Why do birds legs' not get frostbite?

Why do birds legs' not get frostbite?

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In a nature documentary I watched a while ago, there was a scene where a flock of flamingos slept in a lake that froze overnight. Each morning they just had to wait until the lake defrosted sufficiently to release them so they could get on with their day.

How do they not get tissue damage etc doing this?

Birds have a few adaptations and behaviors to keep their legs warm in cold weather:

  • Blood vessels (veins and arteries) run very close together or are intertwined:

    The arteries and veins intertwine in the legs, so heat can be transferred from arteries back to veins before reaching the feet. Such a mechanism is called countercurrent exchange. Gulls can open a shunt between these vessels, turning back the bloodstream above the foot, and constrict the vessels in the foot. This reduces heat loss by more than 90 percent. (Wikipedia)

  • Birds have specialized scales on their feet and legs which act as an insulator.

  • Birds often alternately tuck their legs into their body feathers to minimize heat loss. (Audubon)

  • Other survival mechanisms such as shivering and torpor come into play with extreme cold.

Other sources: Mother Nature Network. I don't have my texts handy, but can highly recommend Manual of Ornithology: Avian Structure and Function by Noble S. Proctor and Patrick J. Lynch.

Birds are warm-blooded animals that have a much higher metabolism, and thus higher body temperature, than humans. While the exact measurement varies for different bird species, the average bird’s body temperature is 105 degrees Fahrenheit (40 degrees Celsius). Bird body temperature can fluctuate during the day depending on climate, diet, and activity, but it can be a challenge for birds to maintain such a high body heat when temperatures dip severely. Smaller birds are particularly at risk, since they have a proportionally larger surface area on their bodies to lose heat but a smaller core volume to generate it. Even the smallest birds, however, have several ways they can efficiently keep warm even in extreme cold.

Birds have many physical and behavioral adaptations to keep warm, no matter how low the temperatures of their surroundings may be.

Physical Adaptations

  • Feathers: Birds’ feathers provide remarkable insulation against the cold, and many bird species grow extra feathers as part of a late fall molt to give them thicker protection in the winter. The oil that coats birds’ feathers from their uropygial gland also provides insulation as well as waterproofing.
  • Legs and Feet: Birds’ legs and feet are covered with specialized scales that minimize heat loss. Birds can also control the temperature of their legs and feet separately from their bodies by constricting blood flow to their extremities, thereby reducing heat loss without risking frostbite.
  • Fat Reserves: Even small birds can build up fat reserves to serve as insulation and extra energy for generating body heat. Many birds will gorge during the fall when food sources are abundant, giving them an extra fatty layer before winter arrives.

Behavioral Adaptations

  • Fluffing: Birds fluff out their feathers to create air pockets for additional insulation in cold temperatures. This can make them look fat and puffy while they are toasty warm.
  • Tucking: It is not unusual to see a bird standing on one leg or crouched to cover both legs with its feathers to shield bare skin from the cold. Birds can also tuck their bills into their shoulder feathers for protection and to breathe air warmed from their body heat.
  • Sunning: On sunny winter days, many birds take advantage of solar heat. They will turn their backs to the sun (therefore exposing the largest surface of their bodies to the heat) and raise their feathers slightly. This allows the sun to heat their skin and feathers more efficiently. Wings may also be drooped or spread while sunning, and the tail may be spread as well. The more surface area birds can expose to the sun, the faster they will heat up.
  • Shivering: Birds shiver to raise their metabolic rate and generate more body heat as a short term solution to extreme cold. While shivering does require more calories, it is an effective way to stay warm in extreme conditions, as least for brief periods or in areas where rich food sources are easily available.
  • Roosting: Many small birds, including bluebirds, chickadees, and titmice, gather in large flocks at night and crowd together in a small, tight space to share body heat. They can roost in shrubbery or trees, and empty birdhouses and bird roost boxes are also popular locations to conserve heat. Even individual birds choose roost spots that may have residual heat from the day’s sunlight, such as close to the trunk of a tree or near any dark surface.

Fun Fact

Emperor penguins are some of the most resilient birds—they keep warm in frigid temperatures by huddling together in large groups.

Why Don’t Ducks’ Feet Freeze?

The Short Answer: It’s all about heat exchange, and the smaller the temperature difference between two objects, the more slowly heat will be exchanged. Ducks, as well as many other birds, have a counter-current heat exchange system between the arteries and veins in their legs. Warm arterial blood flowing to the feet passes close to cold venous blood returning from the feet. The arterial blood warms up the venous blood, dropping in temperature as it does so. This means that the blood that flows through the feet is relatively cool. This keeps the feet supplied with just enough blood to provide tissues with food and oxygen, and just warm enough to avoid frostbite. But by limiting the temperature difference between the feet and the ice, heat loss is greatly reduced. Scientists who measured it calculated that Mallards lost only about 5% of their body heat through their feet at 0 o C (32 o F) 1 . To put this in perspective, the rest of the duck is covered with feathers and in contact only with air, not ice, but because the body is relatively hot, 95% of the heat loss is from the head and body. Meanwhile, the cool feet sit on ice and give up very little heat.

More Info: In the diagram to the left, without counter-current heat exchange, warm blood makes it all the way to the foot. This keeps the feet considerably warmer than the ice the duck is standing on. Remember that heat flow is roughly proportional to the temperature difference. With a large temperature difference, there is a large flow of heat from the foot to the ice.

In the diagram to the right, you’ll see that multiple branches of the artery are in close contact with branches of the vein. This intertwining of arteries and veins is called retia (singular – rete tibiotarsale). Because heat flows from the arterial blood to the venous blood, the arterial blood becomes colder and the venous blood becomes warmer. Less warm blood gets to the foot, keeping the foot cold and reducing the temperature difference between the foot and the ice. This reduces the flow of heat from the duck to the ice.

Bird’s legs and feet are relatively free of soft tissue and even the muscles that operate the foot are mostly located higher up in the leg and connected to the bones of the feet with long tendons. Because there isn’t much soft tissue in the lower legs and feet, there is less need for warm blood. Many birds also have valves in their leg arteries that control the blood flow, and there is evidence that some birds can pulse the blood to the foot. Every once in a while, warm blood flows to the foot to make sure it doesn’t suffer from frostbite. And when you see a duck or other bird standing on one foot in the cold, this probably serves to protect tissues and reduce heat loss even further because the one leg that is tucked up under insulating feathers gets a chance to really warm up and recover from any potential cold damage, and with only one foot on the ice, the surface area in contact with a cold surface is cut in half!

Interestingly, this same system can function to good effect when a bird is standing in excessively warm water. In strong sun, the shallow water some birds forage in can heat up well above the body temperature of the bird. This presents the bird with a cooling problem. However, counter-current blood flow in the legs can reduce this problem. Overheated blood coming from feet standing in water at 45 o C (115 o F) transfers some of that heat to the arterial blood flowing to the feet. This reduces the temperature of the blood flowing in the veins before it enters the main part of the body, and keeps the temperature of the feet somewhat above normal, closer to the temperature of the water the bird is standing in. Again, the small temperature difference reduces the flow of heat, this time from the environment into the feet.

This may explain why out of 66 bird species dissected by Uffe Midtgard 2 , the one with the most elaborate retia was not a cold weather bird, but the Greater Flamingo, with 62 arterial branches intertwined with 40 venous branches, compared to 19 arterial branches and 24 venous branches in a Mallard duck. That fact aside, for the most part, Midtgard’s findings fit what we would expect, in that birds that live in cold weather habitats tend to have more elaborate retia. Many ducks have relatively complex retia, as do Mute Swans, Ptarmigan and Partridge. Songbirds don’t technically have retia, with intertwined arteries and veins, but they have a system that probably functions in a similar way. As many as eight veins returning from the foot are arranged around one main artery.

By the way, birds aren’t the only animals to have evolved this counter-current blood flow trick. It can also be found in the flippers of whales and sea turtles, some reptiles, and some have even suggested that the proximity of the veins and arteries that supply human arms is a simple counter-current heat exchanger.

Something to Think About: One thing I’ve often wondered about, is whether an animal like a duck is uncomfortable in the cold. We see them sitting on the ice, with their feathers fluffed up, and we imagine that they must be suffering. But maybe not. Our bodies tell us we are uncomfortable when we are outside the range of temperatures at which human beings are safe – say 15-25 o C (60-80 o F). We experience pain or other discomfort so we do something to get warmer if it’s too cold, or move to somewhere cooler if it’s too hot. But for a Mallard, sitting on ice is not dangerous. They don’t lose much heat through their cold feet, and their feathers keep the rest of their body toasty warm. There’s no reason for them to move or change their situation, so I’m guessing they don’t feel discomfort at all.

1 Kilgore, D.L. & Schmidt-Nielsen, K., Heat loss from ducks’ feet immersed in cold water, The Condor, 77:475-517, 1975.

2 Midtgard, U., The rete tibiotarsale and arterio-venous association in the hind limb of birds: a comparative morphological study on counter-current heat exchange systems, Acta Zoologica, Vol. 62, No. 2, 67-87, 1981.

Why birds need no fluffy bunny slippers

If you’ve ever chewed on a chicken foot, you’ll have noticed that it doesn’t have much meat. It’s pretty much all tendons and bones. So, unlike your human foot, which contains plenty of moist muscle tissue, a bird’s foot contains only very little fluid in its cells. This means that in a bird’s foot there simply isn’t much that could freeze. Sure, blood circulates through Tweetie's foot, but it’s unlikely to turn into ice. Given a bird’s frantic little heart, blood simply rushes by way too fast to freeze.

But without freezing, piercing ice crystals can’t form, which in turn means that no tissue damage and frostbite can occur. So it’s not that birds don’t get cold feet. They do. It’s just that their feet are unlikely to suffer much damage from the coldness.

Birds, however, being warm-blooded animals, still need to keep the rest of their body, and particularly their core, at a toasty temperature of somewhere between 34–44 degrees Celsius (93–111 degrees Fahrenheit). Otherwise, they’ll risk turning into dead feathery popsicles with chicken feet for a handle.

To prevent heat loss, birds get the equivalent of goosebumps: their feathers puff up and trap air within them. Their bodies then warm this air up in much the same way as your body warms up the air trapped in a sleeping bag when you’re out camping in the cold. Being thusly surrounded by a shell of warm air, birds can then simply crouch down and surround their cold feet with that warm plumage.

But birds also use another nifty trick to prevent heat loss through their feet, namely, a mechanism called countercurrent heat exchange. This mechanism is illustrated below:

Warm blood is pumped down a bird’s leg through an artery and returns back to the heart through a vein. That’s nothing unusual and it’s how it works in your legs as well. However, in your case, if you were to stand barefoot on snow, the warm blood would reach the soles of your feet and transfer much of its heat to the snow, cool down, and then flow back to your heart having lost most of its warmth. Over time, as you foolishly keep standing on that snow, you would lose more and more body heat through your soles and eventually risk experiencing hypothermia.

In birds, meanwhile, this is much less likely to happen because the arteries going down to the feet and the veins going back up are very close together, almost hugging each other. They are in fact so close that most of the warmth going down to the feet is transferred to the cold blood going back up. So by the time the blood reaches the sole of a bird’s foot, it has already transferred most of its heat to the blood going up and so not much heat is lost to the snow. Instead, most of the heat is preserved internally.

And if you’ve ever wondered why birds like to stand on one foot, here’s one answer: by alternating the foot on which they stand and by pulling up the other one into their warm puffed-up feathers, birds reduce how much of their body is in contact with the snow and thus preserve even more body heat.

Finally, if all that doesn’t help, I hope a bird will be smart enough to eventually stand on something other than the cold snow.

Scientists balanced a dead flamingo on one leg to unlock the bird’s standing secret

Paul Rose is affiliated with the University of Exeter and the Wildfowl & Wetlands Trust.


The Conversation UK receives funding from these organisations

Flamingos can stand on one leg for far longer than humans can. They can even do it while asleep. Now scientists have shed some more light on just how these pink birds manage such a balancing act without getting tired.

The researchers from the Georgia Institute of Technology in the US focused on one of the main theories used to explain this behaviour, the muscle fatigue hypothesis. The more a muscle is used, the more likely it is to become tired and so most animals standing on one leg need to regularly switch. But flamingos can use one leg for much longer periods of time without needing to switch. So the theory is that the leg holding them up doesn’t get fatigued.

The two scientists wanted to test if it was possible for flamingos to remain stable on only one leg without the need for active muscular effort. To see if a flamingo could do this, they used a novel method involving two dead flamingos, obtained from a local zoo.

Go on, push me. I dare you. Shutterstock

The researchers positioned the bodies on one leg using clamps and measured how well each cadaver could hold its body weight and maintain balance. They also dissected the leg structures to see if muscle control was used when the birds were stood on one leg. And they collected information from living flamingos to see how much body sway was affected by how many legs the birds stood on.

They not only found that a flamingo could support its body weight passively (with no need for muscular activity) on one leg, but also that it was impossible for the bird to hold a stable, balanced position on two legs. They concluded that a flamingo standing on two legs uses more muscular energy to maintain a steady posture.

So why is a one-legged posture more efficient, and why does it use no active muscular movement? Apparently, it is down to the weight of the bird itself. When a flamingo is standing on one leg, its bodyweight forces the joints in its leg into a fixed arrangement. By moving the dead flamingo, the scientists noticed that there must be a group of muscles and ligaments that lock into place (known as a stay apparatus) in the proximal (near centre) part of the limb.

This stay apparatus resists certain types of movement and keeps the flamingo stable, without the need for it to use leg muscles to keep balanced. The efficient balancing action is only possible when the bird’s foot is placed directly below its body, the position the birds naturally adopt. This actually becomes even easier when the flamingo is asleep because it moves less and so there is less variation in the centre of pressure.

On balance, this seems like a good way to keep warm. Shutterstock

This is the first evidence of a passive, gravity-driven bodyweight support mechanism in a bird’s proximal leg joints. That means the bird supports itself without conscious effort because of the anatomy of the joints in its leg. What they cannot demonstrate is any other explanation as to why a sleeping, unipedal flamingo should benefit from being so stable and secure based on their behaviour. This requires further investigation.

For example, research has shown that birds can lose a significant amount of heat through their legs and this can help them maintain the right body temperature. Even more heat escapes if the birds are stood in water (as flamingos often are) and so being able to easily stand on one leg would help to reduce the amount of heat lost. This would be particularly beneficial for those flamingos who live in cold climates and areas where water temperature is close to, or below, freezing.

The heat loss theory is plausible and makes sense, but is probably supported by the muscular activity hypothesis, too. What is clear is that flamingos, as familiar and fascinating as they are, still challenge our understanding of their physiology, biology and evolutionary history. Many birds stand on one leg but the flamingos’ balancing act may appear more noticeable because they are such strikingly shaped and coloured animals, which adds to their sense of being weird and wonderful. So the debate about exactly why they stand on one leg is sure to continue well in to the future.

Why don’t ducks get frostbite?

While New York is currently having amazingly warm weather, I still remember last winter in Central Park. Then I watched the ducks landing on the pond and skidding across the icy surface. Looking rather peeved, they eventually found a liquid patch of water.

But think of their feet. They are not thick enough to have an insulating layer of fat, nor are they covered in feathers. Thus blood must flow close to the skin, cooling rapidly in the freezing water, or on the ice Why don’t they get frostbite like humans do?

Frostbite is caused by severely reduced blood flow to the extremities in cold weather. Over extended periods, tissues in the fingers and toes do not get warmth or nutrients from the blood and die, causing gangrene and other nasty problems.

The secret for ducks is in the blood flow system. To maintain healthy tissue, and prevent frostbite, you need to provide nutrients to the tissue and keep it warm enough so that it doesn’t freeze. In ducks (and other cold-weather birds), this is done by a physiological set up called “countercurrent”. Think of venous blood, cold from exposure to the air, flowing back into the body from the feet. Too much cold blood will bring the core body temperature down, leading to hypothermia. Then think of warm, arterial blood rushing from the heart. In animals adapted to the cold, the veins and arteries run very close together. As cold blood runs up the leg from the foot and passes by the artery, it picks up most of the heat from the artery. Thus, by the time arterial blood reaches the foot, it is very cool, so does not lose too much heat in transfer with cold water. Blood flow is carefully regulated to maintain the delicate balance of providing blood but maintaining core body temperature.

In this way, the blood in the foot of a duck remains very cool at all times, yet warm enough to keep the tissue healthy. By maintaining blood flow, nutrients required by the foot tissue are also provided. That being said, ducks can still get cold if they stay in the water too long.

It turns out that birds are not the only creatures to use countercurrent to survive in the cold. Marine mammals such as whales, seals and dolphins have arteries surrounded by a web of veins. This makes heat transfer between arterial and venous blood even more efficient, protecting flippers which do not have a juicy layer of blubber to insulate them. People, too, have a rudimentary system for countercurrent. Deep in the arms and legs, arteries and veins run together. When cold, only these protected arteries and veins are used. This restricts blood to extremities and causes – yes, frostbite. However it protects our core body temperature so that we survive (minus a few appendages). The reason our system is less developed is that we just don’t need the system that often – we are more used to trying to dissipate excess heat (by sweating or running blood close to the skin).

Back to ducks. Living in a winter climate is very costly, with an enormous amount of energy needed to reheat ducks after a cold swim or an icy meal. However ducks have adapted to gain advantages from the chill.

Cooling may allow ducks to dive deeper and swim further. By cooling the brain, less oxygen is required and thus a duck can stay underwater longer. In one study, ducks diving in 10 degree centigrade water could stay under 14% longer than those diving in 35 degree water.

Though looking at how irritated they appear when their pond freezes, I personally think ducks prefer summer.

Caputa M, Folkow L, Blix AS. (1998) Rapid brain cooling in diving ducks. Am J Physiol.275(2 Pt 2):R363-71.

de Leeuw JJ, Butler PJ, Woakes AJ, Zegwaard F. (1998) Body cooling and its energetic implications for feeding and diving of tufted ducks. Physiol Zool. 71(6):720-30.

Koeslag JH. (1995) Countercurrent mechanisms in physiology. Continuing Medical Education 13: 307-315.

Reite OB, Millard RW, Johansen K. (1977) Effects of low tissue temperature on peripheral vascular control mechanisms. Acta Physiol Scand.101(2):247-53.

Schmidt-Nielsen K. (1981) Countercurrent systems in animals. Scientific American 118-128.

Nature News: Do ducks and gulls get cold feet in winter?

During our last bout of super-cold weather, a gull flew by my classroom window. These are great windows. Six huge, multi-paned windows that span the back wall of my fourth floor classroom. So, a herring gull flew by and one of the students, distracted in the middle of an exam, wondered how it managed to stay here all winter. Why didn’t it fly south? How did it manage to stay warm? Since we were in the middle of midterm exams I couldn’t start excitedly expounding on this subject, but can’t wait until classes resume this week to bring this up. Temperature regulation in animals is one of my favorite topics.

There are two great vocabulary words concerning how animals regulate their temperature: Endotherms (warm-blooded animals) generate their own heat – these are usually mammals and birds. Ectotherms (cold-blooded animals) don’t generate their own heat, so, when they need to regulate their internal temperature they use outside sources (like the sun). Reptiles, amphibians, fish and insects are usually ectotherms.

These aren’t black and white categories - this is biology so there are many shades of gray. For example, when a bumble bee wants to warm up in the morning it will do so by vibrating its flight muscles (that’s actually what makes the buzzing noise bees make – not the beating of their wings as most people think) to generate heat. So even though they are ectotherms, technically, they are being endothermic when they do this.

This time of year you won’t see any ectotherms out and about, and many endotherms travel south to escape the cold because it takes an enormous amount of energy (food) to maintain a constant internal temperature in freezing conditions. Why is body heat necessary? Primarily for enzyme function. Enzymes regulate everything in our bodies and most enzymes are made to operate best at specific temperatures – if it gets too hot or too cold, they don’t work as well. So, animals that remain here year-round have a variety of adaptations that help them retain body heat - thick fur, feathers, extra body fat and the like.

My curious student was particularly perplexed by the gull’s legs. The gull’s feathers do a great job insulating their bodies, they are the perfect winter jacket - waterproof outer feathers, fluffly inner feathers that trap air next to the body. But what about their legs? They are thin and spindly, no extra fat, no feathers, nothing between them and the cold. I was watching ducks hang out on some ice in the harbor and wondered the same thing. Do they feel cold like we do? Why don’t they get frostbite? Are they miserable?

Humans get frostbite when cold conditions cause reduced blood flow (because our bodies are trying to maintain a constant core temperature) to our extremities. Our fingers and toes don’t get the warmth and nutrients they need from the blood and die. Ducks and gulls avoid this through something called countercurrent heat exchange. As warm arterial blood is pumped from the heart and circulates out into the legs, it passes by cool venous blood which is returning back to the heart. The cold venous blood is warmed by the arterial blood (because heat always flows from warm to cold) in turn, the warm arterial blood is cooled by the venous blood. By the time the arterial blood reaches the feet, it won’t lose much heat to the surroundings and the returning venous blood won’t cool the core too much because it has already been warmed.

Those ducks and gulls you see standing around on ice have cold feet. They’re generally just above freezing. This helps the bird stay warm because heat flow is generally proportional to the temperature difference, so very little heat is lost from those feet (typically only 5 percent of heat is lost through the feet). And, unlike our fingers, the tissues in their feet are adapted to function at close to freezing temperatures.

My final question: Are they suffering from the cold as they stand around on ice? I don’t know. We feel discomfort when we are outside our range of tolerance — pain is what makes us recoil, pull our hand out of icy water or away from a hot stove. Since local gulls and ducks are built for freezing conditions, my guess would be that they are fairly comfortable out there on the ice. I personally would love a pair of boots that did the job of those bird’s feet.


Birds are generally digitigrade animals (toe-walkers), [7] [10] which affects the structure of their leg skeleton. They use only their hindlimbs to walk (bipedalism). [2] Their forelimbs evolved to become wings. Most bones of the avian foot (excluding toes) are fused together or with other bones, having changed their function over time.

Tarsometatarsus Edit

Some lower bones of the foot are fused to form the tarsometatarsus – a third segment of the leg specific to birds. [8] It consists of merged distals and metatarsals II, III and IV. [6] Metatarsus I remains separated as a base of the first toe. [4] The tarsometatarsus is the extended foot area, which gives the leg extra lever length. [7]

Tibiotarsus Edit

The foot's upper bones (proximals) are fused with the tibia to form the tibiotarsus, while the centralia are absent. [5] [6] The anterior (frontal) side of the dorsal end of the tibiotarsus (at the knee) contains a protruding enlargement called the cnemial crest. [2]

Patella Edit

At the knee above the cnemial crest is the patella (kneecap). [4] Some species do not have patellas, sometimes only a cnemial crest. In grebes both a normal patella and an extension of the cnemial crest are found. [2]

Fibula Edit

The fibula is reduced and adheres extensively to the tibia, usually reaching two-thirds of its length. [2] [7] [8] Only penguins have full-length fibulae. [4]

Knee and ankle – confusions Edit

The bird knee joint between the femur and tibia (or rather tibiotarsus) points forwards, but is hidden within the feathers. The backward-pointing "heel" (ankle) that is easily visible is a joint between the tibiotarsus and tarsometatarsus. [3] [4] The joint inside the tarsus occurs also in some reptiles. It is worth noting here that the name "thick knee" of the members of the family Burhinidae is a misnomer because their heels are large. [2] [8]

The chicks in the orders Coraciiformes and Piciformes have ankles covered by a patch of tough skins with tubercles known as the heel-pad. They use the heel-pad to shuffle inside the nest cavities or holes. [11] [12]

Toes and unfused metatarsals Edit

Most birds have four toes, typically three facing forward and one pointing backward. [7] [10] [8] In a typical perching bird, they consist respectively of 3,4, 5 and 2 phalanges. [2] Some birds, like the sanderling, have only the forward-facing toes these are called tridactyl feet. Others, like the ostrich, have only two toes (didactyl feet). [2] [4] The first digit, called the hallux, is homologous to the human big toe. [7] [10]

The claws are located on the extreme phalanx of each toe. [4] They consist of a horny keratinous podotheca, or sheath, [2] and are not part of the skeleton.

The bird foot also contains one or two metatarsals not fused in the tarsometatarsus. [8]

The legs are attached to a very strong, lightweight assembly consisting of the pelvic girdle extensively fused with the uniform spinal bone called the synsacrum, [7] [10] which is specific to birds. The synsacrum is built from the lumbar fused with the sacral, some of the first sections of the caudal, and sometimes the last one or two sections of the thoracic vertebrae, depending on species (birds have altogether between 10 and 22 vertebrae). [9] Except for those of ostriches and rheas, pubic bones do not connect to each other, easing egg-laying. [8]

Fusions of individual bones into strong, rigid structures are characteristic. [1] [7] [10]

Most major bird bones are extensively pneumatized. They contain many air pockets connected to the pulmonary air sacs of the respiratory system. [13] Their spongy interior makes them strong relative to their mass. [2] [7] The number of pneumatic bones depends on the species pneumaticity is slight or absent in diving birds. [14] For example, in the long-tailed duck, the leg and wing bones are not pneumatic, in contrast with some of the other bones, while loons and puffins have even more massive skeletons with no aired bones. [15] [16] The flightless ostrich and emu have pneumatic femurs, and so far this is the only known pneumatic bone in these birds [17] except for the ostrich's cervical vertebrae. [13]

Fusions (leading to rigidity) and pneumatic bones (leading to reduced mass) are some of the many adaptations of birds for flight. [1] [7]

Most birds, except loons and grebes, are digitigrade, not plantigrade. [2] Also, chicks in the nest can use the entire foot (toes and tarsometatarsus) with the heel on the ground. [4]

Loons tend to walk this way because their legs and pelvis are highly specialized for swimming. They have a narrow pelvis, which moves the attachment point of the femur to the rear, and their tibiotarsus is much longer than the femur. This shifts the feet (toes) behind the center of mass of the loon body. They walk usually by pushing themselves on their breasts larger loons cannot take off from land. [10] This position, however, is highly suitable for swimming because their feet are located at the rear like the propeller on a motorboat. [2]

Grebes and many other waterfowl have shorter femur and a more or less narrow pelvis, too, which gives the impression that their legs are attached to the rear as in loons. [2]

Why do birds legs' not get frostbite? - Biology

So, what's with those crows with colored wings anyway?

Or Reporting sightings of tagged/banded crows.

All photographs (c) Kevin J. McGowan and not to be used without express written permission.

What are those crows doing with tags on their wings?

I have been studying crows (both American and Fish) in the Ithaca area since the summer of 1988, and marking birds since 1989 (color bands only in '89). I am trying to gather data on social behavior, reproductive biology, dispersal, and survival (especially after exposure to West Nile virus) on both these poorly-studied species (see my project overview). In order to gather such data I needed to have some way to know individual crows as individuals, hence the tags and bands. Because crows spend so much time walking around on the ground where even short grass hides their legs, the wing tags have been invaluable for finding and identifying individual crows. On a good day I can identify an individual crow up to a half mile away. (A good day being one with the right atmospheric conditions, little fog or heat distortion some tagged crows visible across an open space, say a field of corn stubble and me having my trusty Swarovski ATS 80 spotting scope with me, with its superb 20-60x zoom lens and incredible optics.)

Don't the tags and bands hurt the birds?

No. They are designed to be as innocuous as possible. Birds' "hands" are connected to their shoulders by a flap of skin (called a propatagium or simply patagium) that makes up the front edge of the wing. The flap is relatively thin, contains no muscle and only a few blood vessels. The tags are attached by a small piece of nylon sticking through the patagium. The ends of the nylon pin are melted to hold the tag on (with washers in place to decrease abrasion on both sides of the wing), and the tag sits on top of the wing. The tag does not interfere with any movement and does not pinch or rub any skin. Crows often preen the tags into place just like a feather. When I pierce the patagium to attach a tag the crow usually does not even flinch. They get more agitated when I measure their tail than when I stick in the pins.

Such obvious markers seem like they must increase the likelihood that predators will attack the crows or that other crows will shun them, don't they? To the best of my ability to detect it though, neither thing appears to be true. Unfortunately I cannot make direct comparisons on the survival of tagged and untagged crows because survival cannot be determined without marked individuals! I can say, however, that survival of tagged crows is extremely high. Fully half of all young crows that I mark in the nest are alive one year later. That may seem like a low survival rate, but in fact it is one of the highest known for birds! Breeder survival is on the order of 93% per year, again an incredibly high survival rate for birds. (See McGowan 2001 for published survival rates.)

Crows with tags do not appear to be at any disadvantage in relations with other crows either. Again, it is impossible to have comparison data on unmarked crows, but tagged crows are accepted perfectly fine into their family units. They do not seem to be more involved than unmarked crows in fights and chases in big flocks (either as the pursuer or pursuee). And, they DO successfully compete for breeding spots. I have had over 75 tagged individuals successfully become breeders in my study.

No scientist manipulates their study subjects in any way without a great deal of thought and concern. Every researcher at a public institution in this country must have their proposed protocol approved by their Institutional Animal Care and Use Committee (my approved protocol number at Cornell is 88-210-04). But quite apart from the regulations and laws, good science requires that animals under study are interfered with as little as possible. And that is on top of the personal ethical decisions that each researcher must make about their comfort level with any action taken (or not). In my study I am interested in keeping the crows alive and visible, and if I knew that anything I was doing adversely affected the birds I would stop. I am fortunate to be able to state that the colleague who taught me the marking technique I use is very active in the Humane Society and People for the Ethical Treatment of Animals. Although I made my own evaluations, it seemed to me that her standards were likely to have been even more stringent than my own. I reasoned that if she was happy with the technique, then I probably would be too. And I have been.

What do the different colors and letters on the tags and bands mean?

The colors of the tags represent different years, as does the specific arrangement of the colored leg bands relative to the metal band. The combination of letters indicates the specific individual (its NAME), as does the specific sequence of leg bands.

All crows marked in the study (except those banded in 1989) received two wing tags, colored leg bands, and a USFWS aluminum band. Each individual crow banded before 1998 got tags with a unique two-LETTER (no numbers) combination (same on both wings) and a unique sequence of colored leg bands. Starting in 1998 I began to use a combination of LETTERS AND NUMBERS on the wing tags (I had used up all possible two-letter combinations). Note that "zero" is used, and to distinguish it from "O" the letter, the zero has a small slash inside. The first character was a letter and the second a number from 1998 into 2001. Starting in 2001 the number came first and the letter second. I used up all available number-letter combinations in 2003 and had to start on two NUMBERS. (I'm not sure what I'll do when I use those up.) I did not number them consecutively, but rather chose to spread out the numbers within a family.

Until 1998 when I started repeating tag colors, each year class had received a different color of wing tag. I now have used all my available tag colors and am repeating them. So far I have an 8 year gap between colors, and I rarely have had a tag last 8 years. Each year cohort still gets a different arrangement of the colored bands relative to the metal band. I alternate dark tags with white letters and light-colored tags with dark letters each year. The dark tags all tend to look similar, appearing white at a distance. Differences between year classes can be seen in the amount of wear of the tag more readily than in the color. Recent tags (one or two years old) are bright in color and look neat and sharp on the edges. At about 3 years of age the tags start to get a little frayed at the edges. By four years many tags are severely frayed and many have fallen off (although I have taken steps in the last 3 years to minimize this problem). The tags were made of herculite, which is a light plastic covering a nylon mesh. In 2003 I changed to Cooley, a similar material. As the tags age they crease, cracking the plastic and allowing the white nylon to show. Sometimes this wear can result in white lines of dark tags that have no relation to the painted letters. Reading old tags is something of an acquired skill. Note that all crows originally received a tag on each wing and four leg bands (at least one on each leg), but some are missing one or more of each.

The leg bands are read from the top of the leg to the bottom, the bird's right leg first, a dash to indicate the change of legs, then the left leg. The color designations are W = white, B = "blue" or dark blue, Y = yellow, L = "lime" or light green, O = orange, P = purple (sort of a lavender), R = red, A = "azure" or light blue, F = "flesh" or light pink, G = "green" or dark green (not used anymore), S = "silver" or the metal band. So the little guy in the photo on the right is WS-YO. As the bands age, unfortunately they change color somewhat. W, Y, and F converge on a dirty white. A used to become rather white as well, but now it turns light green. The other colors stay fairly true, but R can fade to resemble O. Some old birds have lost a few colored bands, and the oldest (up to13 years old at the time of writing this) have lost most of them.

The specific age classes of tags and bands are as follows (in the leg band key, S is the metal band, C represents a colored band, and the dash "-" is as explained above):

If I see a tagged crow, do you want to know about it?

Yes! Reporting sightings is useful to me for several reasons. If a tagged bird is reported I know that one of my birds is in that location (which I may or may not know about). If the tag is read and the bird identified (as explained above), then I know that that individual was alive at that time and where it was. I routinely look for tagged individuals and try to keep track of who is where. I have marked about 750 crows over the study, however, and I cannot find them all. Crows can travel large distances (at least to Pennsylvania, West Virginia, or Boston from Ithaca), and there are a lot of other crows out there, so reports from other people have been very valuable. I may or may not have recently seen the crow you report, but I will always be interested in hearing about it. Through reports by others I have found out that some young Ithaca crows spend the winter in Pennsylvania (even though siblings from the same nest of one stayed with the parents on territory all through the winter, and the PA wanderer came back to help its parents during breeding season), and that some have dispersed as far as Geneva, NY to breed. Volunteer sightings have allowed me to calculate (minimum) survival data, data previously unknown for these two species, perhaps the least studied of all North American game birds.

What kind of information do you want?

The most important bits of information I am interested in are WHO you saw (letter combination, color of tag, leg band colors and sequence), WHERE you saw it, and WHEN you saw it (date and time). I also would like to know how many other crows it was with, how many were tagged, what were they doing, and any other information about their behavior or anything else interesting about the sighting. I would appreciate having your name and some way to contact you if I have further questions. Also, if I can I will provide you a little bit of information about the life of that particular crow.

I saw a crow with white in its wings. Is that one of yours?

Maybe. All the dark tags with the white letters, as well as the white tags (and some of the colored ones as well), look white at a distance. The pattern should be on the "shoulder," in the same position that the red is on a Red-winged Blackbird. In flight, the pattern will be on the body side of the wing, rather near the front edge. If you don't see the white like this, but only in the "fingers" of the wings when it flies, it is probably not a tagged crow, but rather one of the relatively frequent abnormal crows that turn up. Check out my discussion page on this topic for more details.

Why do birds legs' not get frostbite? - Biology

Project title or topic of activity

Author(s): Lisa Davidson, Elizabeth Simon

Date : Fall 2000

This activity is geared towards encouraging the conservation of marine communities through exposure to marine birds, organisms who are integral to the ecological web of marine life. Specifically, students will learn about several evolutionary adaptations of marine birds and how these unique characteristics play into basic survival. A portion of the lesson will also focus upon specific birds and exactly how they function in the ocean. This will be done through activities that will help the kids become more aware of the need to do their part to protect these birds. Through listening, inquiry, and hands-on activity, the kids will consider whether protecting the habitat of aquatic birds is as important as saving a better-known animal, such as the dolphin. Finally, the ecological importance of marine birds will be discussed as well as protection and endangerment issues. The station will provide information about how humans negatively affect the lives of sea birds and what can be done to protect them.

This activity is geared towards 3-5th grade children and can accommodate groups of 15 to 45 students.

Background information

Marine birds do not get wet when they enter the water. All birds have what is called a preening gland. The preening gland secretes waxes and fats that a bird spreads throughout its feathers in order to make itself waterproof/insulated. Birds also have powder downs, special feathers made of keratin that break into small dust-like pieces. This dust is spread throughout the feathers, aids in waterproofing the bird (because keratin is waterproof).

Many marine birds have what are called salt glands. Because ocean-bound birds often have no choice but to drink salt water, they need a special mechanism by which to evacuate extra salt from their systems. Salt glands concentrate salt from blood in an area near the sinuses. The bird then can rid itself of excess salt by "sneezing" the salt out. Some non-marine birds have facultative salt glands. When these transient, migrants drink salt water, their normally atrophied salt glands increase in size allowing them to excrete extraneous salt, as needed. The majority of the fresh water that marine birds need comes from their prey.

Many predatory sea birds, such as penguins and cormorants have bills with curved projections at the tips that help to direct fish towards the esophagus. The different lengths and curvatures of shorebird bills determine which prey they can reach by probing in the sand. Differences in bill dimensions influence the rate at which food can be eaten.

Pelicans, cormorants and frigate birds have a distensible pouch between the branches of the lower mandible that they use to capture fish. Pelicans dive and scoop fish up in their pouched bills and drain the water before swallowing their catch. Cormorants pursue fish under water, seizing their prey with their hooked bills. Anhingas spear their fish. Frigate birds steal food from other fish-eating birds. Flamingos have beak lamellae that filter small organisms out of the water. They can eat small invertebrates and even blue green algae. Long billed, long legged birds wade in shallow water or along the edge of the water using their bills to probe in the mud or sand to pluck prey items out. Black skimmers skim the surface of the water to catch fish. Penguins dive to great depths to get their meals while terns and gulls will drop from a vantage point in the sky to catch a fish near the ocean’s surface.

There are several lengths of legs and types of feet found on sea birds. Those birds that spend most of their time on the ocean usually have short, stocky legs and palmate or totipalmate feet (partially webbed or totally webbed). The short legs work well as "oars" and the webbed feet work great as the paddle at the end of the oar. Birds that do a lot of swimming have counter current exchange in their feet and legs. Because ocean water can be very cold and even damaging after extended exposure, marine birds need to compensate for the fact that a lot of heat is lost through their feet to the surrounding water. Birds use counter current exchange to warm the cold blood returning from the feet back up. Counter current exchange works by having the arteries pass close by the veins. The warm blood that is in the arteries heats the cold blood in the veins so that it is not exceedingly cold when it reaches the core of the body.

Tube nosed birds have great noses for smelling food—petrels, albatross and shear waters can smell food for up to 30 km!

Birds, in this case aquatic birds, play an essential, and often overlooked role in the ecosystem. They help to keep the ecosystem at a natural equilibrium state by helping to consume the large population of fish in the oceans and lakes, are able to assist in the dispersal of seeds to new environments, and most importantly keep us awe of their beauty and grace. However, it is astounding how quickly their presence can be taken away from us if we infringe too greatly upon their environment. Five examples of humans disturbing their environment include loss of habitat because of human invasion, unnecessary deaths due to by-catch, oil spills, disturbed migration patterns because of global warming, and loss of predatory instincts.

Ecologists worry that oil spills in the ocean will affect fish and other organisms beneath the surface. Oil spills can also have devastating effects upon organisms above the surface. One of the most poignant examples of birds being hurts by oil spills, is that of Exxon plant oil spill in New York Harbor on January 1, 1990. "In all, over 600 wintering birds were killed outright from the spill" (Birds). The birds’ feathers soak up the oil to the point that the birds’ wings are so heavy that they are unable to fly away or even move well. As the oil continues to soak into their feathers, the birds lose the ability to fight off the cold and eventually freeze to death on the water. In addition, "Birds, who preen, and therefore ingest oil, will have membrane damage and dehydration" (Birds).

Loss of habitat and predatory instincts due to human invasion are essentially interrelated. When birds become too dependent on humans, they will lose their ability to obtain food for themselves. One poignant example of this is that the ducks on Lake Chatauqua in Western New York State do not fly south for the winter. They remain on the lake through the coldest depths of winter because they know that the residents will continue to feed them bread every day. If there were ever a period when the people stopped feeding these ducks, the ducks would most likely not know how to fend for themselves and die.

Another danger that seabirds face is death due to entanglement in fishing nets--in other words, becoming by-catch. Death often occurs because the birds see bait dangling from fishing lines and lurch for it. "In fact, in the Southern Hemisphere, it is estimated that more than 40,000 albatross are hooked and drowned every year after grabbing at squid used as bait on longlines being set for bluefin tuna" (The World’s Imperiled Fish). Many sea birds are also killed because they get tangled up in long drift nets, which are pulled through the water and succeed in catching anything in their path.

In recent years, an increase in global temperature, linked to the increased emission of fossil fuels, has been blamed for a decline in the population of sea birds. Global warming has caused both a rise in the average temperature of the open ocean, as well as a melting of the ice caps at the two poles. The warmer water in the open ocean has caused a decrease in the plankton population, which has significantly impaired the diet of seabirds. The melting of the ice caps at the poles means that birds who have depended of the ice environment (a source of algae) are needing to find new ways to obtain food (Climate change harms ocean life).

Credit for the activity

Parts of this lesson plan were modified from a lesson plan on the teacher’s guide web site,


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