More Minutiae – Habitat Quality and Population Density in the Singer TractPosted: December 30, 2013
Last week I linked to this image and pointed out that the caption described it as the “Third ivorybills’ nest”. That discovery sent me back to the online archives. What I found raises some interesting questions about the Singer Tract, the population density of ivorybills, at least around John’s Bayou in 1935, and about habitat quality in general.
Most ivorybill aficionados are familiar with images from the Singer Tract. I think most of us know the ones that show dramatic, ‘primeval’ forest, but I suspect we are all prone to overlooking those showing less impressive habitat (just as I am prone to taking photographs of the biggest trees and the most dramatically mature forest), even if we are familiar with the debate about how much actual old growth existed in the Tract in the 1930s. In browsing through the Louisiana Digital Library, I came across an image that I had missed, one of the bridge over John’s Bayou taken in 1940. Tanner (p.32) includes an ivorybill sighting from this immediate vicinity, just northwest of the bridge. What I find interesting about this photograph is that the forest along the road appears to be fairly even-aged and does not have the characteristics typically associated with old growth. It is similar to what can be found in many parts of Louisiana today. An image from along Sharkey Road taken in 1937 shows similar characteristics, although a 1939 shot shows more mature looking habitat and was probably taken to the east of the others.
What makes this even more interesting is that the “Third ivorybills’ nest” appears to have been found to the south of Sharkey Road in an area that was either outside or on the very edge of what Tanner considered to be one of the “Best areas for Ivory-bills” (p. 91). This map shows the location of the nest tree, which is designated “Tree III Squirrel”. (also mentioned in Bales p. 45, “two miles to the south of the first nest” and fifty feet up p. 45.) The image of the snag itself suggests it might have been outside of prime habitat, since the cavity was 45′ (caption), 47′ 8″ (Tanner) or 50′ (Bales) up, and the snag is considerably taller than the surrounding trees.
On Edit: The oak snag could have been a super-dominant tree, which would account for the height difference and be consistent with the location being in mature forest. It’s also possible that the tree was an older tree that had been left behind. A close look at the two maps suggests it was outside the area Tanner designated as prime in 1941; it is approximately a half-mile from the John’s Bayou bridge.
Things grow even more puzzling since what appears to be the “Elm Rock” nest, the famous one, is listed as “Nest II” on the 1935 map, and there is no “Nest I” (Nest I was near Mack’s Bayou. See following post for correction.) There is also a “Nest IV” shown. This is somewhat closer to the “Elm Rock” nest and about 2/3 of the way toward Methiglum Bayou.
In his monograph, Tanner apparently recognized nests II and III but not nest IV, and why there is no reference to a nest I remains a mystery. It seems that 1935 was a bad year for ivorybills, and there’s nothing to suggest that any of these three or four nests succeeded. It’s possible that there never was a “Nest I” and that Tanner later decided that “Nest IV” was not a nest after all. Nevertheless, it appears that at least two pairs of ivorybills attempted to nest in the John’s Bayou area in 1935 and that one of those nests was in or near the border of what Tanner would have deemed to be inferior habitat.
This raises many questions and may have some implications in terms of how the IBWO might have been able to persist. At the very least, it calls into question Tanner’s estimate that “six and a quarter square miles” per pair is “probably close to the maximum density.”