I have just been observing a number of grazing brent geese. When an aircraft passed about 200 feet above and 200 yards horiziontally the geese lifted off.
They rose to a height of 20 feet and then settled within 5 seconds.
All an audible alert would have done in this case was ensure they lifted before the aircraft passed over head.
As for an audible alert for airborne geese, have you listened to a skein of geese. They might be brilliant flyers but in human terms they have lousy RT discipline. If they heard an audible alert, at best, they might scatter and increase the chances of a bird strike.
See, be seen, and avoid is one approach (lights on) and a bird detection system is probably the best. Our surface search radar only detects skeins of 1000 or more geese.