Birds "Transform" Themselves in Preparation for Migration

As fall approaches, millions of birds across North America prepare for their migration to warmer climes. Many species fly for thousands of miles, often without stopping.

At their baseline state, migratory birds are already equipped with evolutionary tools such as hollow bones and optical compasses and lungs that provide purer oxygen than is breathed by any mammal. But when dwindling sunlight triggers their migratory physiology, they undergo many more fascinating changes.

In just a few days, their food intake rises by multiple orders of magnitude — the equivalent of having a hamburger for lunch on Monday, and 100 hamburgers on Friday. Even seed-eaters switch to insect-heavy diets rich in energy-dense polyunsaturated fats.

The fat is packed on aerodynamically, tucked in the lower back and wherever else it won’t add much drag. Some species, such as warblers, which can weigh less than an ounce but fly 2,500 miles without resting, double their body weights in preparation for the voyage.

In order to keep pace with the dietary influx, the birds’ digestive organs expand. Even the cells of their stomachs swell. But shortly before takeoff, with no more need for this extra bulk, their guts shrink back to size.

In the meantime, the birds’ pectoral muscles become thicker and denser. The pecs of the red knot, a shorebird that makes 2,000 mile-long migratory flights, swell by 40 percent.

Much of this added muscle mass will be burned for in-flight energy, but most of their fuel comes from fat. Unlike mammals, who fuel endurance exertion with protein and carbohydrates before switching — a transition felt as “hitting the wall” — birds start by burning fat, and use only the minimum of protein needed to keep their brains running. They never hit the wall.

To better turn fat to fuel, their bodies boost production of fat-metabolizing proteins. In sparrows, levels of the proteins double from their usual rates. To further feed their cells, extra oxygen-carrying hemoglobin protein is pumped into the blood, right up to the limit where it would be too thick to flow.

Some species do rest during migration, stopping for a few weeks before crossing some especially vast and barren expanse, such as the Sahara. Even in this brief time, red knots’ pectorals will shrink at first, and their legs and stomachs swell. When the birds are done eating and ready to fly again, the process will be reversed. Their hearts grow the whole time.

Read more in the original article here.