>>Society may well impose conditions on drones that could be serious technical challenges. For example, I can foresee ordinances that say "thy drone shall not be noisier than a standard delivery van". With the latter becoming increasingly hybrid or even all-electric that might be tough for today's quadcopter-type drones to meet.

Maybe the drones will need to autorotate quietly down to drop their payloads before accelerating noisily away to escape detection. ;-)

>>For high drone densities you'd need to look at the behaviour of flocks of birds or schools of fish but emulating those imposes some restrictions:
>>- uniformity of the individual units - difficult to achieve with drones from competing manufacturers
>>- uniformity of direction or goal - not what you want with 40k drones ;)

The brief views of the skies over Coruscant may be a good model. ;-) I recall streams of quick traffic with individual units peeling in and out as required to reach their destination, with a couple of pesky Jedi the only disruption to otherwise smooth travel. ;-)