"When we try to pick out anything by itself, we find it hitched to everything else in the universe." -John Muir
One thing we know about natural history is that we don't know everything and there are always new discoveries.
Steve Shively, a wildlife biologist with the Calcasieu Ranger District of the U. S. Forestry Service, was tasked with an ominous job July 24, 2013. He has experience climbing trees to work on red-cockaded woodpeckers, so it seemed logical for someone to assign him the job of climbing 100 feet up a loblolly pine tree near Kincaid Lake, Rapides Parish, to the nest of a Bald Eagle. Climbing that high might be a daunting experience to most of us, but the real challenge is when one gets to the top of the tree under the nest - the next step is to cling to the underside of the nest, leaving the "safety" of the tree trunk, and slowly inching around to the surface of the nest. Not a good adventure for acrophobics!
Well, Steve was successful. What an exhilarating experience for a seasoned naturalist. He was sitting on top of the world in the wide-open sky, with an active Bald Eagle nest in front of him. He said he stayed in one quadrant of the nest - didn't move around - but what he saw was not anticipated.
Obviously, eagles bring food to the nest, especially when young are present. Although not part of Steve's mission, he made informal observations on the remains of the food items the birds brought to the nest.
Although fish remains are expected, the only one Steve found was a flathead catfish skull, from a fish estimated at two feet in length. No mammal remains were obvious, but there was an earlier report at this nest that an eagle brought a cat (with a collar) to the nest, and a biologist later found cat mandibles on the ground below.
Steve's surprise was the number of turtle remains, and the species that he found. There were only three species that he could see: razor-backed musk turtles (Sternotherus carinatus), small false (Mississippi) map turtles (Graptemys pseudogeographica), and softshell turtles (Apalone sp., the largest having an 8 inch long carapace). He saw hundreds of turtle pieces-parts, with their abundance in the order they are listed above. The surprise was that there were no remains of the most common turtles in Kincaid Lake - red-eared sliders (Trachemys scripta elegans) and river cooters (Pseudemys concinna).
He did find a number of turtle shells on the ground that had fallen from the nest.
Larry Allain, a USGS biologist at the National Wetland Research Center in Lafayette, climbed to a nest in a cypress tree near the Davis Pond Diversion Structure in St. Charles Parish a few years ago (see http://pubs.usgs.gov/ds/605/downloads/DS605.pdf). He saw a couple of pieces of unidentified turtle remains, but the birds on his nest were feeding mostly on fish and nutria.
Larry thinks there may be significant learned behavior involved, and it is not odd that nests, especially in different regions, may have different indications of feeding choices.
Regarding the species found in the Kincaid nest, there may be a simple reason for the dominance of the three species found. The softshell turtles are obvious - the eagle's talons can easily pierce the soft tissue covering the skeleton. The two species with hard shells share features that might make them more easily grabbed than other species on the surface of the water. The razor-backed musk turtle is basically triangular in cross-section due to the sharp keel down the back and the angled lateral margins around the edges of the carapace. The map turtle has a series of enlarged knobs down the center of the carapace, creating the same effect. These features make it very easy for eagle talons to gain purchase on an otherwise hard, somewhat slick shell by wrapping around two of the three angled regions.
Steve thinks razor-backed musk turtles may be chosen due to having easy access to their flesh since they have a relatively small plastron.
Red-eared sliders and river cooters are mostly rounded, and it would be harder (not impossible) for the bird to grab them and hold on.
Jeff Boundy, herpetologist with the Louisiana Department of Wildlife & Fisheries, says that razor-backed musk turtles feed at certain, predictable times of the day in shallow, often clear, water. When doing so they are oblivious to what is going on around them and may make easy targets for eagles that recognize them and their habits as a food source.
They, and map turtles, often sun on logs above the water, and this may make them both easy targets. Softshell turtles do the same, but also often rest in shallow water and are probably vulnerable to eagle predation.
Of course, maybe the latter are not as tasty as the former!
Steve also saw a single western ratsnake (Pantherophis obsoletus) egg in the nest. It appeared viable, and had no fungal growth. Ratsnakes are characteristically active in crowns of trees, and it is not surprising that one living in and around an eagle nest might lay its eggs there. I would think, however, that a baby ratsnake, at about 7 inches in length, would be at a disadvantage sharing the platform with keen -eyed, voracious baby eagles.
Now we naturalists have a new ecosystem to monitor - bald eagle nests.
Postscript: Both Shively and Allain have sworn off climbing eagle nests. It's not the height, they say, it is the shortest part of the journey - from trunk around the edge of the nest to the upper surface. I, for one, will never know the sensation they experienced!
Knowing their curiosity, I won't be surprised to hear of their next arboreal adventure.
Turtle shells found around the base of the pine tree. Photo by Steve Shively.
|Photo taken on the nest. Note the turtle shells across the bottom of the photo, and the white ratsnake egg about in the center. Photo by Steve Shively.|
|The eagle nest, with turtle shells visible around the margins of the seating platform. Note the nest's proximity to Kincaid Lake. Photo by Steve Shively.||A healthy appearing egg of a western ratsnake (Pantherophis obsoletus), in the bald eagle nest next to the shell of a razor-backed musk turtle. Photo by Steve Shively.|
|Loblolly pine tree near Kincaid Lake, Rapides Parish, Louisiana. The huge bald eagle nest is visible top left, and the adult eagle is perched just to the nest's left. Photo by Steve Shively.|