Bird Digestion

Florida Scrub Jay

Florida Scrub Jay, Aphelocoma coerulescens

 

I know this may be an unappetizing topic for some, but being a physician I find the comparative anatomy and physiology of avian digestion fascinating.  Don’t confuse my title and posting with the venerable and recommended periodical “Bird Watchers Digest”, mainly for their sake.  Check it out at http://www.birdwatchersdigest.com.

Herring Gull with lunch

Herring Gull with crab, Larus argentatus

I reckon that a bird spends the majority of his life eating or hunting for food.  Even the apparent sedentary perching owl or hawk is likely planning his next attack and contemplating the next meal.  And this is time well spent since the survival of these warm-blooded, active birds, with very high metabolic rates requires a constant source of energy.  Reproduction (breeding, nest building, and rearing of the young) along with migration are also time consumers, but take a back seat to eating and daily survival.

Piping Plover

Piping Plover, Charadrius melodus

Given the requirements of flight, birds do not have the luxury of storing heavy layers of fat or foods internally, with the one exception being the preparation for migration.  For this some songbirds increase their body weight by 40% and need every last ounce and calorie for the rigors of migration.  But generally most birds need a steady and constant inflow of food and energy to survive.  This is even more critical in the cold of winter.

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Little Blue Heron, Egretta caerulea

Luckily birds have evolved a rapid and efficient digestive system, able to cope with a varied diet.  For some birds and food types the transit from beak to cloaca can be as rapid as 30 minutes.  The beak and toothless mouth are for stabbing, carrying, crushing, and tearing the food, quickly sending it downstream to the tubular esophagus.  Fortunately, given their diet, birds have a small tongue with few, if any taste buds.

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Gull demonstrating the edentulous mouth, small tongue, and no taste buds.

Many have a widened area in the mid-esophagus called the crop.  This is the site of short-term parking for a big meal as is often demonstrated by the tell tale neck bulge of the heron who recently swallowed the large fish, always head first.

Great Blue Heron

Great Blue Heron, Ardea herodias

The bird stomach is very different than ours.  It is a two-part affair with a glandular first sac called the proventriculus.  Strong acid, enzymes, and mucus start the digestive process here, before transporting the food to the second part called the gizzard.

Snowy Egret

Snowy Egret with large insect, Egretta thula

The gizzard is a thicker muscular sac with a rough sand-like lining, perfect for grinding and mixing.  Some contain sand and stones further aiding the process.  Pellets containing the non-digestible waste such as bone fragments, hair, shells, and feathers are passed and often mark the roosting sites of owls and other birds-of-prey.

Peregrine Falcon

Peregrine Falcon eating a Lesser Black-backed Gull, Falco peregrinus and Larus fuscus

The actual absorption of nutrients occurs in the small intestine where food is mixed with the enzymes from the pancreas.  Birds-of-prey have a relatively short small bowel, whereas herbivorous birds have a longer one, needed for the slower digestion of the tougher cellulose-rich food.  Multiple small sacs off the small bowel are called caeca and harbor beneficial bacteria, further aiding digestion.

Limpkin with Apple Snail

Limpkin with Apple Snail, Aramus guarauna

A bird’s colon or large bowel is short, just serving as a conduit to the final cavity, the cloaca.  The cloaca empties to the outside world via the vent, sometimes onto the unsuspecting birder.  As you know the cloaca is the common chamber for both sexes receiving the products of the gastrointestinal, urologic, and genital tracts.  The close and rapid contact of the vents and cloacae is when and where the genetic material is exchanged.

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Its a Blue-gray Gnatcatcher so it must be eating a gnat. Polioptila caerulea

The Cattle Egret below was finally fed up with his diet of insects and mice and got in the drive-thru lane at McDonalds thinking that they might offer a better menu.  I’m not so sure.

cattle-egret-at-mcd

Cattle Egret, Bubulcus ibis

27 thoughts on “Bird Digestion

  1. What a fascinating and well illustrated article! Thank you for this well presented information. Of course, the Cattle Egret picture is a wonderful suggestion of the transition we have all followed: hunting and gathering ,,, fast food take-aways.

    Liked by 1 person

    1. Greetings to my dear friend. Good point, but maybe they do suffer from it. Perhaps this is a future scientific study for your retirement. You could do Barium swallows on birds and revive your professional skills before they’re lost forever.

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    1. Dear Dawn, Thanks for your comment and interest. I am a long time fan and subscriber to BWD and think I may have acquired some of my irreverent style from its contributors. I’m flattered that I may be linked on April Fools Day, or any day for that manner. I’ll check out Out There With The Birds.

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  2. Amazing photos. I laughed remembering a personal memory of the excretory system of birds. While studying by the lake for an ornithology test in college — chapter on the excretory system — a gull flew over and deposited a sample on the page. My friend thought this quite amusing and was laughing heartily, until a gull flew over and deposited a sample in her mouth. I especially like the photo of the Florida scrub jay — the detail is wonderful.

    Liked by 1 person

    1. Thanks for you comment and bird story. It’s the revenge of the birds. I had a similar experience on my first day of medical school, walking to class–a direct hit and caused me to return home, clean-up, and walk into the first class 1/2 hour late.

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