Comment by nyanpasu64

I've read that birds used to have penises (and paleognaths and waterfowl still have them today), but the Galliformes (including chickens) developed atrophied penises which are difficult to see (hence chick sexing and feeding males to the meat grinder), and the Neoaves lost them entirely. Though https://prumlab.yale.edu/sites/default/files/brennan_etal_20... suggests the exact nature is more variable between birds in lineages that "have" them