鸠 is a bound form (b.f.) — it cannot stand alone as an independent word. You'll mainly see it in compound words and idioms. The most famous one is 鸠占鹊巢 (jiū zhàn què cháo, 'a turtledove occupies a magpie's nest'), which means usurping someone else's place or property. It also appears in bird names like 斑鸠 (bānjiū, 'spotted dove') and 鸠鸽科 (jiū gē kē, 'Columbidae, the dove/pigeon family').