The U.S. Department of Defense has done some work on defeating enemy reconnaissance UAVs. This is a prudent move, as the American success with UAVs has been noted and most major nations are acquiring them.
The Department of Defense study found that current American models (from the tiny Raven to the huge Global Hawk) when used as potential targets were all quite vulnerable. While the 2 kg (4.4 pound) Raven, with its 130cm (4.5 foot) wingspan, and 109cm (three foot) length made it a small target when flying at several hundred meters altitude, U.S. snipers found it could be consistently hit. In Iraq, very few enemy gunmen were good shots and Raven losses to bullets were few. The Afghans have done a little better but not by much. Wind, equipment failure, and birds are still the biggest source of Raven losses.
The larger UAVs (Shadow, Predator, Reaper) are target practice for anti-aircraft missiles, although not usually the shoulder fired variety. Most large UAVs fly at 6,000 meters or more, while shoulder fired missiles can go no higher than 3,000 meters. But there are a growing number of vehicle mounted anti-aircraft missile systems available and these would quickly clear the skies of Predator class UAVs.
The Department of Defense is now quietly seeking electronic countermeasures that might be used by large UAVs to defeat guided missiles. At the same time, stealthier UAVs (RQ-170, Avenger) are being developed. Also at the same time, air force and navy researchers are seeking to increase American capabilities to detect and defeat enemy UAVs. That is being done quietly, since anything discovered in that effort could be used against U.S. UAVs.
Against a well-equipped opponent the U.S. will have to rely more on space satellites (thus the great fear of Chinese attacks up there), higher UAV losses, and the use of things like one-use rockets equipped with cameras. Ironically, the smaller UAVs like Raven will become even more important because the micro-UAVs are much cheaper and built to take a beating (and be regularly lost and replaced).
In the meantime, orders for the older UAVs (Predator, Reaper, Global Hawk) are being cut to provide money for new, more survivable, models.