FUGLAþúFUR: AVIAN INFLUENCED SLOPE STABILITY

Bird-tussocks (Icelandic: Fuglaþúfur) are locally well-known, but poorly documented, features of the Icelandic landscape. Comprised of mounds or pillars of turf ~0.5 m in diameter and up to 1 m high, they typically occur at the very highest points of local relief. Their formation is the result of focused fertilization of turf grass by the excrement of birds roosting on these local high points. It is likely the birds select these high points for their visibility, a mechanism driven in part by the positive feedback of bird-tussock growth. Bird-tussocks are preferentially located in areas with hummocky local relief on the order of 1-10 m, spaced 10-100 m apart. Bird-tussock development does not appear to be species-dependent; numerous shorebird species are observed utilizing and fertilizing the tussocks.

Hummocky glacial moraines and landslide deposits, composed of unconsolidated sediment, typically display convex profiles produced by slope-dependent sediment transport processes. However, where capped by bird-tussocks, concave profiles characterize upper hummock slopes, with the apex characterized by a bird-tussock standing in prominent relief to the lower, convex slopes. The contrast in slope profiles indicates bird-tussocks impart a significant influence on slope morphology, namely the well-developed turf serves as an erosion resistant point at the hummock apex. This relationship is further evidenced by the presence of unconsolidated sediment internal to bird-tussocks in significant relief to the immediate surrounding slope.

Bird-tussocks and their influence on slope morphology develop rapidly; convex upper slopes capped by bird-tussocks are a prominent feature of Little Ice Age (LIA) maximum moraines deglaciated within the last century. The longevity and maximum size of bird-tussock development is uncertain, as bird-tussocks on LIA moraines do not appear outwardly different in size or morphology from those developed on early Holocene moraines.