Purebred Pigeon Sept/October 2025

12 – Purebred PIGEON PIGEON BEHAVIOR by Mike McKaig Original Art by Trent Smith All animals sleep, even insects and fish. But only birds, including pigeons, engage in the two sleep states that character- ize the sleep of humans and other mammals: slow-wave sleep (SWS) and rapid eye movement (REM) sleep. SWS is char- acterized by deep sleep. REM sleep is characterized by eyes twitching, reduced muscle tone, and increased brain activity. During REM sleep the animal is less responsive to its envi- ronment and not easily aroused. Birds and mammals switch between REM sleep and SWS when sleeping. SLEEPING WITH HALF A BRAIN Researchers at the Max Planck Institute for Ornithology in Germany have unlocked many mysteries about sleep in pigeons. They were the first to show that pigeons, like other birds, are able to sleep with one half their brain while remain- ing vigilant with the other. The eye corresponding to the sleep- ing half of the brain is closed while the other eye remains open and alert. This remarkable behavior is called unihemispheric slow-wave sleep (USWS). Aquatic mammals such as whales, dolphins, seals, and manatees rely on USWS to allow them to simultaneously sleep and surface to breathe. Pigeons and other birds engage in USWS under some circumstances but put both brain hemispheres to bed at other times. Why do birds sometimes choose to make use of USWS? The answer seems to be for protection. The bird’s one open eye, controlled by the awake hemisphere of the brain, allows the bird to monitor the environment for predators. The re- searchers in Germany discovered that mallard ducks sleep with half a brain to watch for threats. Ducks located at the outside edges of a group showed higher levels of USWS than ducks located toward the center. And the open eyes of the guardian ducks at the outside edges were directed away from the group in order to detect approaching predators. The researchers called this behavior the “group edge effect”. Birds have the amazing ability to increase their use of unihemispheric sleep as the risk of predation increases. SLEEPING LIGHTLY In a related experiment the researchers attached electrodes to the heads of six pigeons and recorded their brain waves over the course of three nights. They discovered that when the birds had access only to low perches close to the ground, they slipped into REM sleep less often and slept more lightly. When they were given access to higher perches, they indulged in more REM sleep and slept more soundly. The pigeons’ behav- ior can be explained by the fact that REM sleep is a potentially dangerous state because animals are not easily aroused from it, leaving them vulnerable to predators. The researchers conclud- ed that the pigeons slept more soundly on high perches because they perceived them to be safer. PIGEON PUPILS In another study the researchers observed changes in the eye pupils of pigeons during wakefulness and sleep. Using miniature cameras attached to the pigeons’ heads, they were surprised to discover that the pupils of male pigeons became smaller during courtship. In mammals the opposite occurs: during arousal the pupils dilate. And unlike mammals, whose pupils constrict during SWS and dilate during REM sleep, the pigeons showed the opposite pattern. Another surprise was the discovery that during REM sleep pigeons’ pupils rapidly constrict and relax over 1,000 times a night, a phenomenon the researchers called rapid iris movements (RIMs). Some pigeons had more RIMs than others, and this did not depend on the birds’ sex. The researchers think it’s likely that the rapid iris movements experienced by pigeons might be common among many bird species. This research indicates that pigeons and mammals have opposite pupillary activity during wakefulness, SWS, and REM sleep. These differences are notable because in many other respects sleep in birds and sleep in mammals are quite similar. GET A GRIP Finally, it has long been known that many sleeping birds, including pigeons, make use of a remarkable device known as the perch lock. When a bird lands on a perch, the weight of its body causes the tendons that flex the toes to tighten. The tight- ened tendons pull the toes inward creating a strong grip around the perch. The perch lock is a reflex triggered by the weight of the bird’s body. As long as the bird’s weight is opposed under its toes by the perch, the toes will retain their grip. The perch lock automatically maintains the bird’s grip, allowing it to ARE PIGEONS AWAKE WHEN THEY SLEEP?