Transport and Breakage of Intertidal Bivalves by Shorebird Feeding Activity: Craig Bay, Vancouver Island, British Columbia

Broken shell debris in grain-supported and matrix-supported conglomerates are routinely interpreted as wave or current mediated hydromechanical accumulations. Fragmentary bivalve shell material is common in low-energy coastal bays along the eastern shore of Vancouver Island, Canada. At Craig Bay, a high proportion of this shell material occurs on or adjacent to gravel-dominated portions of the intertidal flats and is related to predation by resident and migratory shorebirds. These accumulations are abundant and of moderate taxonomic diversity

Two taxa, Western Gulls (Larus occidentalis) and Northwestern Crows (Corvus caurinus) were observed utilizing shell-dropping on hard substrata to obtain food. Western Gulls were observed to pick up shallow-burrowing, mid-sized bivalves in their beaks from various intertidal substrates and drop them on exposed intertidal gravel bars. Northwestern Crows were observed to obtain bivalves from the upper intertidal and drop them on hard substrates (logs, gravel, cement) above the high tide line.

Avian predation activity by both bird taxa resulted in cross-facies transport of bioclastic material. As well as shell breakage of resident infaunal bivalves on intertidal gravel flats, predation activity by these birds resulted in significant cross-facies shell transport. This included transport of bivalves from sand-dominated intertidal flats to those characterized by gravel or cobble substrata by Western Gulls as well as transport to supratidal and intervals well above the maximum high tide line by Northwestern Crows.

Shorebird-generated intertidal shell accumulations have significant implications in the analysis of ancient shell accumulations, particularly those in late Mesozoic and Cenozoic to recent successions.