Zebra finches learn to sing from a robot in a university lab

Prince and John Lennon are standing in a soundproof room at Leiden University. They can sing like the best. The lower part of the beak rises and falls rhythmically, the head moves from left to right, the plumage is brightly colored: two adult male zebra finch in the prime of their lives.

So it seems. But if you step into the room in the Sylvius building of the university to view the birds up close, you will soon see: these are not real birds. zebra finches. These are robot finches. Legless model birds on a gray PVC tube, which contains cables that are connected to a box full of electronics. Prince and John Lennon sing tunes on command, for the benefit of science.

“Zebra finches are incredibly vocal,” says Katharina Riebel, researcher at the Institute of Biology Leiden, part of the university. “The group we have here chatters all day long. But those are mostly short calls. In this species, only the males can sing really extensively, to impress a partner. We now want to find out how exactly the transfer of the repertoire from males to young birds works. How do zebra finches learn to sing?

Initially, Riebel and colleagues did research using sound recordings, which they played to young zebra finches. “That is the standard way in biology: if you want to know how an animal learns sounds, you play sounds. The visual aspect is completely ignored.”

Stuffed bird

“And that while it is known how important facial expressions are for learning a language,” adds linguist Judith Varkevisser, who was involved in the project as a PhD student. “Babies, for example, pay close attention to lip movements.”

That is why Riebel, together with researchers from VU University Amsterdam (VU) and VU University Berlin, decided on a new approach: designing a robot finch. For example, it would be possible to study the learning process in detail, for example by playing finch songs in combination with specific head movements. Or by investigating whether they also learn from a bird that moves its beak but does not make a sound.

A young male zebra finch learns to sing in the first three months of its life. Between 35 and 65 days they are receptive to a melody, research shows. Riebel: “In the following month they start practicing and it seems more and more. When they are around 100 days old they have their own song.” After that it doesn’t work anymore.

It was important that the robot finch would look real and not be rejected. “We even visited a taxidermist to ask if he could help us build a robot into a stuffed bird,” says mechanical engineer and instrument maker Rogier Elsinga in the workshop at the VU. “But he didn’t like that. Then we just started copying one.”

Based on (naturally) deceased zebra finches from the freezer, Elsinga made a detailed scan with ecologist Wouter Halfwerk, the basis for a 3D print. “We have ensured that the shape corresponds to that of living finches. We even refined the bulge of the head until we had the right curve.”

Each male finch was painted faithfully: stripes on the chest, characteristic circle around the eyes. “We asked someone for that who paints those tiny action figures in his spare time,” says Elsinga.

High-speed camera

For the interior, he drew inspiration from an unexpected corner. “My daughters had a new toy: a Digibird, an interactive bird that sings back when you whistle at it. That bird can also move its head and open its beak, and that mechanism turned out to fit perfectly into our design.” Just like real birds, the RoboFinch only moves the lower part of the beak; the top part stays in place.

To know exactly which movements head and beak make, and how often, Judith Varkevisser filmed real zebra finches with a high-speed camera, at 120 frames per second. “Birds can distinguish more details in the same amount of time than humans.”

Elsinga’s electronics colleagues Peter Wiersma and Wesley Delmeer from the VU workshop built a microcontroller into the box under the bird, enabling the robot to make lifelike head and beak movements. The tunes are also true to nature: each robot sings a song by a different bird from the Leiden lab. Elsinga: “Actually, we also wanted to have the chest move up and down realistically, as if the bird is breathing, but that became too time-consuming.”

Aggressive behavior

There are now eight robot zebra finches in use. Initially, they were all males, because the research focused on the learning process of young finches. Riebel: “But in the meantime we have also become curious about the interaction between male and female birds.” For example: what tunes do female finches find attractive?

During the experiments, a real bird is placed in the same room as one of the robot finches and subjected to three ‘singing sessions’. In a session, vocals can be heard, but the robot cannot be seen. In another, the robot moves, but without sound. And in a third session there is sound and movement. Riebel: “We deliberately vary the order of those sessions, so that we are sure that there is no habituation.”

Such a fake bird also smells very different, of course

Roger Elsinga mechanical engineer and instrument maker

“In advance we were a bit nervous about the interaction between the zebra finches and the robots,” says Varkevisser. Riebel adds: “We were concerned that the birds would damage the mechanism if they touched the robot with their beak. Zebra finches are known for preening their peers and sometimes offering seeds.” Elsinga: “That is why we placed a transparent partition between the RoboFinch and the birds. When we saw that they reacted mainly curiously, we removed the shot.” Two horizontal sticks were mounted on either side of the robot, on which the finches could sit at a distance. “To our surprise, that distance was hardly necessary. They almost immediately sat close to the robot finch, as if they accepted it as a companion.”

The real feathers and the up-and-down chest were therefore less essential than thought. “And you have to remember: such a fake bird also smells very different, of course. But that turned out not to be a problem either.” Riebel: “We specifically investigated whether the soft clicking sound that you hear while the robot is singing would have a disruptive effect on the learning process, but that was not the case. The clicking sound was also not imitated by the young birds.”

Although the first research results have not yet been published, the researchers are already positive about the experiment. „We have purposely published the bird’s programming open source, in the hope that other researchers will build on our design,” says Elsinga. With a smile: “The robot finch can still develop further. In that respect, it is like evolution.”