Geophagy, the ingestion of earth varying in particle size from stones to soils rich in clay, is a relatively widespread behaviour across avian taxa. We reviewed its occurrence in birds and its hypothesised adaptive functions in birds. Of the ∼30 avian orders, 23 exhibit geophagy. However, it has only been documented in ∼260 species (∼2% of birds) and therefor is relatively uncommon. Ingestion of stones and other large particles (grit) is recorded in 54 extant families across the avian phylogeny and appears to be ancient in birds and has evolved several times. Clay ingestion is recorded in 14 phylogenetically scattered families and might have evolved repeatedly. Furthermore, at least nine families exhibit both clay and grit ingestion. Six hypothesised functions of avian geophagy involve digestion and nutrition. Ingested grit might provide (1) essential minerals, particularly sodium or calcium, but it appears to primarily (2) improve mechanical digestion of food in the gizzard, where ingested stones and sand particles are known as gastroliths. Consistent with this gastrolith hypothesis, ∼86% of species ingesting grit consume “hard” food: seeds, leaves, insects or other animals. In contrast, ingested clay appears to be for sodium or other nutrient intake and/or protecting birds from ingested plant secondary compounds, by (3) protecting the digestive tract and (4) adsorbing these compounds. Consistent with this, ∼88% of species ingesting clay eat fruit that is often rich in secondary compounds and low in sodium, and clay ingestion is associated with frugivory in a representative sample of genera of which many have been shown to source sodium. The other two hypotheses are buffering gastric pH (5) and acquiring antidiarrheal agents (6), but no documentation of this in birds was found. We suggest additional tests of these hypotheses and additional investigation of these proposed benefits of geophagy in birds.