In Bird Box, as the main characters on the book have to cover their eyes the entire time before even going outside due to the creatures that linger outside. The characters can't explain or even know why these creatures mess with their minds and make them go mad. I believe it is something beyond scientific understanding since the characters believe these creatures take over a human body and drives that host to its death. The characters assumed that these creatures existed since people were sent to their death bed as soon as they looked at a person who went mad. Even though the people in book didn't really have evidence of a creature existing, but they knew they would die if they were to open their eyes upon a person who'd gone mad because the news had informed people around the world before everything went to a disaster. You really can't explain why and how these creatures appeared around the world but they just did. Towards the ending of the book, Malorie ends up going to a sanctuary where she discovers most of the people are blind which didn't affect them since they couldn't see.
Towards the middle/end of the book we know Gary could be around the creatures and it didn't affect or harm him in any way. In the book it is known that people kill someone or themselves if they stare at someone who has gone mad, but the real question is why didn't Gary go insane when he saw a creature? Gary's brother believed that the creatures couldn't harm them and that it was all in the mind. Gary carried his brother's notebook as he told his story to the other characters in the house when he first arrived but he did not tell them he had the notebook with him. Malorie suspected Gary most of the time and finally one night she stole his suitcase and found the notebook and believed that Frank was just a made up story. When she read the notebook she knew he was a mad man and needed Gary to go out of the house. Gary believed he shouldn't be the only one who can be around the creatures so he pretty much killed everyone in the house thanks to Don who let the creatures into the house. Gary obviously left the house afterwords knowing his "experiment" didn't work.
The characters believing that creatures are real is supernatural since it's hard to understand why these creatures have destroyed the human population. Why do humans die just by eye contact if they see a creature? Mainly what is the cause for these creatures against humans. Animals are also affected by the creatures as Malorie rows down the river with boy and girl, they hear birds tweeting and all of a sudden, dozens of dead birds start falling into the river and hitting them everywhere. Tom was known to have brought two huskies home to see if they could help alarm and guide the housemates as they went outside with blindfolds for supplies. One huskie died as Don opened up the windows and doors to let the creatures in and the other huskie died as it heard a creature and tried to follow it leading to seeing the creature at the bar while Malorie looked for mics, amplifiers, and speakers for the house. These items were used to help Malorie hear anything outside the house for the safety of the babies and herself. As the babies grew up, she trained them to not open their eyes and to listen carefully to any sound.
Gary believed being around the creatures was psychological since he wasn't affected by the creatures. Gary also believed the creatures weren't there to harm them and that people only went insane because they were weak minded and easily fragile. As a result the housemates were killed because Gary believed and thought it was "all in the mind." Just by seeing the creatures you would die by getting killed by someone or you would kill yourself by what the people would hear from the news because they didn't want to see a creature or else they would die, so many hid in their homes or went to other houses. Gary might not be the only person to be able to be around the creatures and he might not be a normal person like the people that do get affected by the creatures. I think Gary was lonely at some points living by himself outside surrounded by the creatures and wanted some company. He was a bit delusional to start off with and then when Malorie was giving birth he went insane and told her that he has been around the creatures and that he was always watching them from the outside.
Dead Bird as an example of the dead birds falling from the trees while Malorie is rowing in the river.