Symbolism, Imagery, Allegory
Those fear landscapes? They're straight up symbol factories, like dream journals and cheesy poetry. Everything means something else.
But not all of these fears are symbolic in the way we usually think about symbols. When Four shows that his fear landscape includes his father, we don't say "what could his father be a symbol of?" No, in that case, we know that Four's fear landscape is pretty straightforward: his dad is there because Four is still afraid of his dad's abuse; Four is afraid of claustrophobic spaces because he used to be put in a closet as punishment. Those are some pretty literal interpretations.
Paging Doctor Freud
By contrast, Tris's fears are a little less literal and a little more symbolic. She's afraid of being attacked by birds in a field, but she doesn't have a bird-phobia (unless she's seen Hitchcock's The Birds one too many times, like Shmoop). As Tris notes when she's going through her fear landscape, the bird fear "isn't about the birds. It's about control" (30.3); the drowning in the box fear "is not about the water; it is about my inability to escape the tank" (30.9); etc. For each of her fears, Tris gives us the breakdown of what each symbolizes. This is probably the most intense therapy session Tris will ever go through. And like a good therapy session, we learn more about who Tris is by seeing what her fears are and what symbols they come in.
For example, there are lots of ways that a lack of control could appear to Tris, but it appears to her as a dangerous flock of crows. Why crows? This is just a theory, but it might have to do with the fact that Tris symbolizes her family as birds (ravens) when she gets her first tattoo. And maybe the fact that it's a flock of crows (instead of just one big crow) has to do with her feelings about crowds, which she doesn't seem to love: the Abnegation all blend together, but crowds of Dauntless aren't always a great time, unless they're working together as a community, like when she goes zip lining.