The NavMesh is just for ground navigation, it is not used for flight. The setting your looking for that should work for your situation is in the Bird Parent blueprint. open the “BP_NPC_Bird_Base” in the ParentClassBlueprints folder. Look for the variable “StoreTargetsInThisRange” and lower that number way down (its in Unreal Units). Save and compile.
When the Level Begins the Bird checks its environment and only stores targets that are in this range of their starting location. Just be careful setting it too low. keep in mind that there needs to be at least one target in this range, if not you will get “Found None” errors because the Bird has no targets stored. I’d say start by setting it to 30,000 and start from there. So when the Bird starts the level they will look out 30,000 Unreal units for Targets to store. If there are other targets further away they will be ignored.
So this number is useful for limiting the range of the Birds. This number (Distance) can be changed in the Parent class to affect all Birds, or you can set it on a per Bird basis as well. Also you probably want more than one SkyTarget for each set of Birds. They look a little unnatural always circling one target.
I hope that is helpful, let me know if you have any issues.