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

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.滨鸟（Shorebirds）可形成一系列与其取食行为相关的生物沉积构造，其中最为常见的是探食行为所形成的、于沉积基质（sedimentary substrate）表面产出的形态简单的印痕或凹陷。本文记述了一种由取食鸥（鸥属未定种，Larus sp.）形成的异常生物变形构造：造迹生物（tracemaker）通过原地往复移动足部揉动沉积物。该过程可使沉积物液化，从而更易于捕捉无脊椎猎物——其中最常见的类群为紫石房蛤（Nuttallia obscurata）——这类猎物会被搬运至液化沉积物的表层。当沉积物发生液化时，鸟类会缓慢向后退移，这一行为被解读为在富含猎物的沉积物基质中持续开展取食活动。最终形成的构造由一系列嵌套式凹凸沉积丘与一处末端碗状印痕构成。本文的研究目的在于记录该造迹-造迹生物关联（trace–tracemaker association），并为同类构造的对比分析提供参照。尤为关键的是，该构造与鱼迹（Piscichnus）存在相似性：鱼迹代表鱼类通过水力喷射型牧食迹（fodichnia）或筑巢行为相关居住迹（calichnia）所形成的鳐鱼取食痕迹。然而，鱼迹并不具备造迹生物为系统性觅食而后退所形成的嵌套沉积丘。