Bird Sleep

 

Just after sunset, with fading light and falling temperature, wave after wave of Canada Geese circled our cove and gracefully landed.  They joined a raucous flock of geese, perhaps 500 or more, apparently judging the cove to be a safe haven for the night.  But with all the honking I wondered if any, myself included, would ever be able to fall asleep.  With darkness, however, they did quiet down, except for the occasional honk from a vigilant sentry goose proclaiming all is well.

Canada Geese, Branta canadensis

As one ages sleep patterns become an issue, and sometimes even a topic of conversation and concern.  Being a curious birder I decided to do a little research, emphasis on little, as to the sleeping habits of our feathered friends.  What’s their sleep pattern, how much do they need, where do they go at night, can they sleep while flying, etc.?  I also scanned my photo archives looking for pictures of sleeping birds.  Unfortunately I usually delete pictures of birds with their eyes closed, but did find a few suitable for this post.

Eastern Screech Owl, Megascops asio

On my bedside nightstand there is a fascinating book by Matthew Walker entitled “Why We Sleep”.  It’s mostly about humans but does include a great chapter about the evolution of sleep.  According to the author a biologic sleep requirement must have evolved very early, as all animals, even insects, demonstrate sleep cycles.  You can confirm this with the characteristic brain waves on the EEG’s of sleeping animals and by periodic cycles of non-arousal of small insects.

Yellow-crowned Night-Heron, Nyctanassa violacea

Although all animals require some sleep, the amount and style vary considerably.  Walker states that the length of the restorative sleep requirement is determined by the complexity of the animal’s nervous system.  Both the length and type have evolved separately for every species and are balanced by the equally important need for wakeful hunting, eating, nest-building, and blog writing.

Dunlins, Calidris alpina

We are all familiar with the two types of sleep, REM and non-REM, identified by their characteristic brain waves.  It’s interesting that REM, the shallower sleep stage associated with dreaming, only occurs in mammals and birds.  It is, therefore, a later creation in the evolutionary sequence.  I consider it an “eye opener” to think of birds actually dreaming.

Common Nighthawk, Chordeiles minor

Although there are similarities between avian and human sleep, there are also many differences.  Birds demonstrate hemispheric sleep, the amazing ability to let half the brain sleep while the other half stays wide awake, perhaps as a defense for lurking predators.  At some point this split reverses and the other half falls asleep.  It’s interesting that this hemispheric sleep only occurs with non-REM sleep; REM for some reason, requires total brain participation.

Barred Owl, Strix varia

Frigatebirds are amazing seabirds that can stay aloft without landing for up to two months.  They have one major deficit–they cannot swim.  If forced to land at sea they quickly become water-logged and drown.  So curious Niels Rattenborg and others from the Max Planck Institute for Ornithology figured they would be the perfect bird to evaluate for in-flight sleep.

Magnificent Frigatebird, Fregata magnificent     photo by A. Sternick

Rattenborg fastened EEG leads to the skulls of 15 frigatebirds and attached a device to monitor flight speed.  The study confirmed that birds do indeed sleep while flying, but not in the expected manner.  They slept only in short bursts of 10 seconds and only for a total of 45 minutes each day, a much shorter duration than their sleep cycle on land.  They also only used hemispheric sleep while flying, and only slept while gaining altitude in a thermal.  They were completely awake and alert in every gliding descent, perhaps to avoid a lethal crash landing at sea.

Black Skimmers, Rynchops niger

Birds assume many different sleeping positions on land, but I’ve not yet seen one on its back with feet pointing heavenward.  Shorebirds sleep standing up, often on one leg, and usually facing into the wind.  Night herons, owls, and woodpeckers sleep  perched upright.  Their leg muscles in a relaxed state result in a clenched claw, firmly grasping the branch.  Many birds such as the nighthawks sleep horizontally, while some parrots sleep hanging upside down in a bat-like manner.  Many cavity nesters seek out a vacant cavity for the night.

Bonaparte Gull, Larus philadelphia

Birds, like humans, are susceptible to sleep deprivation.  Walker reports that the U.S. government has spent millions investigating the sleep pattern of the lowly White-crowned Sparrow.  If you deprive this bird of sleep during the season it would normally be migrating, it experiences no ill effects.  But similar sleep deprivation at any other time results in catastrophic physiologic brain and body dysfunction.

White-crowned Sparrow, Zonotrichia leucophrys

I’m not sure how they deprived the little bird of sleep; perhaps with bright lights and continuous Barry Manilow songs at high volume.  In any case, this bird has apparently evolved some protective mechanism for sleep deprivation that the U.S. government would love to uncover.

Black-crowned Night-Heron, Nycticorax nycticorax

Have you noticed how difficult it is to sleep the first night in a new hotel and bed?  I now believe this is a throwback to my evolutionary past.  Is there a Sabre-toothed Tiger lurking in the bushes or a Wooly Mammoth lumbering past my cave?  Just like the birds I require safe sleep, but haven’t yet mastered that hemispheric trick.  I guess I need that sentinel goose, standing guard and signaling all is well.

The Flight of Birds; Fair or Foul?

I was minding my own business at the desk by the window when WHACK, a Cardinal crashed into the glass.  I rushed outside to look for a body in the hedge, or at least a stunned bird, but found nothing, not even a red feather.  He must have survived.  It got me thinking about flight.  It’s marvelous and amazing and we terrestrial-bound species are jealous of the birds, but it does come with risks and at a price.  What are the risks and what exactly have the birds given up when they evolved this specialized skill.

Northern Cardinal, Cardinalis cardinalis

I count five groups that have acquired the ability to fly, (omitting the gliding frogs and squirrels).  They are the myriad insects, the extinct dinosaurs–Pterosaurs, the mammalian bats, the birds, and Homo sapiens, since Kitty Hawk.  You must admit that at least with insects and birds, flight has been a successful strategy, with Aves flying around for 150 million years since Archaeopteryx, and insects for even longer.  This compares with a meagre 20 million years for Hominids on earth, with flight mastered by us just 115 years ago.

Brown Pelican, Pelicans occidentalis

There are, of course, obvious advantages of bird flight.  They can get from point A to point B quickly, whether its to find food, escape a predator, or chase a prospective mate.  The destination may just be across the yard or a migration of thousands of miles. Their flying skills include, hovering, take-offs and landings, on either land or water, soaring, gliding, and high speed dives.  They can catch a fly on the wing and even copulate in mid-air.  Very impressive.

Red-shouldered Hawk, Buteo lineatus

There are, however, obvious physical risks to flight.  My office window, multiplied by millions is an example.  Add to that the glass of towering skyscrapers, burgeoning wind farms, and power lines, and you have some real flight hazards.  Fall migration itself takes a huge toll on the young birds.  That’s why the spring migration is less crowded, returning to us just the survivors.

Limpkin, Aramus guarauna

But I’m more interested in the anatomic and physiologic adaptations that have evolved and made flight possible, and what price Aves have paid for this specialization.  The upper extremity of birds has reduced the five digits of its ancestors to three and these serve as the anchors for the primary flight feathers.  The wing is a wonderful and highly specific adaptation for flight, but useless for grasping a tool or playing the piano.  No matter; birds have evolved a flexible neck and versatile beak and tongue to partially offset these deficits.

Rose-ringed Parakeet, Psittacula krameri

What about size?  It does matter for birds.  Flight requires the birds to be relatively small and light.  When you double the length of a bird you increase its weight 8-fold.  Even though the large Golden Eagle only weighs 15 pounds it requires an 8-foot wingspan to fly.

Black Vulture, Coragyps atratus

The physics of flight applies to the birds, just as it did for the Wright brothers.  There must be air flowing over the wing or airfoil to create enough lift to overcome the drag.  Flapping adds greatly to the lift, but weight is still a limiting factor.  Just recall the spectacle of the heavy swan or goose, beating its wings while running across the pond, in its onerous fight to become airborne.

Osprey, Pandion haliaetus

Experts debate how the Pterosaurs and ancient birds “learned” to fly.  One camp suggests a “tree-down” approach, falling or gliding from a height, similar to flying squirrels.  Another group suggests a “ground-up” technique, running or leaping into the air, similar to our struggling swan.  I doubt we’ll ever know for sure.

Laysan Albatross, Phoebastria immutabilis

Birds have also solved the weight issue by their light, hollow bones, ideal for flight but lacking somewhat in strength–another compromise.  “Light as a feather”, the saying goes.  The evolution of the feather figures centrally in the history of flying animals.  Experts now believe feathers evolved long before flight.  Once we pictured dinosaurs as hairless, leathery reptiles, but now learn that some were actually adorned with colorful feathers.  The only question is whether their feathers were for insulation or for sexual ornamentation, but clearly they were not, at least initially, useful for flight.

Belted Kingfisher, Ceryle alcyon

The weight restrictions of flight also require that a bird brain remain relatively small, and surrounded by only a thin skull.  Most of its brain is devoted to eyesight, so highly perfected in raptors, and much of the rest to the regulation of basic functions and the intricate movements of flight.  Although much has been written about the intelligence of birds, (primarily the Corvids), don’t get carried away.  They will not be writing a Beethoven symphony any time soon, or even running for political office.

Prairie Warbler, Dendroica discolor

The warm-blooded, hyperactive, flying birds are massive consumers of energy.  Their high metabolic rates require a never-ending search for food (using energy in the process) for both themselves and their young.  It is a bird’s greatest mission everyday.  The avian respiratory system is also a unique and complicated adaptation of rigid lungs, multiple air sacs, and unidirectional air flow, all designed to supply richly oxygenated blood to meet their high energy demands.

Sandwich Tern, Sterna sandvicensis

It’s interesting that some birds have given up flight completely.  You wonder why.  For Penguins the rudimentary wings are now used for swimming, while the large Ostriches of the savannas of Africa use their downy feathers and wings for shade.  The flightless Dodo birds of the Mauritius Island in the Indian Ocean were doing just fine on the ground until discovered by Dutch sailors in 1598.  The vulnerable bird was easy prey for man and his contaminants and the Dodo is now extinct.  Unfortunately its name has become synonymous with naiveté and stupidity.

Wood Stork, Mycteria americana

So the birds have paid some price for their lives in the sky.  We humans need to keep this in mind as we stretch our frontiers upward, even to the Moon and Mars.  I consider Homo sapiens now a flying animal, similar to the birds.  We are part of nature and not just an outside observer looking in.  Never mind that our “wings” are metal and rivets and computers; they are merely our adaptations, the products of our brains, and our unique ticket to the wonders of flight.

The Wright brothers, Homo sapiens, 1903