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Nature: Birds' weight less than most would believe

Sunday, April 28, 2002

By Scott Shalaway

The body weights of birds are not something I have cataloged in my short term memory, but I know that birds weigh much less than most people suspect. A recent e-mail reminded me of this.

A reader commented that the turkey vultures roosting near her home weighed between "10 and 15 pounds." I knew that was high, but I had to check a monograph in my library entitled, Body Weights of 686 Species of North American Birds. Before I reveal a turkey vulture's average weight, take a guess. Imagine one of these large birds soaring overhead. What do you think one weighs?

The mean weight of 20 Florida turkey vultures listed in the monograph is a mere 3.2 pounds. Another source reports turkey vultures weigh up to 5 pounds.


Send questions and comments to Dr. Scott Shalaway, R.D. 5, Cameron, WV 26033 or via e-mail to sshalaway@aol.com, and catch Scott on the radio Saturday afternoons from 2 to 4 p.m. on 1360 WPTT.


Using the monograph as the data source, here's a list of mean weights for some common birds: male trumpeter swan, 26 pounds; male wild turkey, 16.3 pounds; female bald eagle, 11.5 pounds; male great blue heron, 5.7 pounds; female great horned owl, 3.4 pounds; female red-tailed hawk, 2.7 pounds; mallard duck, 2.4 pounds; male ruffed grouse, 1.4 pounds; rock dove (common pigeon), 1.2 pounds; American crow, 15.8 ounces; pileated woodpecker, 10.8 ounces; bobwhite, 6.3 ounces; mourning dove, 4.2 ounces; killdeer, 3.4 ounces; blue jay, 3.1 ounces; American robin, 2.7 ounces; hairy woodpecker, 2.3 ounces; male red-winged blackbird, 2.2 ounces; northern cardinal, 1.5 ounces; Baltimore oriole, 1.2 ounces; eastern bluebird, 1.1 ounces; downy woodpecker,27 grams (28.35 grams equals 1 ounce); tufted titmouse,21.5 grams; Carolina wren, 21.5 grams; dark-eyed junco, 19.6 grams; American goldfinch, 13 grams; house wren, 11 grams; black-capped chickadee, 10.8 grams; yellow warbler, 9.5 grams; ruby-throated hummingbird, 3.2 grams.

I suspect these numbers will surprise some readers. Birds look so much bigger, especially on a cold winter day. But birds can fluff their feathers to stay warm when temperatures drop.

Compared to mammals of comparable size, birds weigh surprisingly little. A chickadee, for example, is about the same size as a deer mouse, yet it weighs only a third as much. And a downy woodpecker and chipmunk are about the same size, but a chippie can weigh four times as much as a downy. Because birds fly, they must be light.

Birds are defined in large part by a variety of weight-reducing adaptations that promote flight. They lack teeth and associated heavy jaws. Their respiratory system includes a network of branching air sacs. They excrete concentrated urine undiluted by large amounts of water. Furthermore, birds lack a urinary bladder, so they are never burdened with a heavy organ filled with liquid waste. By expelling waste frequently (ever been around a chicken coop?), birds further minimize body weight.

Birds' sexual organs atrophy during the nonbreeding season. Why migrate or overwinter with nonfunctional gonads? Their digestion is rapid and efficient, so food doesn't stay long in the gut. Their bodies are covered with lightweight, aerodynamic feathers. And perhaps most important, bird skeletons consist mostly of thin hollow bones.

Though feathers are indeed lightweight, their sheer number contributes significantly to a bird's total weight. Counts of the contour feathers that cover a bird's body range from 940 for a ruby-throated hummingbird to 25,216 for a tundra swan. On average, a bird's feathers weigh two to three times as much as its skeleton. A bald eagle's feathers, for example, weigh about 17 percent of its total mass; its skeleton weighs only about 7 percent of its mass.

And even during the nesting season, female song birds keep their weight to a minimum by manufacturing only one egg per day. Larger birds take two days or longer to make an egg. Consequently, a female bird never flies around with an entire clutch of eggs inside its body.

So if you see a really fat robin this spring, it's not because she's pregnant. And she's probably not really even fat. She's just fluffing her feathers to preen or stay warm.

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