Warning: Major spoilers for Bird Box Barcelona below!Bird Box's monsters are offscreen for the entire movie because looking at them induces death.

Bird Box received mixed reviews overall, but the restraint the filmmakers showed in keeping the creatures entirely offscreen was the best possible choice. Bird Box Barcelona adopts the same tactic, as the monsters are never seen full-on. That said, one major character has seen them and believes the beings to be angels. Bird Box Barcelona also puts forth a fascinating "observer's effect" theory to explain how they function - and why it might be impossible to ever depict them onscreen.

Related: Bird Box Barcelona Review: A Compelling, Thrilling, Frightening Horror Spinoff

Bird Box Barcelona's "Observers Effect" Creature Theory Explained

Close up of an eye in Bird Box Barcelona

cast Diego Calva as Octavio, a former deliveryman who also has a degree in physics. Around the midway point of the spinoff, the survivors take shelter in an abandoned building after losing another character to the monsters. This is when Octavio puts forth his theory that the creatures might be "quantum beings," and that they have no true shape of their own. He explains that in quantum mechanics, particles are in a state of "indefination," with endless possibilities existing at once until they are observed.

By the act of being seen, those possibilities all collapse down to one to take a definite shape. This is called "observers effect," and Octavio believes that if his notion is accurate, Bird Box Barcelona's monsters have no permanent shape of their own. They are formless entities who by the act of being seen, then take on a shape tailored to the person looking at them. While Bird Box Barcelona doesn't show what any of these beings look like, it does reveal what one major character sees them as.

What Form The Creatures Take In Bird Box Barcelona

Mario Casas as Sebastian in Bird Box: Barcelona

Bird Box Barcelona's "hero" Sebastián (Mario Casas) lost his young daughter Anna (Alejandra Howard) to the creatures, and after he witnesses one himself, he comes to see them as angels. To be more specific, he sees them as six-winged seraph angels, just like the one found on Anna's necklace. Bird Box Barcelona makes it clear that Sebastián's religion is very important to him, including the fact he and his wife sent Anna to a catholic school. Going back to the notion that monsters are quantum beings, that means they morph into a form based on the people observing them.

Sebastián sees his first creature in Bird Box Barcelona shortly after Anna's death, so it makes sense it would take on the appearance of something deeply personal to him and his daughter. He later encounters another "seer" - as the army dubs those who've seen the creatures - who claims the beings resemble aliens that then beam people up to their mothership. Sebastián himself sees the souls of people leaving their bodies after they've been "saved" by the monsters. Bird Box Barcelona's observers effect argument opens up a wide range of possibilities about what people see them as; it could be angels, demons, loved ones and so on.

Is Bird Box Barcelona Quantum Theory Correct?

Claire holding onto Sofia while they are both blindfolded with fire behind them in Bird Box Barcelona

Octavio puts forth a compelling argument for his observer's effect theory, and applying this to both the original film and Bird Box Barcelona, it makes a certain kind of sense. That said, it is just a best guess from a physics student, and the movie wisely never weds itself to the idea. The next Bird Box movie could easily ignore this premise entirely and conceive a different explanation for the creatures and where they came from.

In fact, Bird Box Barcelona's ending suggests a future outing might take that very route. The final scene reveals that the army has captured one of these entites, and are running experiments to see if they can develop a vaccine against them. This is achieved in part by capturing so-called "seers" and trying to develop a compound from their blood, which is genetically altered by the effects of having looked upon these mystery beings.

This could be a game-changer for Bird Box 3, though in explaining them too much, this approach runs the risk of taking away what makes the franchise's monsters so frightening. Bird Box Barcelona's observer's effect theory is about as close as the series should go in attempting to explain what they are and how they function. It also means the films will never have to worry about unveiling what they look like one day.