Only caged hummers risk obesity

Can hummingbirds get fat?

Hummingbird expert Sheri L. Williamson answers this question on her blog, “Life, Birds and Everything.” The author of A Field Guide to Hummingbirds of North America (Peterson Field Guide Series, 2002), Williamson writes that migratory hummers, including the Rufous and Ruby-throated hummingbirds, prepare for their migratory treks by socking away “huge amounts of fat.”

“A hummingbird ready to migrate looks like a little butterball, with an undersized head perched atop a barrel-shaped body,” she writes, adding, “a bird that has added 50 percent or more to its lean body weight will have fat bulging out of the hollow of the throat, under the wings and in the abdomen. These fat deposits often expand beneath the skin to cover the entire chest and throat, creating what we call the ‘Wall O’ Fat.’”

She cites research that found Ruby-throated hummingbirds can double their weight before they depart tropical wintering grounds to return north, and they will stop along the way to fatten themselves. But 1 gram of fat - more than a third of the lean body weight of a ruby-throated hummer - could power it in flight for more than 300 miles.

“Two grams of fat would be enough to make a nonstop flight from the northern coast of the Yucatan Peninsula to the Gulf Coast of the United States [450 to 600-plus miles] with energy to spare,” she writes.

Williamson says changes in day length appear to trigger a freeding frenzy. The fat is burned off during long-distance flight. And once the bird reaches its destination, its appetite and weight return to normal.

“Only in captivity do hummingbirds have a tendency to become permanently overweight and suffer as a result,” she writes.