Archaic expression for chicken. The exact historical usage is uncertain; it may appear in old texts or poetry. Not used in modern Japanese.
The expression 関路の鳥 can sometimes be found in old documents.
The derivation is uncertain. The kanji 関 (barrier) and 路 (road) combined with 鳥 (bird) may refer to a bird associated with a checkpoint or route, but the exact origin is unclear.