The unfeathered lower parts of birds' legs have a very high surface area, just the ticket for losing heat to the cold winter air. To keep the legs as warm as the rest of the body, birds would have to pump a huge amount of heat carrying blood to those exposed areas. Instead, birds allow the legs to cool, pumping a modest amount of blood into the extremities. In Herring Gulls, the upper part of the exposed leg may have a temperature of only 40 F and the bottom of the foot is just barely above freezing!
These birds are unusual among North American songbirds in that the males are polygamous, that is a male may have many mates. It is not uncommon for a male red wing to have seven or more females in its territory and at least one male had 15 females in his harem. DNA analysis indicates that not all of the offspring produced by females on a male's territory are sired by him. That means that females sometimes engage in "extra marital affairs" with other males when their mate is not looking. In fact, these dalliances, referred to as extra pair copulations by ornithologists, are the rule rather than the exception in most species of birds in which paternity has been tested by DNA analysis.
Birds have a trick to keep their legs from losing too much heat. They arrange the arteries that bring blood to the leg and the veins that return blood to the heart as a counter current system. You can see counter current systems in many heating plants in large buildings. The furnace is situated at the center of the building. Cold air from outside is pumped into the furnace through a long pipe. The warmed exhaust air in the building is pumped out through a pipe parallel to the intake pipe. As the exhaust air moves toward the outside of the building, much of its heat is transferred to the colder intake air moving in the opposite direction. By the time the intake air reaches the furnace, the air has been warmed significantly by the exhaust air. Much of the heat of the exhaust air is recovered before the air is pumped outside. The counter current arrangement of bird arteries and veins accomplishes the same energy savings.
In the last column, I described some of the ways that birds are able to survive the challenges of cold weather. I did not have enough space to describe how birds keep their unfeathered lower legs from freezing. Several readers inquired about this problem for birds in cold environments. I'll tackle that problem this week, including a foray into some chemistry.
Outside of the nesting season, Red winged Blackbirds are well known for forming large flocks that roost together. Sometimes, other species like Brown headed Cowbirds, Common Grackles and European Starlings may roost with them. These flocks pose significant threats to agriculture. A large flock of red wings can devastate crops of corn, sunflowers or rice. Farmers have invested large amounts of time and effort in ways to keep red wings away from their crops. The result of these efforts is that humans now pose one of the most significant sources of mortality for Red winged Blackbirds.
For the Birds Cold and Unfeathered Legs
You can see the effect Discount Women Canada Goose Expedition Parka Red Singapore of these types of bonds in your kitchen. Butter is made of fatty acids with no double bonds; it is a saturated fat. At room temperature, butter is a solid. Not the condition you would want for your membranes! Olive oil, liquid but viscous at room temperature, has one double bond in its fatty acids. In other words, it is a monounsaturated fat. Safflower oil is even thinner than olive oil because it has polyunsaturated fatty acids (two or more double bonds).
By adding double bonds to the fatty acids of the cell membranes in the leg, birds ensure that their membranes remain fluid in cold temperatures. The kinkiness caused by the double bonds keeps the fatty acids from packing too close. The membranes remain fluid.
For the Birds Adaptations to the Cold
Red winged Blackbird is one of the most abundant birds in North America. On some Kansas Christmas Birds Counts, over 6 million red wings in massive flocks have been counted in a 175 square mile area! Abundant breeders throughout Maine, red wings depart for more southerly climates during the winter. Their breeding range extends northward to southern Alaska and southward to Costa Rica. In North America, Red winged Blackbirds nest in every one of the lower 48 states and all of the Canadian provinces.
At night, bird tend to roost in trees, often coniferous ones, close to the trunk. This choice of a bedroom has two benefits. First, the bird is protected from wind chill. Wind blowing across an animal body causes heat to be lost far more rapidly than when the air is still.
Red winged Blackbird males arrive on the breeding grounds ahead of the females. The dominant males stake out territories that they proclaim as their own with songs and displays. One of the most common territorial displays is called the Song Spread. The male exposes the red patches on its wings and sings is metallic con ka ree song. Younger and weaker males are usually not able to establish a territory and breed.
Con ka ree! The Red winged Blackbirds have returned; spring must be on the way. The March arrival of these beautiful birds is a welcome sight as a Maine winter starts to give way to spring. The males are all black except for a brilliant red patch, edged in yellow on each wing. The smaller females are drab and inconspicuous, in their mottled brown and white plumage.
Red winged Blackbirds breed in marshes and other areas with dense, grassy vegetation. The males perch on shrubs and the tops of cat tails. Red wings are therefore easier to see than woodland birds. The abundance and high visibility of Red winged Blackbirds make them good subjects for ornithological study. Not surprisingly, they are one of the most studied North American birds.
Birds can maintain their body temperatures in the face of severe cold in three basic ways. First, they can try to reduce the loss of heat. The plumage of birds in the winter is often twice as heavy as the weight of the summer feathers. In addition, the feathers of birds can be raised to create improved insulation. The erected feathers trap pockets of air. The insulating quality is improved by lowering the density of a material (light wool is a better insulator than dense metal, for instance). I am sure you have seen Mourning Doves and other birds at your feeder that are puffed up greatly to increase the insulating properties of their feathers.
2007 March Maine Birds
Winter arrived with a vengeance over the past three weeks. From the warmth of our houses, many of us marvel at the ability of birds to survive the challenges of winter weather.
Why do most birds not have feathered legs? The answer lies in the fact that a bird's metabolism must be raised to provide the energy for flight. Excess heat is produced by this rise in metabolism and must be eliminated, even in the coldest weather, so a bird doesn't overheat. The unfeathered legs are the sites where that heat is lost.
[Originally published on March 17, 2007]
The rate at which heat is lost from a bird body in the winter depends on the difference in temperature between the bird body and the environment; a bird will lose more heat on very cold days than on mild days. Small birds have more of a disadvantage than larger birds. A bird loses heat based on its total surface area, the portion of the body that is in contact with the cold air or water. The heat produced by a bird is proportional to its total volume; every cell in the bird body is capable of producing heat. A result of simple geometry is that the ratio of the surface area to volume decreases as an object gets bigger. This geometric rule is cruel for small birds like Golden crowned Kinglets because they have a relatively large surface area over which heat is lost and only a modest body volume to produce heat. A kinglet is living more on the edge than a jay or crow because of its large surface/volume ratio.
[Originally published on March 3, 2007]
Similar behavior can be seen in schoolyard bullies. A 10 year bully adopts a swagger to try to intimidate smaller children, uncovering his behavior or However, in the presence of a 13 year old bully, the 10 year old bully alters his behavior, covering his so as not to offend the bigger bully.
When you see a male Red winged Blackbird at your feeder or in a marsh, you may not be able to see the red on the wings. Sometimes, even the yellow margin of the red patch is not visible. The red patches are called badges by ornithologists. When a dominant male is approached by another male attempting to drive the first male off of its territory, the territory owner will expose its badge. This aggressive display tells the intruder to back off. Subordinate males make sure they don uncover their badges in the presence of dominant males to avoid being attacked.
For most birds only a portion of the legs is unfeathered. The long bone you see without feathers is called the tarsometatarsus. This compound bone corresponds to the bones of your foot. The tibia (shin bones) and the femur are usually well feathered. So, it's the elongated foot bones and toes of birds that have no insulation.
The females arrive a week to several weeks later than the males. They then choose a mate based not on the characteristics of the male but rather on the quality of the territory. Usually, the healthiest and strongest males lay claim to the best territories. Only seven percent of second year males are successful in establishing a territory and siring young. The percentage rises to 57% for males in their third year of life.
The body temperature of birds is normally between 104 and 109 F; the fire of life (the rate of metabolism) burns hotter in birds than in any other animals. Birds must stoke their internal fires with more food than mammals of similar size whose body temperature may be only 100 F or so. To say that someone like a bird really means that person is a ravenous glutton!
Interestingly, the color red seems to signal a willingness to fight in other species of Maine birds. Male Ruby crowned Kinglets expose the red on their crowns only as an aggressive behavior. Similarly, Eastern Kingbirds flash their red crown feathers as warnings to other kingbirds during the nesting season.
Allowing the tissues of the lower leg to get so cold requires some fascinating modifications of the cells of those body tissues. Every living is covered with a membrane. This membrane is mostly made of molecules called phospholipids. Each phospholipid has two strings of carbon atoms, called fatty acids, pointing toward the center of the membrane. To function properly, the phospholipids of the membrane need to be able to move past each other. The membrane needs to be fluid enough to allow small molecules like oxygen to diffuse in and carbon dioxide to diffuse out. When a membrane is exposed to colder temperatures, the phospholipids pack in tightly. Oxygen cannot get into the cell; frostbite and often cell death occur.
Each carbon atom in the fatty acids of a phospholipid has four binding sites. Usually, two of the binding sites are used to bond to the two adjacent carbon atoms. The other two binding sites usually bind to a hydrogen atom. However, it is possible for two adjacent carbon atoms to each lose one of their hydrogen atoms and establish two bonds with each other. When these double bonds occur, they introduce a kink in the fatty acid.
For the Birds Red winged Blackbirds