Sedimentological and ecological significance of a biodeformational structure associated with an unusual feeding behavior in gulls (Larus sp.)

Abstract

Shorebirds produce a range of biogenic sedimentary structures related to their feeding behaviors, the most common of which are probing behaviors that result in morphologically simple impressions or indentations on the surface of a sedimentary substrate. Here we describe an unusual biodeformational structure made by a feeding gull (Larus sp.) produced as the tracemaker kneaded the sediment by moving its feet back and forth in place. This process liquified the sediment making it easier to catch invertebrate prey — the most common of which is the varnish clam (Nuttallia obscurata) — that are advected to the top of the liquified sediment. The bird retreated slowly backward as the sediment was liquified, an effort that is interpreted to represent continued and ongoing feeding in the sediment of prey-rich substrates. The resulting structure comprises a series of nested, concavo-convex sediment mounds and a terminal bowl-shaped impression. The aim of this paper is to document the trace–tracemaker association and provide a means of comparison with similar structures. Most notably, the structure bears similarities to Piscichnus, which represents ray feeding traces produced by hydraulic jetting (fodichnia) or nesting behaviors (calichnia) in fish. Piscichnus, however, lacks the nested mounds that result from the tracemaker systematically moving backward as it searches for food.