Geophagy, the ingestion of small round stones (gastroliths, or rangle as they are known colloquially by falconers), has been documented for at least 260 species of birds representing 23% of 231 avian families (Downs et. al. 2019). But geophagy has only been observed in the wild for 12 raptor species (Downs et. al. 2019). I here provide the first report of this behavior by a wild Merlin (Falco columbarius).
The function of gastroliths in wild raptors is unknown; Downs et al. (2019) suggested that they may aid in digestion or provide missing minerals. Fox (1976, 1995) observed that raptors consume rangle separately from food and speculated that the stones helped to loosen mucus and clear the koilin lining of the gizzard via casting, rather than aiding in digestion. Grit regurgitated by Peregrine Falcons (F. peregrinus) was covered in mucus (Albuquerque...