Taphonomy of arboreal nesting in great blue herons
McGrath, Ashley Cathryn
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This research supports the growing number of modern bird taphonomic studies and furthers our understanding of the taphonomy of birds and non-avian dinosaurs. The purpose was to broaden modern taphonomic models available for the interpretation of fossil localities of both dinosaurs and birds through examination of the taphonomy of great blue heron (Ardea herodias) nesting materials found below their arboreal nests. Investigations took place during the 2012 and 2013 breeding season where heron generated materials were described within the colony and compared to Controls outside of the colony. The Cascade Heron Colony located along the Missouri River in central Montana produced heron eggshells, heron nests, heron prey, and heron bones. The heron diet greatly consisted of fish, small mammals, and crayfish. Of the 1,788 elements discovered, great blue herons made up 52% (N=934), of which at least 83% (N=776) were considered juvenile. Broken, disarticulated juvenile heron bones dominated the heron skeletal assemblage. The heron bone assemblage exhibited a strong and statistically significant bias for hindlimb elements (74.09%) in comparison to wing elements (25.91%). Of the bone assemblage, nearly 40% (N=702) of the materials were found in the subsurface survey. I compared the surface sample versus subsurface sample and found herons largely dominated both samples, but more mammal remains and fish remains made up the taxon-specific subsurface sample. All data were compiled for taphonomic interpretation, reporting and testing for significance. Using a 2x2 Fisher's exact test significant difference between the abundance of materials between surface and subsurface was found. Using a X superscript 2 for independence there was difference between plot types (sparsely vegetated, densely vegetated and controls). In the heron eggshell assemblage, ample eggshell was found (N= 947), but little was buried in the subsurface (N=31). Eggshells were described as small eggshell fragments (>1cm but <1/4 of an egg) and large eggshells (>1/4 of an egg). For small and large eggshells, a X superscript 2 test was used to compare against the null 1:1 'concave up'(CU): concave down'(CD) (CU: CD), and further against the alternative hypotheses for hatched (60:40) and predated eggs (70:30) (Hayward et al, 2011). Outside of plots the large eggshells were found in CU (77:23) positions and small eggshell fragments were found in CD (30:70) orientations. Inside plots small eggshells were found in CU (58:42) orientations. Eggshell orientations significantly differed from the null prediction and for large hatched eggshells (N=106) favored CU at 77.40%. Tree-based nesting relative to ground nesting localities had similar nesting material inputs as studied in ground-nesting birds (Hypothesis 1). Controls outside the colony provided the materials described her were generated by heron nesting activities. Hatched large eggshells preferred CU orientations above the hatched 60:40 expected ratio (Hypothesis 2). This work showed burial features of heron materials through the subsurface survey (Hypothesis 4). Tree-based nesting relative to ground nesting showed herons had a higher representations of leg elements than forelimb elements (Hypothesis 5), and the findings from this study showed that vegetation and pH did not influence the density of biological materials below heron nests (Hypothesis 6). Future work into tree-based nesting should examine fallen nest orientation trends and quantify the spatial patterns of juvenile carcasses in proximity to tree bases, tree-based nesting (Hypothesis 3). This works supports great blue herons as plausible models for past-life nesting locale reconstructions.