Introduction

Birds are relatively lightweight animals, and many species spend much of their life cycles in the air, in aboveground nests, and/or in the water. Nevertheless, distinct geomorphic effects can be attributed to some birds. Some are relatively minor geomorphic curiosities, whereas others induce widespread alteration of the landscape in which the birds live. This chapter categorizes the geomorphic effects of birds and examines some specific cases by which these effects are produced.

The geomorphic effects of birds include internal clast transport as gastroliths; geophagy; external clast transport for use as tools (“bioports,” sensu Johnson 1993); clast and mud transport for use in nests; mound building and surface scraping; vegetative removal; and burrowing and nest–cavity excavation. Each of these processes is examined below.

Internal clast transport as gastroliths

It is well known that birds ingest sediment to act as grinding tools in the gizzard, in a fashion similar to that employed by reptiles (see Chapter 3). An almost complete void of quantitative data exists, however, as to the amount of erosion and transport involved in avian lithophagy. Milton et al. (1994) reported on the stone contents of ten ostrich gizzards taken in southern Africa. Stone mass in the adults averaged 0.646 ± 0.266 kg, and in subadults averaged 0.444 ± 0.266 kg. Johnson (1993, p. 74) provided data on the number, size, and weight of stones from five ostrich gizzards.