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’ll be returning to Louisiana in late February and hope to make a couple of more trips during peak search season. Frank has retrieved the cards from our game cams and is in the process of going through several weeks of images. In contrast to the last set, there have been no intriguing hits thus far.
Last month, I came across a very interesting post on the Woodpeckers of the World Facebook group. The link took me to a French website that features some videos of the Black Woodpecker (Dryocopus martius), a Eurasian species, and one of the largest woodpeckers in the world, surpassing the Magellanic in size. In another analogy to the Magellanic, the Black Woodpecker appears to have certain features that are more Campephilus-like than others in genus Dryocopus. This includes flight style; acording to Gerard Gorman’s monograph on the species, Black Woodpeckers don’t generally undulate in flight. More importantly, the size and appearance of the bill certainly evoke the IBWO – generally larger, thicker, and heavier than a Pileated’s. Bill length can reach over 70 mm (although the average is 53-56 mm). According to Tanner, the mean length for ivorybills ranged from 67.8 mm to 74.1 mm depending on sex and region.
For my purposes, this is the most interesting clip. It’s an outstanding sequence that shows a Black Woodpecker scaling bark from a medium-sized hardwood branch. I see the following aspects as being significant and supportive of the hypothesis on feeding sign I’ve discussed in several posts, including here, here, here, here, and here.
For the most part, the bird is removing bark with direct strikes, not the lateral blows of a Campephilus woodpecker. This is possible because the limb is relatively thin, and the bird is able to position herself so that direct strikes will have the same effect that a more lateral blow would have on the bole; she generally engages in lateral movements to flick away bark after it has been loosened. The clip also seems significant insofar as it reveals the amount of effort involved to remove large but very thin strips of bark. In addition, even though the bark is thin, it seems the bird is still removing it in layers, at least some of the time. I think the video tends to support my idea that the work we think is diagnostic – on boles with thick tight bark – is beyond what PIWOs can do physically. At the same time, the footage suggests that the high branch work that Tanner emphasized is likely within the capacity of a Pileated Woodpecker and is indeterminate as we suspect.
In looking at images of Black Woodpecker foraging sign online, it appears that – bill structure notwithstanding – they typically remove bark in layers, just as Pileated Woodpeckers do, and this is true on both hardwoods and softwoods. I have been unable to find any examples of Black Woodpecker work that closely resemble what we think is diagnostic for ivorybill, but examples of the layered scaling are easy to find, for example: