i'm not sure how your birds gonna be flying, but you might aswell figure everything out.
some explanation!
dont bother about each word since you're not a programmer!
void Start - does shit at the start
void Update - does shit every single frame
center = new Vector3(0;0;0) - point out the center of the map. you might aswell change "Vector3 center" to "public Vector3 center", delete the void start() with everything inside it, and put in coords manually in the editor
rotatearound()

oh, we need math for that
1. we get the axis point, which we gonna have birds flying around at. lets say its coordinates 0;0;0 (center of the map probably)
2. we have a location of birds which is transform.position

we need to move them around that circle. what is the formula of position for circle angle?
WHO CARES! I LIED!
THERE IS AN ALREADY MADE UNITY FUNCTION Transform.RotateAround(); https://docs.unity3d.com/ScriptReference/Transform.RotateAround.html

BASICALLY
you make this script and attach it to a bird!

using unity and other shit;

Vector3 center;
void Start()
{
center = new Vector3(0;0;0)
}
void Update()
{
Transform.RotateAround(center,Vector3.forward,Time.DeltaTime*5);
}

I don't have a script for them to move around the map in circles that travel long distances.

how exactly

I tried, there's no good script I can find to make them actually move around the map like I want them to.

cant you force these birds to fly