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The Brood Patch
Incubating the Eggs
Bill Hilton Jr, who has banded almost 46,000 birds at the Hilton Pond Center, is holding this female robin on her back in his hand. He is blowing on her tummy feathers to show the brood patch. When no one is blowing on them, the outer feathers keep the brood patch hidden. The robin can control her skin muscles to open the feathers, so she can put the brood patch directly on her eggs for incubating.
Photo:
Bill Hilton, Jr.

Birds have feathers for insulation — to keep body heat in, and the cold and rain out. But the same feathers that hold a mother bird's heat in could keep her body's heat away from her eggs and her babies! What can she do to share her body warmth when she's incubating eggs? This lesson tells you about her special adaptation, the brood patch (also called the incubation patch). Learn how it works and how banders can look at the brood patch to get information about a bird.

A Little Background About Feathers
A bird's outer body feathers, called contour feathers, are designed to give the bird its shape and to make a barrier between the bird's inner, insulating feathers (down feathers) and the outside world. These outer feathers work like a windbreaker jacket. The inner, down feathers work like the warm sweater under the jacket, to trap the body's warmth. Birds have down feathers covering much of their bodies, but we usually can't see those feathers under the contour feathers. A tiny muscle attaches to each feather, so birds can fluff out their plumage, or even "part it" in places.

How the Brood Patch Develops
Most female birds, and some males, develop a brood patch during the breeding season. Changes in hormone levels during the nesting season start the process. Down feathers on the bird's tummy, and even some contour feathers, suddenly get very loose. In some species, those feathers just fall out. In other species the mother pulls them out. Ducks pull them out while building the nest and use them to make the nest soft and extra warm. Gulls and shorebirds often have three brood patches. Songbirds and hummingbirds have one that heats all the eggs.

When the feathers fall out, other changes happen too. The tissue in the tummy area swells. The tissues hold more water, and the blood vessels that feed the skin expand. These changes make the bird's tummy skin almost as hot as the body's interior.

Which Parent Develops a Brood Patch?
In most songbirds, only the female develops a brood patch, but in a few species the male also does. In species such as hummingbirds, robins, and jays, only the female incubates eggs. In these species only the female develops a brood patch. In a few species, such as phalaropes, only the male incubates; he's the only one who develops a brood patch. In Barn Swallows, males sometimes help with incubation, but not always. Those male Barn Swallows that do a lot of incubating often develop a brood patch.

How the Brood Patch Works
When an incubating parent sits on the eggs, the skin muscles open up the brood patch. Then the parent sits down and wiggles its body back and forth a bit. The sensitive skin feels for the eggs so the parent can settle where the skin makes best contact with the eggs.


National Science Education Standards

  • Plants and animals have life cycles that include being born, developing into adults, reproducing, and eventually dying.
  • Each plant or animal has different structures that serve different functions in growth, survival, reproduction.


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