The spring bird migration is coming to an end. The last species to arrive are usually black-billed cuckoos, yellow-bellied flycatchers, blackpoll warblers, salt marsh sparrows and Nelson’s sparrows.

We all marvel at the ability of birds to fly thousands of miles to nest and then return to their wintering area each year. These Herculean feats become even more astounding when we consider the physiology of birds.

Like mammals, birds are endotherms; they maintain a constant body temperature in the face of changing environmental temperature. All endotherms have a minimum idling speed, called the basal metabolic rate (or BMR). You are likely at your BMR as you read this column.

The BMR varies inversely with size. A hummingbird or a mouse has a relatively high BMR while a hippopotamus or ostrich has a lower BMR. But here’s the kicker – a bird has a higher BMR than a mammal of the same size. The fire of life burns brightest in the birds.

Maintaining the BMR requires energy to produce heat. Those costs become more severe at colder temperatures.

Consider what happens when you push your body to its limits, like running a 400-meter dash. You breathe heavily, your heart rate increases and your metabolic rate rises. For humans, maximum performance results in about a doubling of your BMR. Other mammals may be able to triple their BMR.

Birds outdo mammals by a long shot. The demands of powered flight are huge. A migrating bird increases its metabolism to 8 to 10 times its BMR. That is one blazing furnace.

The ideal fuel to keep the metabolism high should pack the most energy per unit weight. Fats are the fuel of choice for birds. Breaking down a gram of fat produces about twice the energy of a gram of carbohydrate or gram of protein. The burning of fats also produces water as a breakdown product, which can be used by a migrating bird. This metabolic water is a necessity for birds migrating over the Gulf of Mexico or other bodies of salt water.

How much fat should a bird carry on a migratory leg? Obviously a bird needs enough to fuel its flight, particularly if the journey is over water. But carrying extra weight means that the bird has to work even harder to fly. So there is an optimal fuel load that ensures completion of the flight but leaves little fuel in the tank.

How well are birds able to find the optimal fat load? Semipalmated sandpipers provide a nice example. During the fall migration most depart from the Bay of Fundy, flying over the ocean to reach Suriname in northeastern South America. This flight requires about four days of sustained flight.

Before departing, the semipalmated sandpipers gluttonously feed on the abundant crustaceans in the expansive intertidal Fundy mudflats. Over two weeks a bird doubles its mass from 20 grams to 40 grams or more.

Shorebird biologists in Suriname wait for the arrivals of the sandpipers. Some are captured and their mass is recorded. Most are at or close to their lean weight of 20 grams.

Ovenbirds provide a nice example of fat dynamics on a short time scale. Like most songbirds, ovenbirds start a migratory leg in the early evening. Some banding stations keep their nets open during the night, capturing migrating songbirds as they land. The data indicate that ovenbirds captured around midnight, after a short flight, weighed about two grams more than ovenbirds captured around dawn after a longer flight. With a lean mass of around 18 grams, some ovenbirds therefore burn 10 percent of their body weight in a single night’s flight. We have some good evidence that those lighter birds linger longer than fatter birds before undertaking the next leg of a migration.

Herb Wilson teaches ornithology and other biology courses at Colby College. He welcomes reader comments and questions at

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