Search of the Week: “can hummingbirds get fat”

Her bulging chest show that this young Rufous hasn't missed many meals.

His bulging chest show that this young Rufous hasn’t missed many meals.

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.


5 thoughts on “Search of the Week: “can hummingbirds get fat”

  1. Great post.I especially appreciate the additional information about the Fall migration. That is interesting. I can only appreciate how important the banding is to gain ever more reliable understanding of the migratory hummingbirds.Sometimes new ideas do temporarily ruffle the feathers of established thought.

  2. Up here in the midwest, some female Ruby-throated weigh more than 6 grams when they depart in late summer. That’s almost double their breeding weight! Handling these birds is amazing: there’s SO much fat all over, they feel squishy in the hand. When they perch, rolls of flab bulge from their necks as they move their heads, and the fat store on the back looks like a dowager’s hump. As Sheri says, we’re really happy to see these tubs o’ blubber so well equipped for their journey south.

    As a feeder watcher, the first place you’ll notice fat on hummers is the deposit between the legs and vent. It bulges and stretches the skin, making the white feathers poke straight out. We call that the “lace panties” effect, and it’s a giveaway that the bird has shifted into pig-out mode.

    • Love them squishy hummers! Marshmallows with wings.

      Three grams of fat would theoretically give a Ruby-throated a range of at least 900 miles, but I’m sure the flight efficiency declines as the fat load increases. Still, a hummer that doubles its body weight (from 3 grams to 6 grams) is going to have more of a cushion (literally and figuratively) than one that adds only two-thirds (3 g to 5 g). This could make the difference between life and death if a Ruby-throated crossing the Gulf hits strong headwinds from a cold front.

  3. Have watched closely as David Holmes blew very carefully on the feathers of the Rufous female that was banded at our home on Nov. 11th, 2010. She was not blubbery. She had been nectaring regularly at our feeder since Nov. 1st. She was also hawking for insects around our late blooming shrubs and flower beds. The assessment was that she was regular weight, healthy and fit. She departed on Nov. 13th. Hope she had developed some amount of fat for migration. It was still warm weather then.
    These hummers are surely amazing.

    I cannot get this image of blubber spilling out all around the bird’s body. Lift must be quite a challenge!!!

    I had a question. Do hummingbirds have brown fat? This is fat that can thermogenerate and enable hibernating animals to come out of their long rest. Groundhogs and chipmunks for example. Baby humans have brown fat too. The hypothesis is for protecting the thoracic cavity for vital organs. Hummingbirds go into torpor if nights are cold such as in Alaska. We have an Allen’s hummingbird right now in Virginia that is surviving the winter fine. Goes into torpor each night we are presuming. There are Rufous here too.

  4. Cool about your Rufous visitor! It’s likely, considering how little fat she would have been able to lay on in just two days, that she didn’t move on very far.

    Brown fat is a mammalian specialty, so birds deal with quick access to their adipose energy reserves by some other mechanism.

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