What Are The Monsters in Bird Box?
Bird Box's ending doesn't offer any clear answers as to where the entities came from, whether they're supernatural or alien in nature, or even what exactly people see when they look at them. However, by the end of the movie we do have a fairly clear idea of how they operate, and it's possible that the charcoal drawings (pictured above) that Gary lays out on the coffee table before he goes on his killing spree show what he sees when he looks at the entities.
What we do know about the monsters is that they're invisible when no one is looking at them (or at least, that's how the Bier chooses to portray them), but they are detectable by the way they affect gravity - arriving in gusts of wind that blow leaves around, and even causing leaves to lift off the ground and hover in the air. When they get close to Girl, her hair can be seen lifting from her head. The monsters don't appear to be strong enough to physically harm people or even break through doors and windows, which is why they rely on some people becoming their disciples and trying to force others into looking at them. It's implied that these disciples were people who were already mentally ill, and are therefore affected differently by seeing the entities.
As for what most people see when they look at the entities... well, it's never fully spelled out, but it seems to always be something that makes the person either terribly afraid, terribly sad, or both. Lydia (Rebecca Pidgeon), for example, calls out to her mother before climbing into a burning car. Whatever it is that they see, it's clear that the sight alone isn't enough to drive people to suicide. Looking at these creatures also causes physical damage - like the warping of people's irises - so it's safe to assume that they automatically trigger some kind of suicidal impulse inside the brain. This impulse cannot be ignored, but it can be briefly resisted with great effort, which is what allows Olympia to hand over her baby before killing herself, and allows Tom to take out the marauder before turning the gun on himself.
The characters themselves float some theories as to what the monsters are. Douglas thinks it's some kind of biological warfare, while Charlie (Lil Rel Howery) thinks that humanity has been judged and found wanting, and points to his research on the topic of demons and the apocalypse. He cites Zorastrian myths about Aka Manah (a seductive demonic entity), the Christian demon Surgat ("who opens all locks"), the Chinese fox spirits known as the huli jing, and the mischievous Celtic spirits called púca. Charlie posits that these are all different names for the same thing: a demonic entity that preys on people.
Based on the fact that the entities seem to have supernatural knowledge of people's weaknesses (they know to call out to Malorie in her sister's voice, and in Tom's), and Gary's drawings of them, Charlie's theory that the monsters are demonic in nature seems pretty sound. This is also reinforced by the fact that birds are able to sense their presence, since in many mythologies, cultures and religions birds are associated with psychopomps - spirits that guide people from the land of the living to the land of the dead.