When have you ever seen the words obese and superathletes linked? However, this is an apt characterization of the small migratory birds that undertake 12 to 20 hour flights across the open water of the Gulf of Mexico on their northward spring migration.
Female Black and White Warbler, normally weighs about 10.2 grams, but may bulk up to 15 grams with fat reserves to fuel a trans-Gulf migration.
Trans-Gulf migrants take off from staging areas in the Yucatan peninsula in early evening, fly all night at about 5000 feet using celestial cues for direction, and land on the southern U.S. gulf shores sometime the next afternoon — a flight marathon.
This male Indigo Bunting must have been so busy metabolically turning food into fat stores that he didn’t complete his feather molt before leaving on his northward migration.
At an average flight speed of about 30 mph, birds could travel the 600 miles across the Gulf in about 20 hours; with a tailwind, they might average as much as 45 mph and get there sooner. But they usually don’t stop on the coast, and instead fly inland to the cover of trees and shrubs where they might find the insect, or seed, or fruit resources they need to fatten up for the next leg. Bad weather and southerly head winds that prolong the flight time leave birds exhausted of fuel so that they literally “fall out” of the sky onto oil rig platforms or land on the barrier islands.
Orioles love oranges and grape jelly, but this female Baltimore Oriole apparently craves protein during her migratory stopover. She tore off the butterfly’s wings before consuming its body.
How do birds fuel this flight Marathon?
We know that a 30 gram bird like the Baltimore Oriole flying at 30 mph expends about 4-5 kcal (= 4-5 Cal) of energy per hour of flight, or about 10 times their basal level of metabolism. In flying the 20 hours to cover the 600 mile distance, Orioles would then expend about 80-100 Cal (equivalent to a cup of blueberries or a small piece of raisin bread). Each gram of fat metabolized liberates 9 Cal, so this flight marathon would cost about 9-11 grams of fat. For a 30 gram Oriole, this amounts to 30-36% of their body mass lost over the course of a day, and which must be replenished before the next leg of migration. No wonder they feed so voraciously at their migratory stop-overs.
But small birds like this male Magnolia Warbler typically put on fat equivalent to 50-70% of their non-migratory body mass! The extra fuel reserve takes them farther inland, or may make a difference in sustaining flight against strong headwinds.
Migratory birds are indeed obese superathletes — storing, metabolizing, and turning over 50-100% of their body weight on a daily basis as they undertake their long distance migration.
and how about this metabolic marvel…
Female Ruby-throated Hummingbird feeding on milkweed nectar.
Tiny 3 gram (one-third the weight of a Warbler!) Ruby-throated Hummingbirds annually make the trans-Gulf migration by doubling their body weight, loaded with 3 grams of fat. We know they cross the gulf rather than go around it because they have been found on the Mississippi coastal barrier islands, and occasionally fall out in bad weather on the oil rig platforms.