Aviation researchers at the University of Florida have copied the wing action of seagulls to develop spy drones that can morph shape mid-flight.
The drones could shoot darts with microphones into rooms
The toy-sized drones are being developed for tricky urban missions so that they can zip around tight places.
They could fly into urban environments to detect biological agents.
Funded by Nasa and the US Air Force, the unmanned, sensor-packed craft in development could be on missions in two to three years, say researchers.
By watching how seagulls alter their wing shape, and using morphing techniques, the agile craft can squeeze through confined spaces, such as alleyways, and change direction rapidly.
The micro air vehicles (MAVs) could automatically find their way to monitor locations, such as apartment blocks, where suspicious activity is detected.
Although a relatively new area, it is not such a challenge to get a craft to morph.
It is more of a challenge to do it under autopilot, Dr Rick Lind, assistant professor of mechanical and aerospace engineering, told the BBC News website.
Fly like a bird
"The ultimate aim is to have an on-board autopilot so it can fly by itself through cities to search for bio-agents."
Essentially, the researchers want to take the human out of the loop.
"Autonomy for us involves using cameras or some other sensors to tell us about obstacles in its flightpath. We can assume it will have maps, but we cannot anticipate poles, trees and so on.
"The vehicles will need to identify unexpected obstacles, re-plan the flight path and go on with the mission," explained Dr Lind.
"We realised we needed better agility and manoeuvrability to move in the city so we asked, 'well, how do birds to do it?'" Dr Lind explained.
Research colleague Mujahid Abdulrahim studied the action of gulls in flight to develop the latest version of the drones, which range from 6in (15cm) to 2ft (61cm) in size.
Other US and global teams are also working on morphing drones, such as Darpa (Defense Advanced Research Projects Agency).
Its craft, said Dr Lind, would have wing spans of 20 to 30ft (6m to 9m), however, which are not suitable for urban environments.
The F-14 is the most well-known example of a shape-changing craft which can shift its wing shape for different manoeuvres mid-air, said Dr Lind.
As a former Nasa engineer, he also worked on a special version of the F-18 that twists its wings on command.
But the original masters of flight, the Wright brothers, were also an inspiration for the craft.
Their planes used the pilot's movements on a platform to control wing shape.
Dr Lind's craft are constructed of extremely lightweight and strong carbon fibre. The frame itself weighs about 50g, allowing for about 750g of payload.
On board is a small motor, batteries, GPS (global positioning system), a camera and communications equipment.
Eventually, the craft will be laden with sensors as well as communications to gather and send information back to base.
"A mission payload would include a mission camera, chemical sensors, and, potentially, acoustic sensors for listening outside apartment windows, for example," said Dr Lind.
"We have flown with a video camera but do not have chemical sensors yet - they are being developed by other organisations."
Nature has always been a useful source for flight engineers
What the team hopes is that sensors and communications equipment can be developed to be as light and as power-efficient as possible.
Very small sensor technology is being developed, and some nanotechnology techniques could be deployed to drastically improve the technology.
"There are always smart and nano materials being developed," said Dr Lind. "Currently, they require too much electricity to be useful. But they have great opportunities for the future."
The craft is powered by lithium ion batteries, but there lies the greatest challenge for small vehicles.
"Batteries are being developed all the time, though. Every month there are better ones on the market," he said.
"A lot of development is by the cellphone industry. As they develop, they slowly come to market." Finding small, low-cost circuit boards is also a problem right now.
Swarms and darts
Eventually, the craft will be tiny, allowing them to work in swarms, thus making them even more inconspicuous, the team believes.
"Colleagues have built vehicles as small as four inches across. They are difficult to spot visually. From an audio viewpoint, they are very quiet," said Dr Lind.
Working in swarms, each craft would communicate with each other. A likely scenario would involve a "mother ship" stationed high above a city, he explained.
"It could maybe fly 20 smaller vehicles inside the city. Each small one sends information up to the ship, which can then make decisions about the job and redirect the vehicles to other areas," he said.
One possibility for the morphing craft as they get smaller is using them to plant monitoring devices, such as microphones, into specific locations.
"You could have it shoot a small dart and the vehicle flies away, or the vehicle could change its shape to look like something else. The wings could fall off, for example."
Autonomously controlled drones will be ready in two to three years, Dr Lind said.
"They could be deployed very rapidly," said Dr Lind. "We are comfortable that the vehicles are of consistent quality that they will perform in a variety of conditions."
But in 20 years' time, the vehicles will go from bird size to insect size, the researcher believes. They will, at that point, be able to morph considerably, changing colour and form.
"They will be like biological systems so that they mimic birds much more than they do now."