Devised in 1970 by American psychologist Gordon Gallup Jr, the mirror test, is a behavioural experiment. to assess whether an animal is capable of visual self-recognition.
In the classic MSR test, also known as the mark test, mirror self-recognition test, red spot technique, or rouge test, an animal is first anesthetised and marked with a visible substance, such as scentless paint, dye, or a sticker, on a part of its body it cannot normally see such as its forehead.
Once the animal recovers from the anaesthetic, it is given access to a mirror. Researchers then observe what happens when the marked animal is placed in front of a mirror. Animals that pass the mirror test typically interact with the mark on its own body, by touching or investigating it, which is interpreted as evidence that the animal recognises the reflected image as itself, rather than perceiving it as another individual.
Very few species have successfully passed the mirror test. Among them are great apes, manta rays, dolphins, orcas, and a single Asian elephant.
Humans develop the ability to recognise themselves in the mirror at about 18 months, which coincides with the first signs of social behaviour.
In 2008, scientists from Goethe University in Frankfurt conducted an experiment to determine if any bird species could recognise themselves in a mirror. They chose to test European magpies because of their remarkable intelligence. Magpies are known to hide food and remember its location for later, they have a strong understanding of object permanence – the ability to know something exists even when it’s out of sight – they conduct ceremonies for dead members of the species, and they are naturally curious, making them ideal for such an experiment.
In the study, the researchers placed a red, yellow, or black spot on the throats of five magpies that had been raised in captivity. The red and yellow spots stood out against the birds’ feathers, while the black spot blended in and was virtually invisible.
Before introducing mirrors, the magpies didn’t react to the spots, suggesting they couldn’t feel them or didn’t find them bothersome. However, when the mirrors were introduced, the magpies with red and yellow spots began scratching at their necks, indicating they recognised the spots on their reflections and connected them to their own bodies. In contrast, the magpie with the black spot didn’t respond, as the mark wasn’t visible.
To ensure their findings weren’t influenced by other factors, the researchers ran a series of control tests. First, they placed mirrors in front of the magpies when no spots were applied. At first, the birds exhibited social behaviours such as acting aggressively toward the mirror or jumping at it as if confronting another bird. Over time, these behaviours diminished as the magpies grew accustomed to their reflections. The researchers also turned the mirrors to show their non-reflective sides during the test. In these instances, the magpies did not react to the spots, further confirming the role of the mirrors.
This study challenged the previous assumption that self-recognition is only possible in mammals who have a neocortex, a brain structure absent in birds. Instead, the magpies’ ability to recognise themselves in the mirror suggested that this skill evolved independently in birds and mammals. This phenomenon, known as convergent evolution, occurs when unrelated species develop similar traits due to comparable evolutionary pressures, even if the underlying mechanisms differ.
However, in 2020, another team of researchers attempted to replicate the experiment using a larger sample of magpies caught in the wild as adults. This follow-up study raised doubts about the original findings. When the marked wild magpies were tested, their social behaviours, such as reacting to another “bird” in the mirror, occurred more often in front of the non-reflective side, rather than the reflective mirror. This inconsistency suggests that more replication studies and further experimental work are needed to confirm whether magpies can consistently pass the mirror test.
Following the success of the original magpie mirror test, numerous experiments have been conducted on other corvid species, who, like magpies, are known for their large brains, high intelligence, and advanced skills in tasks requiring perspective and planning.
In 2020, researchers from the University of Delhi published a study in the Journal of Ornithology documenting an experiment in which Indian house crows successfully passed the classic mirror test. However, as with the European magpie, subsequent attempts to replicate these findings have been unsuccessful.
Other corvids that have failed to pass the mirror test include jackdaws, hooded crows, carrion crows, ravens, California scrub jays, azure-winged magpies, and Clark’s nutcrackers.
Some scientists argue that the standard mirror test is not a reliable method for assessing self-recognition in animals. Birds, for example, often respond aggressively to their reflection, leaving little opportunity for them to calmly consider whether they are seeing themselves. Additionally, some birds may not perceive the mark as unusual or may lack the motivation to respond to it.
Eleven years after Gallup demonstrated that chimpanzees could pass the mirror test, renowned psychologist and behaviourist B.F. Skinner of Harvard University set out to challenge its validity as a tool for understanding animal cognition.
Skinner’s experiment involved training a pigeon to use a mirror to locate blue dots. Initially, the pigeon was taught to look in a mirror to identify blue dots on a wall behind it, pecking at them to receive a food reward. Next, it was trained to peck blue dots placed on its own body, again with food as a reward. In the final test, a blue dot was placed on the pigeon’s lower belly and covered with a bib so it could not see the mark directly. Without a mirror, the pigeon did not peck at the dot, but when placed in front of a mirror, it used the reflection to locate and peck at the dot on its body. Technically, the pigeon passed the mirror test, though only after 10 days of training.
In his published paper, Skinner mocked Gallup’s conclusions, arguing that while the experiment showed a pigeon could use a mirror to locate an object on its body, this did not indicate self-awareness. Instead, the bird had simply been conditioned to pass the test through learned behaviour.
Some other species of bird may be capable of recognising themselves in a mirror. While they have not passed the classic mirror test described above, alternative experiments suggest they do possess some level of self-recognition.
In 2023, researchers from the University of Bonn in Germany adapted the mirror test for roosters by studying their alarm calls, which are triggered when they sense a predator nearby. The study involved 68 roosters placed in different scenarios. In one setup, a rooster was positioned in front of a mirror while a silhouette of a hawk was projected overhead. For comparison, the researchers also projected the hawk over roosters in three other conditions: when they were alone without a mirror, when they were next to a visible rooster in an adjacent compartment, and when they were next to a hidden rooster separated by a mirror.
The results showed that roosters gave significantly fewer alarm calls when next to a mirror than when next to another visible rooster, suggesting they understood that their reflection was not a separate bird. They also produced a similar number of alarm calls when in front of a mirror as when they were alone. In the setup where another rooster was concealed behind a mirror, the birds also called less frequently, indicating that they recognise other roosters visually rather than through smell or sound.
Despite these findings, the roosters failed the traditional mirror test. Researchers concluded that it was feasible they may have perceived their reflection as an unusual member of their species mimicking their movements, which may have discouraged them from calling out of irritation rather than recognition.
