Pigeon is a 64-bit chess engine, written by Stuart Riffle. It runs on Windows and Linux, and will plug into any UCI-compatible chess GUI. Here is Pigeon playing as a bot on lichess:


The latest version is rated about 2000 against humans online at ICC and FICS, which is pretty good. For a bird. Playing against other chess engines, it has a CCRL 40/4 rating of about 1750, which is... somewhat less good. But Pigeon always tries very hard.

  Download Pigeon 

Full C++ source code is available on GitHub, under the MIT license.