Bird Box

The nature of the creatures that cause the Problem is a central, and unresolved, mystery in the text. The only reliable information about the creatures includes whatever can be inferred from their direct effects upon people. The rest involves speculation or, in Gary’s case, unreliable sources. Direct, verifiable encounters with the creatures are rare, though it is implied that Malorie has at least two such encounters. Size, shape, appearance, intent, and everything else about them remains unknown, as Malorie realizes while wondering what word best describes them.

At one point, Don even favors a theory that there are no living creatures, just some kind of ray or other scientific phenomena. Tom theorizes that they are beyond human comprehension, like infinity, and Malorie latches onto that theory, thinking of it often. Thematically, the creatures represent the unknown and the unknowable things that lie beyond human understanding, and they therefore tie into each of the main themes, since they pose a threat to civilization, causing difficulties for parents like Malorie, while forcing her to make difficult, risk-laden choices.