Have you ever wondered about how animals sleep? In
particular, do birds slumber and dream? How do they accomplish this when
migrating over long distances for days or even weeks at a time? Learn
about our understanding of sleep as it occurs in birds.
Birds Share Sleep Traits with Mammals
It
is somewhat surprising that birds – that are more closely related to
reptiles – share many sleep characteristics with mammals.
In fact, the members of the avian class are the only animals outside of mammals that are noted to have both non-REM and REM sleep. Despite these similarities, they have a few unique manifestations of sleep behaviors.
When the sleep of birds is studied with EEG
to measure the electrical activity of the brain, specific changes are
noted that reflect various changes of sleep. With non-REM sleep, the EEG
demonstrates a high voltage, low frequency tracing. This similarly
occurs in other animals, including humans.
Birds Can Sleep with an Eye Open or While Flying
Birds
are interesting in that they often keep one eye open during non-REM
sleep. Reptiles do as well. How is this accomplished? It may relate to
the fact that birds, and other migrating animals, are able to sleep with
one half of their brains at a time. This phenomenon is called hemispheric sleep.
By closing just one eye, the bird is able to stay aware of its environment and the potential presence of predators.
The wiring works in such a way that the
eye closed is opposite to the side of the brain that is asleep.
Research demonstrates that birds that are sleeping in riskier situation
spend more time with one eye open while sleeping. (Incidentally, humans
may sleep with eyes open for different reasons.) They will also keep the eye open that is closer to the potential threat.
Being able to sleep with half of the brain at a time has
some noteworthy advantages. Beyond protection from predators, other
activities can be enhanced by maintaining brain productivity over
prolonged periods. In particular, this phenomenon allows birds to fly
continuously for days, weeks, and longer while sleeping during flight.
It is not necessary for them to land to rest. Instead, half the brain
powers down while the other half maintains the course.
What Do Birds Dream About?
Many
species of animals demonstrate rapid eye movement (REM) sleep, and
birds are no exception. As in humans, REM sleep appears similar to
wakefulness when observed with an EEG. In addition, features of the
state are observed: rapid eye movements, twitching of muscles, and
reduced temperature regulation. There are a few observed differences,
however.
During typical REM sleep, the muscles are relaxed to
prevent the enactment of dreams from occurring. If you dream about
jumping out of a window, you don’t want to actually do it while
remaining unconscious.
Abnormalities of this normal sleep paralysis can occur in REM behavior disorder. When observing the REM sleep of birds, the measure of muscle activity via EMG
rarely shows relaxed or paralyzed muscles (a condition called atonia).
Nevertheless, reduced muscle tone can be noted. For example, head
drooping is often observed when birds are in REM sleep.
In
addition, REM sleep is often much shorter in birds compared to mammals.
Each episode is brief, often lasting less than 10 seconds. When the
entire period of sleep is assessed, less REM occurs overall.
Interestingly, songbirds are observed to have more REM sleep. It is
fascinating to speculate on the role of REM sleep in learning and
whether these birds have more utility for this stage of sleep.
In
many birds, but not all, the periods of REM sleep become more prolonged
towards morning, just as occurs in other animals. There is also some
evidence that the amount of REM sleep increases following sleep
deprivation.
No one really known for sure what birds dream about,
but one might surmise that they are re-experiencing their daytime
activities, just as humans often do.
Sleep seems to be important for all known animals, and birds are no exception.
Sources:
Amlaner CJ, Lesku JA, Rattenborg NC. “The Evolution of Sleep.” In: Basics of Sleep Guide, 2nd ed. Amlaner CJ and Fuller PM, Editors. Westchester, Illinois: Sleep Research Society, 2009:15.
Amlaner CJ, Ball NJ. “Avian sleep.” In: Kryger MH, Roth T, Dement WC, eds. Principles and Practice of Sleep Medicine, 2nd ed. Philadelphia: WB Saunders Co., 1994:81-94.
Martinez-Gonzalez
D, Lesku JA, Rattenborg NC. "Increased EEG spectral power density
during sleep following short-term sleep deprivation in pigeons (Columba
livia): evidence for avian sleep homeostasis.” J Sleep Res 2008;17:140-53.
Rattenborg
NC, Martinez-Gonzalez D, Lesku JA. “Avian sleep homeostasis: convergent
evolution of complex brains, cognition and sleep functions in mammals
and birds.” Neurosci Biobehav Rev 2009;33:253-70.
Rattenborg NC, Amlaner CJ, Lima SL. "Behavioral, neurophysiological and evolutionary perspectives on unihemispheric sleep." Neurosci Biobehav Rev 2000;24:817-42.
Rattenborg NC. "Do birds sleep in flight?" Naturwissenschaften 2006;93:413-25.
Rattenborg NC, Lima SL, Amlaner CJ. "Half-awake to the risk of predation." Nature 1999;397:397-8.
