The spring migration is coming to an end. The arrival of Black-billed Cuckoos, Blackpoll Warblers, Salt Marsh Sparrows and Nelson Sparrow’s will mark the end of the spectacle.
Spring migration is a wonderful event. We marvel at the sudden appearance of birds, dressed in their breeding finery with males singing lustily. The spring migration lifts our spirits.
Understanding the physiological demands of migration makes the spring migration even more astounding. Birds are the edgiest of creatures. By that I mean that meeting energy demands is more difficult for birds, particularly small birds, than for any other animals.
Birds and mammals are the only two groups of animals that are truly warm-blooded or endothermic in the terminology of physiologist. Endotherms maintain a constant body temperature in the face of varying environmental temperature.
Endothermy is a tremendous advantage, allowing such animals to find food and avoid predators in harsh temperatures. Living life at a faster pace promotes learning; birds and mammals are the most intelligent of all animals.
The cost of endothermy is great. Heat lost has to equal heat produced. On colder days, the internal furnace has to be stoked at significant cost. Compare a mammal and bird of the same size and you find that the bird’s thermostat is set a bit higher than the mammal’s thermostat. The fire of life burns brightest in the birds.
Small birds have a tougher time making ends meet because of the cruelty of geometry. Heat is lost across the surface of a bird’s body. Replacing that lost heat is the job of organelles called mitochondria that are found in every cell in the body. The amount of heat produced is therefore proportional to the volume of a bird.
Let’s consider a couple of cubes, one with a length of 2 inches and one with a length of four inches. Each side of the smaller cube has an area of 4 square inches. Six sides give a total surface of 24 square inches. Its volume is 2 x 2 x 2 = 8 cubic inches. The ratio of surface-to-volume is 24/8 = 3. The same calculations for the bigger cube yield a surface-to-volume ratio of 96/64 = 1.5.
You can see how a small bird is at a real disadvantage. It has a relatively large surface area where heat is a lost and a meager volume in which heat is produced. An African elephant does just fine outside in a Minnesota zoo in the winter because its surface-to-volume ratio is so small. A hummingbird would have no chance in such cold weather.
As you read this column, your body is idling at its Basal Metabolic Rate (BMR_. If you were to exert yourself as hard as you can, doubling your BMR is the best you can do. For birds, flight is ridiculously expensive. A flying bird has to increase its BMR up to 23 times!
To fuel that elevated metabolism, a bird has to pack on fuel. Fats are the fuel of choice. A gram of fat has twice the calories of a gram of carbohydrate or protein. The breakdown of fats also produces more water than other types of fuel.
The amount of fuel carried is determined by the length of a migratory leg. For Ruby-throated Hummingbirds crossing the Gulf of Mexico, two grams of fat need to be carried in addition to the lean body weight of three grams to provide enough oomph to make landfall in Mississippi.
Prior to migration, the gut of a bird enlarges to allow rapid digestion and absorption of food. But during the migration, the bird shuts down its gut to save energy. When a bird lands after a migratory leg, it takes a day or two to get the gut working again.
Given the energetic demands of migration, it is no surprise that birds wait for favorable winds to continue their migration.