The guys over at the University of Maryland have been working on a flapping wing UAV that uses integrated flexible solar cells in the wings that charge the battery as it flies around! Pretty cool stuff.
The sad thing is that the current efficiency of solar cells (apparently 6%) means that this system is far from being able to fly indefinitely. According to the UMD article, the motor requires 30W, but the solar panels provide 3.6W. Interestingly meat apparently has 20 times more energy than the current battery technology!
Nature has a significant edge over engineered system in other areas. For example, 1 gram of meat stores 20 times more energy than 1 gram of the current battery technology. So in terms of the energy density, we engineers have a lot of catching up to do. In nature, solar energy collection devices (e.g., trees) are not on-board ravens. Hence, ravens ultimately utilize a large collection area to gather energy into a highly dense storage source (e.g., meat), giving them a much longer range and better endurance than Robo Raven III.