The American white Ibis: Working hard all day, sleeping around all night.

As I stared out at the waters in Brazos Bend State Park in Houston, I couldn’t help but notice the liveliness of the whole area. While I normally dread rain, even a small drizzle, I do sometimes find it suitable as a soothing sleep aid and an excellent mosquito repellent. But I liked this place. It wasn’t because it looked all that impressive—after all, it was just another large swamp about a half mile across and surrounded by tall grasses and trees—but rather because I had the cacophony of fellow restless college students to surround me and make me feel just a little less isolated. In fact, if I were a B grade horror film director, this would seem like a perfect opening scene before the disasters start happening—it was calm and just a little bit unsettling. In fact, between talking to students and observing birds, I would find myself staring at the various ripples in the water, half-expecting the alligators that resided there to burst into action.

Imaginations aside, what stood out the most to me was the white ibis (Eudocimus albus) which happened to be the only bright entity within my field of vision. Wading through the outer fringes of the lake, the Ibis had a penchant for the marsh environments as it hunted for food. Luckily, this was not the first time I encountered the bird; I remember three years ago when I took my first ecology module, the white ibis became a staple bird to sight during our field trips.

The white ibis is a long lanky bird with a white coat, a long orange crescent shaped beak and tall orange legs that lifted its body well above the surface of the water. Its habitat extends from the coasts of southern U.S. to the start of South America, and can be occasionally spotted on in various football games rooting for the University of Miami. What was most astounding was its almost mechanical routine – it would wade through the shallow waters for 3-4 steps and stick its beak down to catch its meal of small fish, crustaceans, or worms. Of course, standing more than 50 ft away prevented me from clearly identifying exactly what it was that she was consuming. Yet this method of hunting was strangely enchanting; it would bob its head for every step it took and the prey capturing motion was very quick. But strangely enough, nearly 15 minutes into observing its behavior, I was finally rewarded with a flight. After leaping forward into the air, the ibis seemed to almost glide through the sky, barely using any energy after the initial burst. Its next course of action landed it next to perhaps the only other white bird in the vicinity, the Great Egret, and they seemed to quickly become acquainted with each other as they waded around in close proximity and showed no hint of aggression towards the other individual. In fact, I wouldn’t be surprised if these two individuals have worked together countless times before based on their foraging dynamic. Throughout this half hour display, I couldn’t wait to go back and research more into the behavior and characteristics.

Apparently, I’m not the first to be piqued by this unusual bird. The first article that caught my eye was an in-depth analysis of the effects of sexual dimorphism on the species conducted by Dr. Kushlan in 1997. Females were shown to be around 74% as heavy as males and similarly smaller in most other attributes, yet, there was little difference in their feeding habits or effects on most other behavior. In a later study by Dr. Bildstein in 1987, it was found that males have disproportionally larger bills than females with most other attributes being of similar ratio between the sexes. The explanation found was that males could increase their foraging range to deeper waters due to their larger beaks, but are at the same time size limited by their increased nutritional requirements.

As for the constant dipping of the bill into the water that I had observed, I discovered that these birds don’t hunt by sight, but rather by sensory pits located on their bills. When they feel movement in the soil and water around their bills, it allows them to react quickly to capture the prey they are after. In fact, a recent 2010 study of bill sensory organs, it was found that white ibises had nearly 35% of their beak length that was sensitive to movement as shown in the included graph.

But all this researching of sexual dimorphisms segued into the most traditionally interesting topic – sex. If there’s one thing that I have learned throughout my short period thus far as an Ecology and Evolutionary Biology Major at Rice University it is that monogamy is a rare thing in the animal kingdom (humans included) and in the case of the white Ibis, this is especially true. According to the study by Peter Frederick, while the birds are supposedly monogamous, it was found that extra-pair eggs ­­­­present in almost every brood of eggs laid, suggesting that this is a very common occurrence in this species. He was able to determine this by conducting DNA tests on all the nestlings in a brood and comparing these markers to those of the parents. In particular, this study looked at the different rates of mate guarding in various broods of white ibis colonies and studied the length of time which females left their eggs and nests as shown by the graph at the end of this entry. Of course, many of us now would ask, what is there to gain from this behavior? After all, as humans, we don’t go around secretly impregnating other people’s wives (at least not often or intentionally I hope). In cases like these, I like to refer back to the theories of evolution espoused by Dawkins in The Selfish Gene, a must read for everyone! But to paraphrase Dawkins, what succeeds in nature is the continuance of the individual’s genes and what better way to do that than secretly spread it with multiple mates? And as time passes, this trait becomes a common occurrence in the population and further strategies and methods are selected for that would increase an individual’s chances of spreading his/her gene. Maybe the promiscuity of these white ibis isn’t something that’s incredibly unique, but nonetheless slightly depressing for a hopeless romantic like ­­­­me.

Before I wrap this entry up, I would like to highlight just one more interesting fact I found while surfing the web for information on the Ibis. It seems that according to several studies, white ibises have a working relationship with several other species of birds which include the snowy egret, great egret and the little blue heron. The late Lawrence Kilham had observed similar behavior as I did when passing white ibises became attracted to foraging great egrets and ended up foraging close to them. In particular, he had observed the one egret and 40 Ibis strategically maneuver between each other to capture the maximum amount of food along a 35 meter long beach shore. So my observation of the interaction between the egret and Ibis at Brazos Bend was not merely an accidental coincidence or attraction towards similar looking species as I had thought at the time, but an actual working relationship between birds for their mutual benefit!

Honestly though, If it weren’t for the fact that the white ibis had such striking features and colors, I would definitely not have paid attention to it, but I’m glad I did. So if you ever happen to see a white with long orange legs and beaks, see if it happens to be a white ibis. I have included links to all the white ibis articles that I found interesting below.

References:

  1. Bildstein, K.L., 1987, Energetic Consequences of Sexual Size Dimorphism in White Ibises (Eudocimus albus). The Auk, 104(4): 771-775 (http://elibrary.unm.edu/sora/Auk/v104n04/p0771-p0775.pdf)

The reference covers the possible effects of sexual dimorphism and covers the difference between feeding strategies of the two sexes.

  1. Cunningham, S.J., M.R. Alley, I. Castro, M.A. Potter, M. Cunningham, and M.J. Pyne, 2010, Bill Morphology of Ibises Suggests a Remote-Tactile Sensory System for Prey Detection, The Auk, 127(2): 308-316.  (http://www.bioone.org/doi/full/10.1525/auk.2009.09117)

This reference is about the tactile sensations and sensory organs in the beak of the Ibises. The study was utilized for its graph shown in Fig 4.

  1. Frederick, P.C., 1987, Extrapair Copulation in the Mating System of White Ibis (Eudocimus albus), Behaviour, 100(1/4): 170-201 (http://www.jstor.org/stable/4534581)

This discusses extra pair copulation in white ibises. The figure was used from this article to look at the duration that females left their nests depending on their brood size.

  1. Kilham, L., 1980.  Association of Great Egret and White Ibis, Journal of Field Ornithology, 51(1): 73-74 (http://www.jstor.org/stable/4512519)

This article is related to the commensalism relationship between the Great Egret and the White Ibis.

  1. Kushlan, J.A., 1977. Sexual Dimorphism in White Ibis, Wilson Bulletin, 89(1): 92-98 (http://elibrary.unm.edu/sora/Wilson/v089n01/p0092-p0098.pdf)

The initial study on the basic sexual dimorphisms in White Ibises.

Some pictures of the Ibises taken from Wikimedia and Flickr (Pictures are free for reproduction and commercial resuse).

White ibis standing on the shores of a lake. Take note of the pure white feathers and orange beak and legs.

Group interactions of the white ibis. These birds tend to forage in groups and work together to increase food gathering productivity.

Image taken from Birds of North America collection from Cornell university. This shows both the breeding and non breeding ranges of the white ibis.

This graph discusses the various durations in which the mother is absent from the nest depending on the various clutch sizes depending on the number of days after laying the eggs.

This charge shows the relation between the length of the bill and how many sensory pits it contains.

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