Feeding Sign, Foraging Preferences, and Prey Species: Some Observations and SpeculationPosted: October 13, 2015
Frank and a visiting ornithologist spent this past weekend in our search area. I’m eager to read and will be posting Frank’s report before long. For now, suffice it to say they set up three trail cams, one on the snag where we captured the image discussed here and here and one on this downed sweet gum top found in April:
It most likely fell on April 19th. When I found it a couple of days later, it had fresh green leaves attached and no sign of insect infestation. Since then it has been partially scaled. This is an important data point, as we know the scaling took place within five and a half months of death, and Tanner documented the IBWO’s preference for freshly dead wood. We hope there will be a return visit soon.
They also placed a camera on an even more recently fallen water oak, something that started me thinking about possible patterns in the feeding sign we’re finding.
I’ve counted the examples of feeding sign from our current search area I’ve posted on the blog (which is by no means all the suggestive work we’ve found but is generally the most impressive), and the results for sweet gums are interesting, especially in light of Tanner’s observations suggesting an IBWO preference for sweet gums. Our results also suggest a preference for hickory. (Hickories were scarce in the Singer Tract, and apparently the species present in our area were not present there.) In both cases, the frequency with which we’re finding scaling seems to exceed the relative abundance of either type of tree, although we have not made formal counts. This sign was found between the spring of 2012 and the Spring of 2015, except for the downed top pictured above, which was scaled a little later.
The tally includes a couple of examples of work that falls short of what we consider to be diagnostic for IBWO. It also includes the small sweet gum snag that looks like it was attacked with a hatchet.
Edited November 2021 to add: We now have reason to think this is likely Pileated Woodpecker work.
While there seemed to be a preference for sweet gums prior to the 2014-2015 season, the preference was considerably more pronounced this year when the abundance of fresh scaling on sweet gums in a relatively small area was astonishing. Here’s the multi-year breakdown:
Sweet gum: 25
Presumed sweet gum: 6 (One example possibly PIWO)
Oak species: 3
Willow oak: 2
Maple: 1 (Possibly PIWO)
Ivorybills fed on sweet gums in 42.6% of Tanner’s observations, scaling in 40 instances and digging in 3. Sweet gums made up 20.8% of the forest composition in Tanner’s study area. Next on Tanner’s list of preferred foraging trees were Nuttall’s oaks. By contrast, Pileateds “appeared to have no preference for any species of tree.” Tanner observed PIWOs feeding on sweet gums on fourteen occasions; nine involved digging and five involved scaling. He further noted, “What scaling Pileateds were observed to do was mostly on loose bark and was never as extensive or cleanly done as the work of the Ivory-bills.”
On a more speculative note, I think I’ve been able to identify one species of beetle that’s infesting the sweet gums, including the small one shown above. They’re an invasive, the granulate (formerly Asian) ambrosia beetle Xylosandrus crassiuculus (or another closely related invasive). Ambrosia beetles are tiny, but they are gregarious, with adult females creating chambers and tending broods of larvae in the sapwood. They can kill small trees but also infest larger ones. They have a relatively short life-cycle, and one source suggests they can produce 3 or 4 broods a season in the deep south. It’s worth repeating that I’ve seen signs of ambrosia beetle infestation elsewhere in Louisiana (near our old search area and in upland hardwood forest adjacent to our current one) but did not find work suggestive of ivorybills in either place.
We’ve found known IBWO prey species in our search area, on trees that we suspect were fed on by ivorybills. We also suspect that, contrary to Tanner, they may feed on darkling beetles. Could they also be feeding on an invasive species? We can see no reason to suspect otherwise and will continue our investigations with this in mind. I plan to return to Louisiana Thanksgiving week.