COMPARATIVE ANALYSIS OF AVIAN PERCUSSION FEEDING TRACES BELICHNUS ON NORTHEAST COAST BIVALVES

Shorebirds produce a variety of feeding traces on mollusk shells, ranging from broken valve exteriors to openings of varying shape, which were recently ascribed to ichnogenus Belichnus. In this study, the morphometry of predation traces was characterized using 37 clam valves from a tide-dominated embayment in Massachusetts (Spisula solidissima and Mya arenaria) and wave-dominated open ocean beaches of New Jersey (S. solidissima). With their smaller size (<5 mm) and smooth circular outline, gastropod drillholes (Oichnus isp.) were easily distinguished from irregular (elongated, oval, rectangular) openings on valves ranging from 5 to >20 cm in length. The latter, interpreted as Belichnus traces, had a maximum diameter of 3 to >20 mm, and nearly all exhibited a high interior/exterior diameter ratio (D_I/E; up to 30x) due to the peeling of inner shell layers by the percussive impact from the outside. In contrast, Oichnus D_I/E was 1 or slightly lower (e.g., beveled or countersunk). A greater number of shells with nearly rectangular openings contained radiating cracking, compared to those with gastropod traces. In addition, boreholes were proximal to the umbo, whereas avian traces were distributed more randomly. Several surf clam shells contained more than one Belichnus (highlighting the current problem with ichnospecies designation) and there is a slight correlation between valve thickness (some exceeding 3 mm) and trace diameter. Further research will include field observations and experiments to distinguish between traces made by beak strikes from those resulting from the impact with other shells or gravel (in wave-dominated settings), bird-induced ballistic impact on hard substrates, as well as from characteristic fodinichnia produced by vertebrate (raccoons, foxes, fish) and invertebrate (crabs, nearshore shrimp) predators. Ultimately, this research will aid in establishing diagnostic criteria for identifying unconventional predation traces in fossil mollusks.