Wading birds leave interesting evidence of their dining specialties
- jjvanm
- 4 minutes ago
- 3 min read
Published June, 20, 2026, in the McAllen Monitor
Story and photos by Anita Westervelt, Texas Master Naturalist
We had several white ibis visit this winter; they probed for prey along the edge of the resaca. They eventually travelled on, all but an immature white ibis that stayed behind and continued working the shallows.
Ibis are interesting to watch hunt and catch food – as are other visitors to our resaca: great egrets, great blue herons and night herons. Depending on the size of the bird, they have different eating preferences. One thing they all have in common is a peculiar after-dinner trait: they cast castings.
Many waterbirds frequently cast, or regurgitate, hard pellets to purge their digestive tracts of indigestible matter.
Wading birds, like, herons, egrets, and ibises swallow their prey whole, so their digestive systems take in tough crab, crawfish and insect exoskeletons. Like an automated sorting machine, the birds’ gizzard, a muscular stomach, grinds the food, sending digestible soft contents into the intestines for nutrients to the bird. The indigestible parts are packed into a hard mass – the casting – that gets coughed up, often landing on the shore to be discovered by unsuspecting and curious wanderers.
The first casting I found while searching for “signs of life” during a recent bioblitz was colorful; it had an overall dark pink hue. Odd bits flung around it were light blue colored. Bright red and blue red swamp crawfish claw parts were strung out close by. Another casting a few feet away, atop Texas frog fruit plants, was a pale monochromatic whiteish blue. Both cylindrical shaped castings measured about one and a half inches long.
Ibis find food blindly, by touch, while probing for buried food in shallow water and mud. In water, they wade slowly, swinging their long, curved bill back and forth. When sensitive nerve endings along their bill detect a prey, it triggers a rapid, involuntary reflex that snaps up the prey in a tweezer-like pinch. They swallow small food items whole and will crush hard shells of crabs and crawfish with their mandibles and remove claws from crabs.
Ibis castings reflect the substrate from which they probe; they are mostly composed of mud, sand, grit and indigestible hard parts of crabs, crawfish, snails and insect wings. The castings are one to two inches long, dark colored, irregular shaped, clumpy and earthy looking.
Wading birds generally eject a casting once a day, but frequency varies, depending on the bird’s recent diet.
Herons and egrets generally hunt by sight, using passive ambush technique. They stand completely still or slowly stalk, waiting for a fish or frog to swim by then swiftly strike and spear their prey.
The larger great blue herons have the most varied diet: primarily fish, and also frogs, snakes, rodents, small rabbits, grasshoppers, dragonflies, aquatic insects and fledglings of other marsh birds. Their castings may be four inches long and include fish and snake scales, skeletons, bones, feathers and fur.
Telling the difference between castings depends on size and content. The castings I found may have been left by a great egret but it will most likely remain a mystery as to which species left them.
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These websites were helpful in writing this story: All About Birds, Birds of the World and National Audubon Society.