Naturespeak A naturalist's view of the world

June 2, 2009

The Belly of a Blue-footed Bird

Filed under: Uncategorized — wykes @ 3:47 pm

Unless you are a bird bander, a chicken farmer, or just a farmer with chickens you’ve probably never seen a brood patch. This is the featherless area on a nesting bird’s belly which comes in contact with, and incubates, the eggs. In order to see it, you have to have a breeding bird on it’s back and willing to sit still. Without this crucial  little patch of nakedness there would be no baby birds (well, there would be a few, since not all birds have brood patches, but certainly not as many). So, when I came upon a freshly dead female Baltimore Oriole that had an obvious brood patch I was compelled to share it with you. Here was a breeding bird that was on it’s back and sitting still – very still. A dead bird is never a pretty sight but it is a pretty good opportunity to learn a few things.

Let’s face it, female orioles cannot compete against their dashing orange and black mates for eyeball time. That’s the way it is in the avian world.  The females of this species do have a subtle beauty all their own (see here).  Although possessed with a significant splash of orange against an otherwise greenish yellow body, blue is their color of note . The sky blue beak and perching feet (see above) are not features that show from afar, but do stand out upon close inspection.  Looks definitely take a back seat to talent, however, when one considers how these birds construct their intricately woven basket nests (see here). This spring I saw one of these nests in the process of construction (see here). The female was visible inside the still transluscent bag as she added another strand of plant fiber to the weave.

Keeping to the bird at hand, the delicate female cradled in my palm displayed a raised patch of feathers surrounding a slit-like opening over the belly (see here).  Blowing away these feathers revealed an open patch of brooding skin beneath (see below). This arrangement is very similar to the marsupial “pouch” found on the opossum. In life, the bird can close up the gap by controlling the bordering feathers just as the ‘possum shuts in her young like a coin purse. 

The bird doesn’t carry young in her fold but instead uses the bare skin to incubate her clutch of eggs. She opens the fold, settles onto the eggs,  and makes direct body contact with them – skin to egg. In this way the bird can efficiently transfer her body heat (around 100 degrees F) directly to the developing embryo inside. In the case of the oriole, the eggs are cream colored with splatters of black and they are laid in clutches of four. Constant egg turning by the female insures complete coverage and eventually the circulation of the embryo distributes the nurturing heat. This heat transfer also benefits the hatchlings as the nesting process continues on it’s normal course.

Brood patches – a.k.a. incubation patches -develop from a normally naked portion of the central belly called the apterium. This is one of those million dollar words that simply means “featherless,” and it is good to know that bird feathers grow in tracts and do not cover the entire body. Yes, all birds have narrow naked parts which are visible as the smooth skin between the goosebumps on your chicken meal! As the nesting season approaches, hormones direct the body to shed the feathers around the belly apterium. Some birds, such as ducks, have to pluck these feathers out, but our oriole need only watch as the downy feathers sluff off. Eventually, additional blood vessels begin to engorge the flesh and thicken the skin to the point where it looks like a wrinkled pad. After the season, the process reverses itself and the bird resumes a state of minimal nudity.

In some birds, both the male and the female develop brood patches, but in the oriole world only the females incubate the eggs and develop brood patches.  This brings up the question surrounding this dead female and the fate of her young. Since she is the sole incubator, her passing would mean the end of the line for her clutch. But there are clear indications that she had not yet laid any eggs. In the early stages of brood patch development, before laying and incubation actually begins, the thin skin retains the transluscent look as seen on this bird.  The skin becomes thickened and opaque only when the egg warming begins.  This oriole’s death, therefore, probably resulted only in an empty nest and, of course, a teachable moment.

1 Comment »

  1. lol My little brother saw one of these a few days ago and he completely freaked out, he kept saying “That bird just opened it’s stomach!”
    He was honestly terrified. I missed out on the actual bird so I had to look it up. Now the hardest part is trying to convince him he did not see a demonic bird ripping it’s own chest.

    Comment by nana — May 11, 2013 @ 6:38 pm

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