Remember when it took a large defense contractor and a huge budget to make a UAV? $8.4M buys a few spare parts for a Predator. Hobbyists can build them at home anymore for a couple of hundred bucks. Engineering is getting interesting again...
How does air traffic control manage "small unmanned aircraft" under manual and autonomous control? This technology is expanding from battlefield applications to search / rescue, border surveillance, environmental monitoring, and home hobbyists. While some bird - aircraft strikes may be claimed to be unavoidable, we have only ourselves to blame when the paths of manned and unmanned aircraft intersect.
The advantage of the really small UAVs is that they don't fly high enough to be a problem. They actually have more problems with those in the Predator and Globalhawk class. There is one case of which I have heard where one returned home after losing comms - straight across Class B airspace (think in the traffic pattern at Chicago O'Hare or LAX). From what I hear the FAA was not amused.
It's actually very very easy to control the airspace for these SUAVs. Generally it is a bit more of a headache in areas like the U.S. but in Iraq or Afghanistan it is a breeze. It's just a matter of good tracking and reporting between the pilot and their immediate control point, so they can deconflict the airspace during flight. It does get quite a bit more complicated, but I have flown the Raven for over 900 hrs total. Most of that time was spent having the SUAV within just a couple hundred meters of live piloted aircraft conducting operations in the area. So long as everyone is talking to each other... It's easy. As for the loss of signal, again it gets complicated and requires special training and faith in the onboard navigation instruments. Every unmanned aircraft is preprogrammed to return and loiter at certain points until they regain communication with the pilot. These UAVs and SUAVs have a ton of fail safes. The Raven helped me save lives of Coalition Forces and civilians in Iraq. For that, I am forever grateful.
Small UAVs (like birds) may not typically fly at the cruising altitude of large aircraft. However, landing and takeoff are vulnerable times for big airplanes. Hitting or ingesting UAVs in that airspace will pose "a problem".
Airspace around an airport is a very different matter. Anyone flying anything there is taken very seriously. Responsible hobbyists definitely would not do that. Military operations would also not be conducted there.