Answer: Extremely, but not permanently.
Migratory hummingbirds, notably Rufous and Ruby-throated, lay on huge amounts of fat in preparation for the more challenging legs of their journeys. Ruby-throateds crossing the Gulf of Mexico in spring* may double their body weight before departure from their tropical wintering grounds and at important stopover sites along the way. Research by Robert Lasiewski (The Auk 64, 1962) indicates that one gram of fat (more than a third of its lean body weight) should carry a Ruby-throated over 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+ miles] with energy to spare.
The fattening process begins when changes in day length initiate changes in the brain that send migratory hummingbirds into a feeding frenzy. They take in far more calories than they need for daily survival, storing the extra energy in the form of fat. The fat is burned off during long-distance flight and must be replenished during stopovers where food is abundant. Migrating hummingbirds can put on fat so quickly that they usually spend less than a week at any one stopover site.
Once a hummingbird reaches its winter or summer 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. The concern about overly sweet feeder solutions causing liver disease in wild hummingbirds seems to originate from observations of captive hummingbirds, which often develop liver disease as fatty deposits infiltrate normal tissues and impair the organ’s functions.
A hummingbird ready to migrate looks like a little butterball, with an undersized head perched atop a barrel-shaped body. In the hand during banding, we can see the fat by gently blowing aside the breast feathers. A bird that has added 50% 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.” To banders, fat hummingbirds are a welcome sight. It means they’re finding the resources they need to make their epic journeys.
* Not all Ruby-throateds cross the Gulf of Mexico in spring migration, and a growing body of evidence suggests that very few return south by that route. I was deliberately wishy-washy about this issue in the field guide to avoid upsetting a few members of the hummingbird community who believed that the trans-Gulf route is as important in fall as in spring. Huge concentrations of Ruby-throateds along the central Gulf Coast of Texas in fall migration (but not spring) and multiple recaptures of banded birds due west of banding locations in the southeastern states have helped to change the minds of many former proponents of fall trans-Gulf migration.