The month of November has brought on decidedly cooler temperatures. Admittedly, these recent thermometer readings are not at the subfreezing point of latitudes further north, but there is enough of a drop to cause a change in wardrobe.
Cutoffs, tee shirts and flipflops are being replaced by heavier clothes and shoes. Even visitors from up north, both human and otherwise, are prepared for maintaining their body at a comfortable thermal level in the open environment.
While people can wear two or more pairs of socks and most animals seasonally increase their layers of fur on their legs, many birds are different. If bare legged during the summer, they are bare legged in the depths of winter.
Like many creatures in the wild, avians grow additional insulation during the autumn and winter months. The feathers and down may keep the bird’s core temperature above 100 degrees, an especially useful fact during freezing nights in panhandle Florida.
Bare legs in November are painful enough, but a dip in cold water could be absolutely excruciating. Birds seem not to mind, and it is all because they are equipped with lower limbs which function well at near freezing temperatures.
The featherless legs remain viable during the harsh weather through several design features common to birds. Without these physical qualities, it is likely most, if not all, birds would lose their lower appendages.
Under the leathery skin, bird legs are mostly bone and tendons with a minimum of muscle tissue. Therefore, the cells in that region of the body contain very little fluid in the form of blood or glucose.
The proportionately sturdy bones and strong tendons are critical to the bird’s ability to perch at a variety of sites. Effective hunting, observing and sleeping are all dependent on the ability to stably and securely rest on branches and other places.
A complicated circulatory system is necessary to maintain enough heat in the little blood in the bird’s legs and feet. Known by the scientific name Rete mirabile, which means wonderful net, the adaptation keeps the lower extremities just warm enough to remain functional and protect the remainder of the body from excessive heat loss.
This heat exchange system, which interlaces the arteries and veins, cools the blood moving into the feet and legs and warms the blood returning to the main body. The speed of the bird’s circulatory system is so fast that the little fluid which does pass through its feet does not have time to freeze under average winter conditions.
The arterial system works in combination with the relatively few nerves in the leg region to give the bird the ability to function in severely cold surroundings. Their metabolic rate is far higher than humans so while they stay warmer, they must constantly forage for the fuel to maintain life.
Behaviors which mitigate the effects of cold are also important techniques for assuring complete survival during the cold days and nights. While they may vary some by species, they are common among most birds.
Standing on one leg while withdrawing the other into the feathers and down creates an opportunity to balance the chill. Sitting on the legs in a protected or sheltered spot will provide even more warmth to the legs when necessary.
Fortunately, these time-tested methods are effective, and birds are plentiful in numbers and varieties. Without them the Thanksgiving turkey might never have existed.
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