For the Birds Radio Program: Laura's Quick Weight-Loss Plan: Migration!
Birds don’t migrate because they don’t have enough food: they migrate when food is most abundant. They can lose significant proportion of their body weight in mere hours.
When people first studied bird migration, they assumed that birds leave the north in autumn at the time when there ‘s not enough food left. But for most birds, the timing of their departure actually coincides with a period of food abundance. Swallows, nighthawks, and flycatchers leave the Northland in August and early September when flying insects are everywhere. Most species of warblers leave long before cool nights and falling leaves make it difficult to find insects on foliage. Sparrows from the north arrive here when fallen weed seeds are most abundant, and most of them depart long before snow covers the food supply. And Sharp-shinned Hawks wing through the Northland not after their prey species have left but right as the dickey bird migration is at its peak.
So the actual trigger of migration is obviously something other than starvation. As a matter of fact, one trigger of migratory behavior is actually the very abundance of food. As summer days grow shorter, birds start putting on an incredible layer of fat. When a Ruby-throated Hummingbird is ready to migrate, it weighs in at 4.5 grams, of which fully 2 grams is fat—that is, almost half its weight. When preparing for migration, White-crowned Sparrows can increase their body weights from a starting point of 24 grams all the way up to 34 grams in three to five days time—that would be like an actor, preparing to play Falstaff, going from a starting weight of 150 pounds all the way up to 210 pounds in less than a week.
If birds have an easy time putting on weight in fall, they have an even easier time taking it off. Again, that hummingbird weighing in at 4.5 grams in New Orleans gets down to 2.5 grams 26 hours later when it reaches the other side of the Gulf of Mexico on the Yucatan Peninsula. Birds killed late at night at a television tower in Champaign, Illinois, were found to be much lighter than those that were killed earlier in the night. The birds were estimated to have lost between 2.5 and 4.4 percent of their gross weight every hour while flying. Another similar study showed that Veeries lose 1.3 percent of their body weight per hour and Ovenbirds exactly one percent of their weight while migrating. That would amount to one and a half pounds of weight loss per hour for a 150 pound person trying to lose weight while migrating.
A scientist named V.A. Tucker, studying parakeets or budgies, found that they expend 3 calories of energy with every kilometer flown. Compared with walking or running mammals of the same size, a flying bird is 10 to 25 times as efficient in energy consumption per kilometer traveled. They’re obviously more efficient than me. The 20.7 grams of fat in a McDonald’s quarter pounder with cheese is enough to fuel 10 hummingbirds on a 600 mile non-stop flight across the Gulf of Mexico, but it can barely get me through an afternoon.