I would like to clarify that my english isn't perfect, so i had to seek help to translate some words.
—This post is directly related to sirens originally being bird-women. I directly associated sirens with owls due to the poor eyesight and lack of information at that time. I did researchs and linked the features of owls, with their centrally positioned eyes and easily recognizable features with human faces, with low vision due to the darkness, since they are nocturnal animals. Sailors who saw owls in the dark from afar, with their limited vision, and only heard the animal's call once, due to irrational fear, might associate it with a female voice and with fear that would add more to the brain, spreading rumors of a "night calls from a woman" and eyes in the night staring directly at them would help the thought that there was something hiding. Apart my theory, it also includes the gorgons, that originally are related to Medusa and her sisters, and whose appearance is reminiscent of reptiles and snakes. (Also considering the fact that snakes have facial features that can be associated with something humanoid, since they have mostly a flat face ) since at that time bodies would be seen drowned in rivers and were frequently seen in a state of decomposition, sometimes they had greenish skin and uncomfortable expressions, also floating due to gases trapped in the body and hair moving with the flow of water. Due to mental confusion and the shock of seeing a dead body in the water resembling sea serpents, since they had similar characteristics there would be rumors spreading rapidly. What adds to the theory is the story that would be the origin of Medusa, which follows the idea of death by drowning and becoming a creature that is half snake, half woman (often depicted with greenish skin). These two theories are related 'cause they may have merged due to a change with the translation at a point in history, that would make the mermaids we have today; fish-womans, who sings in the dark ocean to deceive man. It was a theory i had linking human facial structures with animals and how irrational fear can cause a misunderstanding, not saying the existence of these urban legends are false but adding a rational thought. If I'm wrong about something, I'd like to learn more.
( i installed reddit just to share this theory. lol)