 1942). Unfortunately, as discussed in a number of posts, Tanner’s descriptions are somewhat opaque, and most of the published images of feeding sign, including those in the monograph, are not very illuminating. Indeed, some of them are consistent with pileated work that we’ve documented. Plate 8, shown below, is a prime example. The caption reads, “Ivory-bill feeding sign on a slender limb”.
Early on in my study of this subject, I hypothesized that certain kinds of bark scaling on hardwoods might be beyond the physical capacity of the Pileated Woodpecker. I still believe this to be true, a view that is supported by what we’ve documented for pileated and by numerous examples of pileated scaling from online sources. At the same time, the details of what types of work might belong in this category have shifted somewhat, especially as it has become clear that Pileated scaling can look like what’s shown in Plate 8 and that pileateds will scale bark from recently dead sweet gums.
This is not to suggest that ivorybills never scale small and medium-sized branches in a similar manner. According to Tanner they did so frequently; however, I have been focused on what may be diagnostic for ivorybill. It seems likely that there is considerable overlap between ivorybill and pileated work when smaller branches are involved (at least on sweet gums).
The sequences we obtained showing pileateds scaling a sweet gum limb have inspired me to look more deeply at the characteristics of hardwood bark and pursue some research avenues that I hadn’t considered previously. I’ve linked to some of the sources in recent posts, but I’ve had some additional insights that seem important enough to share. Every time I think I’ve run out of things to say on the subject, something new crops up.
Like virtually everyone else, I’ve followed Tanner and focused on two bark characteristics, “tightness” and thickness, but it recently struck me that other features might be important as well. And the literature, mostly from the lumber industry, supports this idea.
Tanner suspected that the Singer Tract ivorybills preferred sweet gums and Nuttall’s Oaks because the bark is thinner, and the thinner barked limbs had “more borers” than thick barked ones. While abundance of food was likely a factor, I suspect that, at least with respect to sweet gums and possibly Nuttall’s oaks, ease of scaling and access to food played a role.
It’s important to point out that in live trees, hardwood bark adhesion varies seasonally, with bark becoming tighter during dormant stages and looser (with considerably less variation from species to species) during the growing season. Bark is often if not always tighter on recently dead trees than on live ones (Stokland et al. 2012).
In addition, “The structural and chemical traits of dead wood, inherited from the traits of living trees, are also major drivers of wood decomposition and these traits vary greatly among tree species.” (Cornellisen et al. 2012). The authors of the linked paper point out that other factors, including size and site, can also contribute to the way that bark loosens post-mortem, but specific traits seem to be paramount, especially since the scaling we deem to be suggestive, whether on standing or downed wood, is on trees that are alive or are recently dead. Because the scaling has a very distinctive appearance, we also deem as suggestive hickory snags and stubs that appear to have been scaled some years ago, even if they are in a considerably later stage of decay overall. Bark attached to hard wood on these longer dead stubs and snags often remains tight for 3 or more years after death.
A 1978 report, entitled Bark and Wood Properties of Pulpwood Species as Related to Separation and Segregation of Chip/Bar Mixtures examined bark morphology and strength properties in 42 different pulpwood species and identified factors that impede the mechanical removal of bark from logs. These include: cellular structure, bark adhesion, bark strength, bark toughness, wood toughness, specific gravity/density, and moisture content. (Institute of Paper Chemistry 1978) One caveat about this report: a subsequent paper gives the sample size for each species, and in many cases (including sweet gums) it was only 2 (Einspahr et al. 1982)
It may be counterintuitive, but the authors found that shagbark hickory was far and away the most difficult bark to remove. (The tightly adhering layer is thin, beneath the dead bark that gives the species its shaggy appearance.) One key finding was that:
“Morphologically, the presence of fibers increases inner bark strength and, when sclereids (a type of cell) are present, bark strength is decreased. Inner bark strength, in turn, has a major influence on hardwood wood/bark adhesion. The multiple regression equation employing wood toughness and inner bark strength accounts for 72% of the wood/bark adhesion variation encountered.”
Sclereids are virtually absent in hickories (Nanko 1980) and a few other species that don’t approach the hickories in bark strength and bark and wood toughness (Eastern cottonwoods, yellow poplars, white ashes, and black willows). These tables are particularly illustrative:
Shagbark hickories are the extreme outlier in this study, in terms of adhesion, as well as in terms of inner and outer bark toughness and strength; there are very few shagbarks in our search area, and we have never found scaling on one. I have been unable to find specific information about bitternut hickory bark strength or toughness, but the industry’s debarking problem applies to all species in the genus Carya due to the near absence of sclereids in conjunction with the other factors. Moreover, the industry does not differentiate among hickory species (Timber Mart South 2016). This 1996 paper is worth quoting at length in this regard (full text is not readily accessible):
The amount of published literature dealing with hickory debarking is very limited. Often it is only mentioned as an example of one of the hardest tree species to debark. One study quantified this by measuring the strength of the bark-to-wood bond of 42 hardwood species, including hickory. According to Einspahr et al., the dormant season bark-to-wood adhesion for hickory is greater than 3000 kPa, which is a tenfold difference from the growing season and nearly three times as great as the dormant season wood/bark adhesion for quaking aspen (Populous tremuloides, L.), a species considered to be extremely difficult to debark in the northern United States.
Einspahr et al. also microscopically examined the failure zone in an attempt to correlate morphological differences with bark-to-wood adhesion. For hardwoods in general, they found that during the growing season, failure occurred in the cambium or in the xylem just inside the cambial zone. Conversely, dormant-season failure occurred in the inner bark. They also found that fibers in the bark increased the inner bark strength while sclereids decreased inner bark strength. Hickory bark can contain between 15 to 20 percent fiber and contains less than 0.05 percent sclereids.
While these studies have confirmed that hickory is difficult to debark, they have not addressed possible solutions to the problem. As a result, hickory is often left behind during harvesting, reducing the total usable fiber from a given stand and, over time, increasing the percentage of the species in the hardwood resource, compounding the problem of future harvests.
When a tree dies, the bark eventually loosens and detaches naturally as the cambium decays. After felling, the cambium remains alive until it has consumed all available food or dries out. Moisture loss, while causing cambial death, initially greatly increases the strength of bark attachment because additional bonding between fibers occurs as the secondary valence bonds with water are broken (Belli 1996).
Thus, even though hickory bark adheres less tightly than sweet gum bark during the growing season, it seems likely that it’s harder to scale year round, given its much greater wood and bark strength and toughness. It is also clear from my observations that sweet gum bark loosens far more rapidly than hickory bark post mortem. Note that we have found fresh scaling on both live and recently dead hickories.
Based on specific gravity of the bark – averaging 0.72 for shagbark and 0.60 for bitternut – and bark moisture content – averaging 34% of dry weight for shagbark, and 60% for bitternut – it seems likely that bitternuts are somewhat easier to debark than shagbarks but considerably harder to debark than virtually any other tree species in our search area.
Comparing bitternut hickories to other species, most oaks have a considerably higher average moisture content in their bark (Chestnut and Southern red, including Nuttall’s oaks, are exceptions) and similar specific gravities. Sweet gum bark has an average specific gravity of 0.37 and an average moisture content of 91% of oven dry weight. (Schlaegel and Willson 1983, Miles and Smith 2009). But oaks and sweet gums have sclereids, and sweet gums and all tested oak species score far lower on bark toughness and strength than shagbark and, by inference, bitternut hickories. Sweet gums and the tested oak species are fairly similar in these regards, but I suspect that the higher density and lower moisture content in oak bark makes it harder to scale and may mean that oak bark adheres more tightly than sweet gum bark for a longer period after death.
I posit that when it comes to woodpecker scaling, dormant season bark adhesion, inner and outer bark strength, and inner and outer bark toughness are all relevant factors. We know that Pileated Woodpeckers remove sweet gum bark with some difficulty and that even on medium-sized limbs, they are not consistently able to remove bark cleanly down to the sapwood. It’s also clear that bitternut hickory bark is very difficult to remove, second only to shagbark hickory in our search area. This further reinforces my view that the work on hickories is not the work of Pileated Woodpeckers.
Click here and here for examples of the hickories that are scaled in a manner we hypothesize is diagnostic for Ivory-billed Woodpecker. Also be sure to watch this YouTube video of a Crimson-crested Woodpecker (Campephilus melanoleucus) foraging. (Thanks to Phil Vanbergen for finding the clip and the scaled hickory at the second link.) I’m reposting the link to the video here because I think it very clearly illustrates many of the characteristics we associate with Ivory-billed Woodpecker work on hickories, although the species of tree being fed on is unknown. Note the striking similarity in appearance and also that the work of the substantially smaller billed Crimson-crested is not as clean around the edges as the work we’re ascribing to ivorybills.
There were no bitternut hickories in the Singer Tract, but there were congeners – pecans and water hickories. Tanner observed ivorybills scaling on these species twice and digging once. For pileateds, there were 4 instances of digging and none of scaling, as opposed to 5 scaling and 9 digging on sweet gums. The relative abundance of water hickory and pecan at Singer was 2.7%; approximately 10% of the trees in our search area are hickories, and hickories are second only to sweet gums in terms of the number of scaled trees we’ve found. While Tanner’s is obviously a minuscule data set, it may support the hypothesis that live and recently dead Carya bark is too tough for pileateds to scale extensively, if at all.
There are a number of hardwood species found in potential ivorybill habitat that are somewhere between sweet gums and hickories in terms of how easily scaled they may be and how soon after death bark decay and loosening set in – eastern cottonwoods, black willows, water tupelos, some oak species, red maples, green ashes, honey locusts, persimmons, and elms – in these species, it seems likely that close examination of the scaling and bark chips can provide some clues.
Previous Ivory-billed Woodpecker searches have focused on bark adhesion and state of decay when considering scaling as possible foraging sign. Bark morphology, dormant season adhesion, inner and bark outer strength, and inner and outer bark toughness, and wood toughness are all relevant to the ease with which bark can be scaled from live and recently dead hardwoods. Specific gravity and moisture content are also factors. Bark from trees in the genus Carya is difficult to remove industrially, and members of this genus are likely the most difficult trees to scale throughout the historic range of the ivorybill. Since Pileated Woodpeckers scale sweet gum branches with some difficulty and do not consistently remove bark down to the sapwood, it may be beyond the physical capacity of Pileated Woodpeckers to scale hickories extensively and cleanly, while leaving large pieces of bark behind. Extensive work on hickories that has a distinctive appearance may be diagnostic for ivorybills; this distinctive appearance of this scaling may also be the key to recognizing Ivory-billed Woodpecker foraging sign on other species.
This may be no more than an aside, but it may be a relevant data point. I recently observed a Pileated scaling briefly on a live 14″ DBH Norway maple in my yard near New York City. The photos show that the sap is flowing. The appearance of the scaling is exactly what I’d expect for Pileated, with strips about half an inch across. Norway maple may be a decent stand-in for sweet gum; while its bark has a higher specific gravity, 53 as opposed to 37, the moisture content of the bark is almost identical, 91% as opposed to 90%.
Bock, Walter J. and Waldron Dewitt Miller, The Scansorial Foot of the Woodpeckers, with Comments on the Evolution of Perching and Climbing Feet in Birds, American Museum Novitates, #1931, 1959
Belli, Monique L., Wet storage of hickory pulpwood in the southern United States and its impact on bark removal efficiency, Forest Products Journal. Madison 46.3 (Mar 1996): 75.
Cornelissen, Johannes H.C., Ute Sass-Klaassen, Lourens Poorter, Koert van Geffen, Richard S. P. van Logtestijn,Jurgen van Hal, Leo Goudzwaard, Frank J. Sterck, René K. W. M. Klaassen, Grégoire T. Freschet, Annemieke van der Wal, Henk Eshuis, Juan Zuo, Wietse de Boer, Teun Lamers, Monique Weemstra, Vincent Cretin, Rozan Martin, Jan den Ouden, Matty P. Berg, Rien Aerts, Godefridus M. J. Mohren, and Mariet M. Hefting, Controls on Coarse Wood Decay in Temperate Tree Species: Birth of the LOGLIFE Experiment, Ambio. 2012 Jul; 41(Suppl 3): 231–245.
Einspahr, D.W, R.H VanEperen, M.L. Harder et al. Morphological and bark strength characteristics important to wood/bark adhesion in hardwoods, The Institute of Paper Chemistry, 1982: 339-348.
Institute of Paper Chemistry, Project 3212, Bark and wood properties of pulpwood species as related to separation and segregation of chip/bark mixtures, Report 11, 1978.
Miles, Patrick D. and W. Brad Smith, Specific Gravity and Other Properties of Wood and Bark for 156 Tree Species Found in North America, United States Department of Agriculture, Forest Service. Northern Research Station, Research Note NRS-38, 2009.
Nanko, Hiroki, Bark Structure of Hardwoods Grown on Southern Pine Sites (Renewable Materials Institute series), Syracuse University Press, 1980.
Schlaegel, Bryce E. S and Regan B. Willson, Nuttall Oak Volume and Weight Tables, United States Department of Agriculture, Forest Service. Southern Research Station, Research Paper SO-l 86, 1983
Siry, Jacek, ed., Species Detail Report, Timber Mart-South, 2016
Stokland, Jogeir N., Juha Siitonen, and Bengt Gunnar Jonsson, Biodiversity in Dead Wood, Cambridge University Press, 2012
Tanner, J.T. The Ivory-billed Woodpecker,National Audubon Society, 1942.
Thanks to Fredrik Bryntesson, Steve Pagans, Chris Carlisle, and Bob Ford for their help with this post.