Mark Lynch
Birds at Rest: The Behavior and Ecology of Avian Sleep. Roger F. Pasquier. 2025. Princeton University Press. Princeton, New Jersey.
“O sleep, O gentle sleep, Nature’s soft nurse” (William Shakespeare Henry IV Part 2, Act 3, Scene 1)
One of the enjoyable aspects of birding is watching birds behave. More than any other group of vertebrates, birds often allow us to watch them as they seek mates, build nests, search for food, compete for territory, and migrate. Mammals are often nocturnal and very wary of human presence. I’m usually happy just to catch a glimpse of a skunk or deer. I have a lifelong interest in snakes, but I rarely see them nowadays. Populations of certain species have seriously declined in many areas of New England, and most of their lives are lived out of human view. Certain species of turtles do sun and are easily seen but quickly head for the water as humans approach. The one exception is coming across a pair of large snapping turtles mating, rolling over and over in the murky water like a pair of dinosaurs in battle. It is something you will not forget. But how often have you seen that? We do hear frogs and toads in spring and summer, and sometimes you can see them mate, but most of their lives are lived away from our casual gaze. Furthermore, reptiles and amphibians brumate in New England and are totally absent in late fall through spring. But bird species are around us throughout the year and, if you take the time to watch, allow us to look into their world. Some of the most spectacular bird behavior is concerned with large flocks heading to roost in winter or during migration. Some species, including starlings or crows, stage dramatic and complex roosting behaviors that can be enjoyed if you know where to look. But one bird behavior is still little known to most birders. That is: how do birds sleep?
Birds at Rest is a welcome compendium and summary of what is known today about how birds sleep, how birds rest, and how birds gather to go to roost. Roger F. Pasquier is a lifelong birder and worked at BirdLife International, The World Wildlife Fund, and the Environmental Defense Fund. He is currently an associate in the Department of Ornithology at the American Museum of Natural History in New York City. His previous book, Birds in Winter: Surviving the Most Challenging Season, published in 2019, was a similar scholarly overview of what is known about bird behavior in the coldest months. Both books take an international view and represent yearslong work. His new book combines the results of laboratory research with field observations.
This book is therefore a mix of modern science—if we think of science as the testing of hypotheses—and more old-fashioned natural history. (p. xii Birds at Rest)
Birds at Rest begins with what is known about the neurology of sleep in birds. Sleep is critical in all vertebrates. Sleep strengthens synaptic connections, and it plays a role in learning and the organization of memory. A lack of sleep can weaken synaptic connections and can seriously affect overall health. There are basically two types of sleep: slow wave sleep (SWS) and rapid eye movement (REM), sometimes known as active sleep. Pasquier does a good job of summarizing the neurology and functions of these two types of sleep. If you would like to learn more about our brains and sleep, I recommend Kenneth Miller’s Mapping the Darkness, which tells the history of how scientists discovered what happens in the human brain during sleep.
In Birds at Rest, Pasquier describes the many things that can affect birds’ sleep. Like humans, if birds’ sleep patterns are disrupted over long periods, it can lead to serious health problems. Melatonin levels can vary, making a bird drowsy. But changes in body and ambient temperatures can also affect birds’ sleep. Birds just don’t sleep in the same way that mammals do. They need to be aware of predators and to avoid falling off a perch through the night. So many bird species sleep for short periods of time.
The short duration of each REM sleep, when muscles are relaxed, was once thought to protect perching birds from falling, but it has since also been found in many birds that sleep on the ground (Lesku and Rattenborg 2014). Domestic geese can continue in REM sleep while standing on one leg if their head is resting on their back; if the head is erect, however, muscle tone is higher and the birds are in SWS (Dewasmes et al. 1985). (p. 7)
Some species of birds sleep in extremely short bouts and also wake for brief periods, then go back to sleep again. One of the most extreme examples is the Chinstrap Penguin.
Recent experiments with wild Chinstrap Penguins (Pygoscelis antarcticus) in Antarctica have shown that sleep can even be a matter of seconds, done frequently throughout the day, and adding up to what for humans would constitute “a good night’s rest.” Using remote sensing monitors, scientists found that during December, when living in constant daylight, penguins slept more than ten thousand times a day, for an average of four seconds at a time. (p. 5)
What humans would call chronic insomnia is a typical good night’s sleep for Chinstraps. Other species of birds exhibit what is labeled unihemispheric sleep in which one side of the brain is operating very differently from the other simultaneously.
The relatively short bouts of sleep typical of birds reflect the balance necessary between the need for neural maintenance and the risks that may be increased by a complete shutdown of the brain. While a complete shutdown might be the safest and most rapid way to perform neural maintenance, it may also make the bird more vulnerable to predation. In addition to sleeping in bouts far briefer than those of mammals, birds have evolved a way to maintain a degree of alertness during these bouts: unihemispheric sleep. In SWS, birds can sleep with both eyes shut or with one eye open. EEG activity in the brain hemisphere opposite the open eye is then intermediate between wakefulness and SWS, while the hemisphere opposite the closed eye is in SWS. (p. 9)
Birders can often see this behavior in sleeping flocks of migratory waterfowl in which some individuals have only one eye open.
Sleep posture is varied among different species. Some birds sleep on one leg, others sleep with their neck relaxed, head facing forward. Some species will turn their head, with their head and neck resting on the back, their bill buried in their scapular feathers. Ostriches can sleep with their body flat on the ground, their legs and long neck stretched out. There is an illustration of this odd position on page 25 of Birds at Rest. If that wasn’t weird enough, Hanging Parrot species actually do hang upside down by their feet while asleep.
Heat and cold in different climates may force birds to find a place to rest during the middle of the day. In tropical or desert climates, birds will seek cool, shady places for rest, thermoregulation, and sleep during the hottest parts of the day. In colder climates, some species, including grouse and Snow Buntings, may dive deep into snow to escape overexposure to wind and subzero temperatures. Some birds may even hibernate or exhibit torpor.
Hypothermia is a deliberate lowering during sleep of the metabolic rate and body temperature, usually by not more than 10°C from a bird’s normal daytime temperature of 40–42C. Its signs are temporary sluggishness and a lack of coordination. Hypothermia consists mainly of slow wave sleep, when birds have more muscle control than during their brief episodes of rapid eye movement sleep. (p. 52)
Torpor is a profounder form of hyperthermia, with body temperature lowered to below 30C, sometimes closer to 20C or even less. Heart, respiratory, and metabolic rates are greatly depressed, coordination is essentially absent, and response to external stimulation is diminished or absent. (p. 57)
Diet can affect how and where birds sleep. Eating too much may force certain species to rest in place before being able to move on.
Common Eiders (Somateria mollissima) wintering in the Gulf of St. Lawrence, Quebec, feed on mollusks, echinoderms, and crustaceans, all of which they swallow whole; their powerful intestines crush the shells and digest the organic matter, but meanwhile the excess weight of the shells not yet excreted makes the birds so heavy they must rest before feeding again or attempting to fly. (p. 41)
On local pelagic trips, if you are lucky, the boat may come across a flock of shearwaters just floating on the ocean. They have gorged on prey items and are waiting to digest their meal before taking off. Only if pressed by a close approach will the shearwaters be seen taking off with some effort. Species of birds that depend on the tides to find food may change their daily sleep-wake cycle to take advantage of low tides. Especially during times of bright moonlight, shorebirds can sometimes be found taking advantage of a nighttime low tide.
Perhaps one of the most intriguing sleep behaviors are those species, including swifts and frigatebirds, that may sleep while high in the air.
Two distinctive features of avian sleep—unihemispheric sleep, in which only half the brain and one eye rest, and short bouts between wakenings—likely evolved as antipredator mechanisms, but these have enabled the evolution of an attribute unique to birds, the ability, real or potential, to sleep while in flight. Of the world’s more than ten thousand birds, only a few are believed—and still fewer thus far proven—to do this. They are found in different orders, so the ability is likely to have evolved separately. (p. 122)
Some birds build special structures to sleep in that are not their nest for raising young.
Alexander Skutch (1961) popularized the term dormitory for a structure birds use for sleeping that is not a nest in which young are raised. He hypothesized that the dormitory-building habit originated in birds that continued using the breeding nest after the young had fledged. (p. 118)
Pasquier cites some observations of two different species that share the same tree cavity.
In New Hampshire, White-breasted Nuthatches (Sitta carolinensis) often sleep in holes excavated by Downy Woodpeckers, which, on other nights, sometimes use the same holes themselves. When the nuthatch leaves the hole in the morning, it regularly removes the feces that accumulated during the night, while the woodpecker defecates when it exits, both, thereby, keep the hole clean for future use. (p. 91)
Many species roost in tree or rock cavities. A few species, such as Burrowing Owls and kiwis, roost in burrows in the ground. Wherever birds choose to roost, the location has to meet three basic requirements: “safety, protection from weather, and proximity to food.” (p. 65)
Group roosts of sometimes thousands of birds offer safety in numbers. Pasquier is fascinated with large communal roosts because of the spectacle they provide for the birder.
The evening gatherings of hundreds, thousands, sometimes even millions of birds to roost are among the most dramatic spectacles provided by birds. Trails through the sky of birds approaching from different directions may begin kilometers away, guiding both other birds as well as observers to the site. (p. 159)
Around New England, during a late afternoon in winter we can sometimes watch large flocks of starlings fly to a roost under a bridge or even a dock. These winter flocks of birds flying en masse to roost are so interesting that bird trips have been led to watch large crow flocks fly to different roosts. Sometimes these roosts are disturbed by predators in the middle of the night, and the birds then move and settle down again nearby. For several winters in a row, several hundred crows roosted on a small hill very close to my house. One morning, pre-dawn, as we got up and were loading the car for a Christmas Count in the dark, we were alarmed by what we later learned were hundreds of crows rustling their feathers overhead. The roost that had been on the hill had obviously been disturbed and had moved to the trees and roof of my house. They didn’t fly off but signaled their displeasure at our presence and disturbing their rest by loudly ruffling their feathers. The sound was the same that the crows made in Hitchcock’s The Birds’ famous playground scene, only multiplied many times over. In fall or early winter, just before dawn, large flocks of blackbirds can first be heard, then seen, leaving their roosts in cattails or phragmites. The sound of them squealing and calling, quietly at first, but growing in volume until it’s almost deafening, and then suddenly bursting into the sky with a whoosh is another roosting spectacle I have been lucky to observe.
Some of the largest multispecies roosts are found in the southern United States. These roosts are used throughout the year by local birds, which are then vastly outnumbered as migrants from further north arrive for the winter. Then, several million icterids, a mix of Red-winged Blackbirds (Agelaius phoeniceus), Common Grackles (Quiscalus quiscula), Brown-headed Cowbird (Molothrus ater), Rusty Blackbirds (Euphagus carolinus), and European Starlings (Sturnus vulgaris) use extensive wetlands, rice fields, or stands of conifers. American Robins (Turdus migratorius) may join, but they stay at the periphery. Over the course of the winter, the populations and sex ratios of each species using the roost may change. (p. 168)
Many traditional large roost locations have been disturbed, torn up, or even built upon, and the birds have been forced to find other locations that may not be as good. Some species have moved their roosts to urban locations.
In South America, wintering Purple Martins (Progne subis) roost in flocks of hundreds of thousands, originally in marshes and in trees on river islands, but in the 1960s some in Brazil began roosting in urban and industrial sites, including an oil refinery in Manaus. (p. 75)
Roosts in urban locations, however, have increased levels of noise and light at night which can have several negative effects on birds sleeping.
Sleeping birds, no more than waking birds, have not escaped the influence of expanding human populations, settlement, and land use. Birds living in proximity to people are exposed to light and noise that has changed their very sleeping habits. In more remote places, birds have suffered from human disturbance at essential roosting sites and sometimes lost those sites entirely. Historically and today, the birds that roost in large aggregations have been harvested for food, often at unsustainable levels, and the proliferation of invasive animal species all over the world has made some birds easy prey while they sleep. (p. 280)
Pasquier cites several laboratory studies that have discovered the physical problems caused by sleeping in a well-lit urban environment.
Looking for other physical impacts, exposing Zebra Finches (Taeniopygia guttata) to dim light comparable to street lights was found to alter their daily patterns of activity and rest. It also induced nocturnal feeding, caused body fattening and weight gain, and reduced melatonin levels. Lipids increased in the liver, as did nighttime levels of glucose, suggesting an impairment of metabolism. (p. 265)
Birds at Rest is an important yet entertaining summary of what is known about the neurology and behavior of birds sleeping, resting, and finding a roost. I have quoted only a small fraction of the material that Pasquier has compiled for this book. There are also a modest number of delicate black-and-white illustrations by Margaret La Farge that perfectly compliment the text. If you are interested at all in bird behavior, this is a book that belongs on your shelf.
To listen to Mark Lynch’s interview with Roger Pasquier for Inquiry on WICN, click on this link:<https://wicn.org/podcast/roger-f-pasquier/>
Literature Cited
- Miller, Kenneth. 2023. Mapping the Darkness: The Visionary Scientists Who Unlocked the Mysteries of Sleep. New York: Grand Central Publishing.
- Pasquier, Roger F. 2019. Birds in Winter: Surviving the Most Challenging Season. 2019. Princeton: Princeton University Press.