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From GizMag (hawk attack is at 1:50 in the above video):

Researchers from the University of Maryland have built a new micro air vehicle dubbed Robo Raven that's such a convincing flyer, it's been attacked by a local hawk during testing. Though numerous other robotic birds have successfully taken to the skies in recent years, including Festo's visually stunning SmartBird, this featherless mechanical marvel is capable of impressive complex aerobatic maneuvers thanks to completely programmable wings that can flap independently of each other.

Dr. Satyandra K Gupta says that if eight years of building experimental robot flappers has taught him anything, it's that "designing and building robotic birds is hard, despite the apparent simplicity of the idea – flap wings to generate thrust to propel forward and use the moving air to generate lift to stay afloat."

The professor of mechanical engineering at the University of Maryland's A. James Clark School of Engineering had his first flight success in 2007, in collaboration with faculty colleague Dr. Hugh Bruck and a team of students. Three more models were built in the years that followed, including one called Big Bird that had jointed wings. The last one carried a video camera to document its flight and was launched from a modified ARL Lynchbot ground vehicle. It was robust enough to fly in winds up to 10 mph (16 km/h).

A lack of simulation tools meant that the researchers had to rely on trial and error testing, and what looked promising on paper didn't always make for a good flyer. Design flaws came at a high price and often ended in catastrophic crashes, with team members literally having to pick up the pieces and start again. Even successes brought their own problems to the table, in the shape of a local hawk that felt convinced enough (or at least threatened enough) to attack the robotic birds while they were in the air, and tear them to shreds.

Though Gupta and Bruck were able to build machines that could fly, only simple flapping motions were achieved due to both wings being driven by a single motor. An attempt to recreate the kind of precision wing control that makes their real-world feathered cousins such compelling viewing for everyone from curious children to dedicated twitchers was not successful, and the project was shelved.

This time last year, the researchers decided to try again. As the end of April 2013 approached, students Eli Barnett, John Gerdes, Johannes Kempny, Ariel Perez-Rosado, and Luke Roberts made a breakthrough.

Robo Raven uses two programmable actuators that can be electronically synchronized to coordinate motion between the two wings. The new design required more power to operate, though, and a microcontroller had to be added. This resulted in a heavier bird, and one that proved too weighty for flight. The design team laser cut and 3D printed lightweight polymer components in an effort to get weight down, but it was still not light enough. The answer was threefold.

First, numerous motion profiles were created so that the wings always achieved an optimal lift and thrust balance. Then a method of measuring aerodynamic forces during the flapping cycle was developed that allowed for rapid evaluation of various wing designs, before finally selecting the best fit for the job. Lastly, the system was optimized to make sure that everything worked together as efficiently as possible.

As well as sporting wings that can beat independently of each other, Robo Raven can be programmed with any number of motion patterns, allowing it to perform impressive aerobatics like back flips, dives and rolls, and breathtaking turns.

In fact, the robotic bird proved so realistic that not only did it get attacked by a territorial hawk (at 1:49 in the video below), but other birds began to follow as it motored round the test site.

Gupta says that much study and many developments lie ahead, and he hopes Robo Raven will inspire others to choose robotic bird creation as their hobby.