The fall bird migration is a protracted spectacle extends well into November or beyond. The presence of numbers of sandpipers and plovers on the mudflats and the scarcity of Tree Swallows and Barn Swallows tells us that the fall migration is already well underway.
In contemplating migration, I continue to be amazed by the ability of birds to fly such long distances. Sure, bats and insects can fly but none can hold a candle to birds when it comes to feats of flying.
We all know about many of the features of birds that allow them to master the air: hollow bones, light yet strong feathers for producing lift and thrust, and streamlined bodies. However, other adaptations for flight are subtler and perhaps unexpected. The various organ systems for birds all contribute to make a bird a consummate flying machine.
Let’s start with the urinary or excretory system. The function of any excretory system is to rid the body of nitrogen-containing wastes from the breakdown of proteins. The most common waste product is ammonia, a toxic material.
For fish and invertebrates that live in freshwater or the sea, it’s pretty easy to get rid of the ammonia by producing a large quantity of dilute urine. Water is not a problem for an aquatic organism.
Humans and other mammals can’t use this same mechanism. We would have to essentially spend our lives drinking water and urinating to flush the toxic ammonia from our bodies.
To solve this problem, we convert ammonia to a substance called urea. Urea is toxic only in very high concentrations and can be dissolved in water. So, the problem is solved for mammals. By converting ammonia to urea, our kidneys can concentrate the urea and get rid of it with a moderate amount of water.
This method of removal of nitrogen waste does not work for birds. Many of the avian adaptations for flight involve making the body as light as possible. If birds produced urea, they would have to carry around an unacceptably heavy load of water to flush the urea from their body.
Instead, birds convert their ammonia wastes to a compound called uric acid. It takes more energy to convert ammonia to uric acid than to urea. However, the cost is worth it for birds because uric acid is non-toxic and also does not dissolve in water.
Birds therefore get rid of their nitrogen waste by using only enough water to push the paste-like uric acid down the excretory system. The white center in bird guano is uric acid.
Let’s consider the digestive system. Living birds do not have teeth. Rather the grinding of food is accomplished by the gizzard, the second of two stomachs of a bird. How can this arrangement contribute to flight?
First of all, teeth are heavy. Particularly for long-distance migrants, a few tenths of a gram can make all the difference. Secondly, teeth would make it difficult for a bird to keep its head in the proper position during flight. A bird’s head needs to maintain a particular position to be aerodynamically efficient.
By having its “teeth” in its gizzard, a bird can lower its center of gravity. The position and weight of the gizzard enable the bird to maintain an efficient posture during flight.
The demands of flight can be seen in the reproductive system. During the reproductive season, the male reproductive organs or testes are quite large. However, once the breeding season is over and migration begins, the gonads of the males shrink to less than 1% of their breeding season weight. That is some weight savings! The difference between the weight of the gonads of House Sparrows between the breeding and non-breeding season is 500 times.
Most female birds have only a single ovary. Exceptions are most hawks and some pigeons and gulls that have a pair of ovaries. The ovary swells during the breeding season and then regresses dramatically during the non-breeding season to reduce the weight of a flying bird.
Finally, we can consider one aspect of the skeleton in birds: the shape of the sternum or breastbone. In flying birds, the ventral side of the sternum has a large sail or carina. This carina serves as the attachment point for the two muscles that raise and lower each wing during flight. Most of the power generated during flight occurs during the downstroke and this muscle is the larger one.
The breastbone of ostriches and other non-flying birds is flat like our breastbones, strongly suggesting the carina is a structure that evolved to facilitate flight.
[Originally published on August 21, 2011]