In falconry, rangle is a term used for small stones which are fed to hawks to aid in digestion. These stones, which are generally slightly larger than peas, are used less often now than they were historically. ...
• (v. i.) To range about in an irregular manner.
Small stones fed to the bird of prey to assist the crop
Ran'gle (răn'g'l) intransitive verb To range about in an irregular manner. [ Obsolete or Prov. Eng.] Halliwell. 
Small, smooth stones ingested by a bird to help clean out her crop. Rangle both refers to the stones used and the action of feeding the stones.
Small stones eaten by birds to aid digestion