The Scarlet Ibis Is What it Eats

During our stay at the Asa Wright Nature Centre on Trinidad, we did a late afternoon, early evening trip to the Caroni Marsh, a forty square mile mangrove swamp which has been protected as a wildlife sanctuary. The marsh is the roosting place for the iconic bird of Trinidad, the Scarlet Ibis. At dusk, Scarlet Ibises congregate by the tens of thousands. They gather from the marshes and wetlands where they have been feeding through the day. Some cross the few miles of water which separate Trinidad from Venezuela. It is an astonishing sight even for people with little interest in birds.

The Scarlet Ibis is a rather small wading bird - about two feet in length, weighing less than 1.5 pounds. Its long decurved beak is pink; its legs and feet are reddish pink. Its plumage is brilliant, bright red. Only the tips of the wings are black, a feature which gives added strength to the long primary feathers.

“Brilliant, bright red” may seem like an overstated description. If “red” needs an adjective, one would think that either “brilliant,” or “bright” would do the job. Not in this case. The brilliant, bright red Scarlet Ibis is almost surreal. Flocks of dozens, or hundreds, come down to the green leaves of the mangrove trees, making the evening sky look like some gigantic red and green Christmas tableau.

As impressive as this evening congregation is, I was most taken during the couple of preceding hours when our guide directed the flat-bottomed boat slowly through the marsh’s channels in search of the ibis, other wading birds, and other creatures of the marsh. The ibis was not difficult to spot. Imagine a brilliant, bright red basketball balanced on a tree branch. Even deep in the tangled mangroves, it is not difficult to spot.

Next imagine those brilliant, bright red basketballs strung together as illuminated lights decorating a carnival tree. As we moved slowly along a quiet channel, we saw more and more of those red lights strung through the branches, resting and preening before their final flight to the roosting area. If the boat drifted too closely, they flew. It was easy to follow the flight of the brilliant, bright red body and wings as the birds wove their way through the mangrove swamp.

Many birds, maybe even most, find some means of camouflaging themselves, either to protect themselves from predators or to hide themselves from the unsuspecting prey they wish to eat. Clearly, the Scarlet Ibis does neither. That suggests to me that as it evolved, this ibis had no predators which could do it serious numerical damage. There were dangers from raptors and arboreal predators, especially to its eggs and nestlings, but since the ibis nests in colonies, the danger to the species was never serious; there is safety in numbers. Apparently the Scarlet Ibis has even managed to avoid the worst depredations of the most adaptable, creative, and destructive of all predators - humans. It is neither endangered, nor threatened.

The Scarlet Ibis does not need to hide itself from its food. Its long bill probes for food in mud and shallow water. It is guided by touch, unseen by its prey except for the brief time when the prey is pulled from the darkness by the tip of the beak and flipped into the gullet.

Like most birds, the ibis diet is varied. The Scarlet Ibis feeds on frogs, fish, reptiles, and crustaceans. But it has a favored food, and that favored food is what gives the Scarlet Ibis its brilliant scarlet-red. The young Scarlet Ibis is a gray and white bird, looking very much like a young White Ibis. The ingestion of red crabs gradually produces the scarlet plumage. There are some scientists who consider the Scarlet Ibis,  Eudocimus ruber, and the White Ibis, Eudocimus albus, to be the same species with diet making the difference. The crabs which are the favored food of the Scarlet Ibis are rich in carotenoids; the more carotenoid-rich crabs the ibis eats, the redder it becomes.

Now I am venturing into a new area for me, one which I still have much to learn about. But basically, birds cannot produce red, orange, or yellow feathers on their own. They are what they eat, and they must have the right diet to produce the plumage that will be most attractive to a mate. For birds like the Scarlet Ibis of Central and South America, or for more familiar birds closer to home, like the Northern Cardinal, or Purple Finch, in order to have the bright plumage which will attract a mate, they need a diet rich in carotenoids.

Carotenoids are a class of organic pigments that are produced by plants. These red, yellow, and orange pigments help plants to absorb light energy for photosynthesis and prevent degradation of chlorophyll. Animals that eat plants rich in carotenoids, or which eat other animals rich in carotenoids, enjoy numerous benefits from these compounds just as the plants do.

From our perspective, the most familiar visible effect on birds is that carotenoids serve as coloring agents. Numerous species of birds (like the cardinal, ibis, or finches) feast on carotenoid-rich foods. As a result, individuals with the best diets are the most colorful and potentially more successful at attracting mates.

The coloring agent which makes carrots orange is a carotenoid. Some of us may have been told to eat our carrots so that we would have good eyesight. Aviation crews during World War II were given carrot juice to improve their night vision.

The benefit to eyesight of the carotenoid in carrots is not just a ploy of vegetable growers. Researchers at Arizona State University have learned that in birds, as in humans, carotenoids are deposited in the retina, improving their eyesight - in particular, their ability to see color; “the more carotenoids you eat, the better you can see color, the better mates you choose, and the redder the foods you choose, thus giving you even more carotenoids for health, attractiveness and vision.” There also seems to be evidence that carotenoids enrich the testes and seminal fluid, preventing sperm cells from oxidative damage and resulting in greater fertilization ability of males.

What this means in our northern climes is that the cardinal with the brightest red is most likely to attract the best females; in addition he probably mates with females which are socially paired with other males. He is more likely to sire more young, and more healthy young that survive.

With regards to the brilliant, bright red Scarlet Ibis of Central and South America, it seems to me that color no longer creates an “attractive” edge. It is simply an evolved and long-established given, although their diet high in carotenoids may keep their eyesight keen to danger and their young strong and more likely to survive.

Carotenoids provide us with some brilliant and colorful birds. There may also be some dietary lessons for us. They improve eyesight, increase attractiveness and the ability to assess attractiveness, and enhance the overall health, especially it seems, of the male. Another way of putting it might be: “Young man, if you want to get the girl, eat your carrots.”

And so you see, good birding is good for lots of things.