Modern ecological studies suggest that avian predation has a significant impact on intertidal mollusc-dominated communities and can potentially bias the fossil record by altering the composition and probability of preservation of shelly fauna in coastal habitats. To assess if bird predation can be detected indirectly using surficial shell assemblages, fourteen samples from an area known for bird predation were compared with an expected model derived from observational studies on bird feeding behavior in the literature. The model predicts: (1) minimal bioerosion and physical decay of mollusc shells; (2) high degree of shell fragmentation; (3) ubiquity of distinct fracture patterns; and (4) shell assemblages dominated by a few species. This model was evaluated using 3 distinct habitats of Argyle Bay (San Juan Island, WA); lagoon, bay, and gravel bar. On the gravel bar, where avian delivery of prey items is observed to be high shell assemblages displayed high fragmentation (>75% of valve missing), dominance of pristine shells, frequent presence (68%) of specimens still retaining ligaments, dominance (72%) of valve fractures that cut straight across growth lines, and nearly monospecific composition. In contrast, shells from the bay and lagoon, where avian export of prey items would be high but processing negligible, have a low proportion (30%) of highly fragmented shells, a dull to chalky appearance, lower frequency (44%) of ligament–preserving specimens, and much more variable fracture patterns. Fidelity analysis, a comparison of species diversity and abundance between the sympatric life and death assemblages, indicates low death fidelity and in all three habitats death and life assemblages differ significantly from one another. However, the gravel bar death assemblage is more similar in taxonomic composition to the life assemblages than to the other two death assemblages, suggesting that the gravel bar approximates the present day composition of the local mollusc fauna ecosystem more closely than either the bay or lagoonal death assemblages. The bar deposit, with a combination of highly fragmented but pristine shells, dominant fracture patterns, and monospecific composition, suggests that supratidal deposits resulting from bird predation can be identified using indirect methods based on damage patterns in shell assemblages. These results offer a promise for applying unique taphonomic and taxonomic signatures of shell accumulations generated by molluscivorous birds to study avian predation from historical, ecological, and paleoecological perspectives.
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Vol. 26 • No. 1