In the Jason Statham film Hummingbird (AKA 'Redemption' & 'Crazy Joe'), who or what was 'Hummingbird'? I guessed it was his operative name in the special forces, but could not confirm that from two viewings.
The hummingbird, literally speaking, was the call-sign of the drone in Afghanistan that was witness to Joe's actions. The imaginary hummingbirds are a motif that represent Joe's PTSD and his guilt, the proverbial albatross around his neck.
Joe believes that he is lethal when he is sober and becomes dangerous to those around him. The summer he spends sober is no different. During this period, Sister Cristina is his hummingbird, bearing witness to his actions.
We see him reverting back to being a harmless homeless man in the end. But we are also told that it won't last as he is again shown to be under surveillance by London's network of CCTV cameras.
(There is a sub-plot with Cristina's own story which also ties into the whole witness/guilt theme.)