Geophagy in New Guinea birds

Geophagy, the ingestion of soil, has been reported for birds and other animals from many regions, but its functions remain debated. We report geophagy for the first time for New Guinea birds: four or five parrot species, two or three pigeon species, one hornbill species, one crow species and possibly one cassowary species. These species ate soil at an area of bare ground created by a landslide on a steep mountain slope. Parrots visited the site in the early morning, pigeons in the late afternoon, flying in from considerable distances. All 11 of these species are frugivores. In the: vicinity, we-recorded 133 other bird species (including 37 other frugivore species) that were not observed to visit the site. Within each taxonomic group of frugivores, those practising geophagy tended to be the largest species of their guild. To test hypotheses regarding the functions of geophagy, we carried out chemical and physical analyses of soil samples from the site. The ingested soil was much too fine-grained to be useful as grit; it contained only modest levels of all 14 minerals analysed; it lacked buffering capacity; and there was no evidence that it protected against diarrhoea. Instead, the soil's high measured cation-exchange capacity, high content of cation-binding minerals and binding of large quantities of tannic acid and quinine suggest a different hypothesis: that geophagy in this case served to bind poisonous and/or bitter-tasting secondary compounds in ingested fruits and seeds. Geophagy thus represents one weapon in the escalating biological warfare between plants and animal consumers an evolutionary arms race at which parrots excel. We discuss five unsolved problems posed by geophagy.