How Birds Stay Slim

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A recent article in the New York Times posits an idea as to why people get fat; it isn’t their fault. Instead of blaming food choices, lack of exercise, slow metabolism, or simple gluttony, the authors believe that the availability of “junk food” makes us fat. High calorie fast food, snacks at the movies and sports venues, and vending machines all make unhealthy food much too tempting and easy to obtain. As a result, 40% of us in the U.S. are deemed obese. There is no consensus as to why these foods make us fatter, but scientists seem to agree that processed foods, rather than whole foods, share the blame, especially carbohydrates.

House Sparrow

Search Google for the term “bird obesity” and you’ll get 27 million hits, but virtually all of them refer to pet birds – fat parrots and chunky cockatiels, or to poultry. It’s easy to see why confined birds, which are fed and don’t exercise much, get fat. But what about wild birds at bird feeders, eating high-carbohydrate seeds such as millet with 6 grams of protein and 36 grams of carbohydrate per quarter cup and 374 calories per 100 grams? Sunflower seeds contain 5.5 grams of protein and only 6.5 grams of carbohydrate and 584 calories per 100 grams. (One ounce = 28 grams.)

Recent research examines the metabolism, diet, and activity of birds, factors that determine their weight. Bird metabolism is fairly high so they use energy at higher rates than mammals. Larger birds lose less body heat because of their size, and so use less energy (per unit of body weight) than small birds. For example, Trumpeter Swans use 46 calories per day per kilogram (2.2 pounds = kilogram) of body weight; White-crowned Sparrows use 346 and hummingbirds 1600! Most birds at your feeder are relatively small, so their basic metabolism rapidly uses the food energy they consume. Plus they are active, feeding, flying, and avoiding predators. Bottom line, any wild birds that are obese and move more slowly will be at a disadvantage; the evolutionary process will weed them out.

But there are times when birds do put on some weight, such as before migration. In order to undertake such a challenging journey, birds eat as much as they can in anticipation of flying long distances and perhaps across large bodies of water. Perhaps the most striking example is that of the “dough birds.” The Eskimo Curlew, now probably extinct, was once an abundant shorebird which bred on the tundra of western arctic Canada and Alaska, migrating to the pampas of Argentina in the late summer and returning in February. They were hunted for food and when shot they hit the ground and often split open, revealing their enormous stores of fat which gave the appearance of dough.

And young birds, nestlings or fledglings, tend to have a lot of fat until they are able to adequately fend for themselves.

Is there a lesson here for us pleasingly plump folks? My doctor’s advice: “eat like a rabbit and run like a deer.”

7 thoughts on “How Birds Stay Slim”

  1. When migrating birds feast on the fruits of non-native plants, they are eating junk food. Calories come from carbs, not protein, and don’t serve the birds well on their journey.

    1. Two responses. When migrating birds move, especially over long distances, they will eat a variety of foods not familiar to them, but that doesn’t make it junk food. Also, calories come from both carbs and protein, 4 calories per gram for each, whether carb or protein. Fat has 9 calories per gram. Birds need all three on migration, especially fat.

      1. Thank you for your comment, Dr. Lederer. I neglected to include “fat” in my post!

        In teaching others about the value of native plants, I use this wording of “junk food” to help them see the advantages of planting natives for wildlife.

  2. Love this post. It was very informative, with words of wisdom from the lives of nature and especially the birds well expressed through a good doctor and shared through your pen( So to speak). So interesting to know the ‘calorie count’ So to speak, consumed by larger vs smaller birds at the feeding stations. We cannot risk over feeding, because they are always always active. Yes, good lesson for us,too.
    Thank you.

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