The male-female fitness disparity of American White Ibis

I just had my first birding trip about a month ago, and I enjoyed it so much. Although I saw many different birds at the park, the first one that I will write about is my favorite among the many I had spotted. But before I start on my bird blog post, I will share with everyone the “birding tip of the day”.

Birding Tip #1 – If you want to take photos of any bird, first take it from afar. Only approach the bird for close-up shots after your birding team has left so that everyone may have the opportunity to observe the bird. If you approach while the team is still observing, there is a good chance that you may scare it away, depriving others the chance for a good observation.

It was an unusually cold Sunday Morning. I had been up all night partly because I was excited about my first birding trip for my Field Bird Lab Course and partly because I was trying to memorize the required birds, their behavior and the sounds they make for my first bird quiz.  Strangely enough, despite not sleeping, I nearly did not make it on time to the meeting point. The drive to Brazos bend state park was an uneventful one and Darren, Maria, my TA, and I just talked about birds and biology in general during the drive.  It started to rain on the way there, just as the weather forecast had predicted. We reached the park at 8am and other than the lab class, there were only a few people at the park.

The first spot we went to look for birds was a lake right in front of the parking lot where our vehicles were parked. And there they were, standing out from the background, two white birds. From afar, it looked as though they could be snowy egrets or great egrets, the two white birds that was on the to-memorize list. I trained my binoculars on the bird and, as predicted, one of them was indeed a snowy egret. It had a slim black bill, yellow lores, long black legs with yellow feet and white plumage. However, the other was a bird whose name I had no clue of. Although it had a body shape similar to the egrets, its beak was orange-red in color and was long and down curved. It was something else. I asked Professor Queller and he told me it was an American White Ibis (Eudocimus albus).

I stayed and continued to observe the bird for 20 more minutes. During the whole time, all the white ibis did was to dip its beak into the water about 1 to 2 times to scoop for food although it didn’t seem to be scooping for fish. Upon returning home and doing some research, I found out that this bird actually feeds on insects and a whole bunch of crustaceans such shrimps and crawfish. The white ibis continued the same process the whole time I was observing it and one interesting thing I noticed was that it would move in a circular motion. I guess such a method allows it have a consistent 360 degrees look out for any predator. Besides the snowy egret, there are also a group black-bellied whistling ducks (2 adults and 3 ducklings) and 2 American coots around the white ibis, all waddling around and feeding. Those two birds were really easy to identity because black-bellied whistling ducks have that distinct red-white-black body color pattern and the coots a dark grey body a distinct red frontal shield.

Excited, I decided to creep nearer to see if I could take some close up shots of the white ibis, as I inched closer, I was sure the bird noticed me but it did nothing more but stare at me for a couple of seconds before continuing its feeding routine. I got my picture!

In an attempt to get an even more close up shot, the bird somehow decided I was too close for comfort and took off by flapping its wings rapidly a few times to gain momentum and then gliding gracefully to the other side of the lake. It is also during this time that I saw that it also had black wingtips. As the white ibis landed on the other part of the lake, I realized it tended to stick close to the shore and amongst vegetation growing at the edge of the lake. I reckon it does so to camouflage/hide itself from predators.

We soon left the lake to continue the birding trip. As we walked to the other side of the lake, I spotted more white ibises. But this time, it was a pair of them, feeding far out from the well-trodden path among the messy growth of trees, barely noticeable by anyone who is walking casually down the dirt path by the side of the lake. As usual, there were just foraging for food – two dips, one look, two dips, one look. In addition, once again, there was an egret close by. But this time, it was a great egret, judging by its very long neck and black legs and feet. It seemed to me that they liked to hang out near Egrets.

As the group left to go back into the forest, I decided to stay behind and observe the ibises a bit longer. To my surprise, the moment the group was out of sight, one of the two white ibises immediately flew onto the dirt path we were walking on. It seemed that they were actually quite aware of what is going on around them, despite appearing to be obsessed with finding food. Because I was hiding behind some bushes, the white ibis did not spot me and continued to walk along the path. It was the first time I saw it out of the water and I observed that their legs are about the same color as their beaks, orange! During this time, I managed to take more close-up shots of it.

As I lay on my bed and thought about the bird that night, I came to realize that from all the time I spent observing the white ibises, all the white ibis did was search for crustaceans in the water. It seems to me that food must really be an important part of the white ibis’s life, so I got out of bed to see what interesting stuff I could find about white ibis and its diet in the primary literature.  As I searched for interesting information about white ibises and their feeding behavior, I found out the males seems to benefit much more than the females in terms of evolutionary fitness in areas like parental care, mating behavior and food foraging. Also during my search for information, I came across the writings of Professor Peter C. Frederick and he has done a lot of extensive research on wading birds such as egrets and ibises. If any of you guys are keen to learn more about these wading birds, I strongly suggest you search for journal articles that he published.

Coming back to the white ibis, let me share about its parental care behavior first.

From the journal articles I have found, the white ibis is actually a sexually size dimorphic avian species which tends to exhibit sex dependent pre-fledgling mortality during periods of food limitation. For many bird species which have sexually dimorphic nestlings, it is usually the larger sized male nestling that often experience higher morality as a result of greater nutritional needs. But guess which gender has a lower mortality rate for the white ibis?

That’s right, the male! Despite being on average 15% bigger and hence having a larger calorie intake requirement, it turns out that in times of food shortage, young male white ibises have a lower mortality rate of survival as compared to the females. Although the underlying factors to why the males tend to have better survival rates have yet to be discovered, it is suspected that that adult white ibis tend to feed the larger male nestlings first; either because they are either perceived by the parents to have a higher chance of survival or the larger male nestlings simply out-compete the small females for food because size. In the event of food shortage where there might be just enough food for one nestling, the male is the one that usually gets fed.

Having read Richard Dawkins’s book, The Selfish Gene, those explanations do make evolutionary sense. The adult parent might practice favoritism in this situation because if it feeds the smaller one in times of food shortage, both nestlings might eventually die because the larger one was not fed and the smaller one was too weak to survive despite being fed small amounts. However, if the adult white ibis fed the larger male nestling, it might be just enough for the male nestling to survive to adulthood as there could be a possibility that males are better able to deal with long intervals between feedings, which is a key characteristic of food shortage. Such behavior increases the chance that at least one of its offspring is able to survive, thereby ensuring the continuation of the genes.

Even in mating behavior we see signs of disparity between how much the males and female with regards to evolutionary fitness. Although the white ibis is predominately monogamous and both sexes do provide parental care to their young, the male would often fly off to engage in extra pair copulation with other nesting females. Very often, the male would first copulate with its primary female partner first. After which, when copulation within the main partnership is more or less done, the male would usually leave the female at her nest to fly off to find other female nesting white ibis and attempt another copulation. These extra-pair copulations are usually done after the within-pair copulations. This allows the males to increase their reproductive success to a considerable extent. Nevertheless, as all males are, although they often seek other females to engage in extra pair copulation, they themselves do not really approve of other males engaging in extra pair copulation with their own primary mate and hence, often guarding its own mate vigorously from other males whenever he is around and being a good mate.

That’s not all; adult male ibises have also been found to steal food from both young juveniles and feeding females instead of foraging its own food by forcing its bill down the throat of its victim and extracting the ball of food that is meant for the young juvenile ibises. And because the adult male ibis are considerable larger and more aggressive than that of the female ones, the female almost never try to chase the pirating male away but instead just try to avoid them. The adult white ibises only steal food from non-mate feeding females and juveniles that are not part of its nest. I suspect that such behavior exist because the males are usually starving due to intensive mate and nest guarding, hence by stealing food from a feeding parent or juvenile from a nearby nest, the adult male ibises is able to obtain food without having to spend long periods of time away from the nest and its mate. This behavior allows the male ibises to obtain food without having to worry that its female mate copulates with another male ibis, thereby reducing its own reproductive success.

On a whole, the bird trip was really fun and the whole experience really encouraged genuine learning about birds. I can’t wait for the next trip. J

** Update!!! – In a recent journal article that was just published recently in December 2010 by Professor Frederick (yet again!), he and his team have found out that the consumption of mercury via its food in white ibises have affected the hormones within the male bird, altering the courtship behavior in males, leading to and high percentages of male birds mating with other males. This homosexual behavior among the male white ibis has resulted in a lower reproduction rate among the white ibises as many female are left out in the mating process. **

And here is what everyone has been waiting for! PICTURES!

Here is a simple distribution diagram of the breeding and non-breeding range of the white ibis in the America region –

This is the lake where I first spotted the white ibis!

The white ibis feeding among vegetation close to the edge of the Lake, accompanied by a snowy egret and a number of black-bellied whistling ducks.

Here a white ibis is landing at its new feeding spot across the lake; once again, among the vegetation. Notice the black wingtips that are only visible usually during flight.

In the picture below, there are two white spots, which are the white ibises. See if you can spot them!

And here is a great egret. Notice its extremely long neck and long plumes at the back of the body. During my bird trip, I observed that the white ibis usually forages around other birds, either along with some wading ducks or an egret. I suspect that this behavior reduces the chance of the white ibis attacked by a predator because there are more birds keeping a look out and if a predator does come, there is a chance that it might go for the other birds.

White ibis walking on the pathway when the EBIO 337 Birding Team left the area.

This diagram shows that the mean mass of the male nestlings are consistently larger than the female ones, highlighting the presence of sexual dimorphism in the young of white ibises –Adams, E. M. and Frederick, P.C. 2009. Sex-Related Mortality of White Ibis (Eudocimus albus) DOI: 10.1675/063.032.0114

And as seen in the tables above, during the time of starvation, the number of male nestlings found was more than the female ones where a total of 103 males were found in comparison to 74 females –Adams, E. M. and Frederick, P.C. 2009. Sex-Related Mortality of White Ibis (Eudocimus albus) DOI: 10.1675/063.032.0114

This diagram shows that extra-pair copulation only increases as within pair copulation decreases, implying that the white ibis usually complete their within pair copulation before engaging in extra-pair copulation. – Frederick, P. C. 1987. Responses of male white ibises to their mate’s extra-pair copulation behavior.

And below are the references which I found all these interesting information about the white ibis for this blog post.

1)      Adams, E. M. and Frederick, P.C. 2009. Sex-Related Mortality of White Ibis (Eudocimus albus) DOI: 10.1675/063.032.0114

This paper introduced the discovery of sex-related mortality in nestlings and some possible explanations to it.

2)      Smithsonian Marine Station – Eudocimus albus (Information Page)

This is a good information package on the white ibis. Many of my observations were similar what was on the website.

3)      Frederick, P.C. 1986. Extrapair Copulations in the Mating System –

This paper discusses about the sexual behavior – extra-pair copulation – of the white ibis and possible benefits to such a behavior in terms of reproductive success

4)      White Ibis – Birds of North America Online

This website provided an overall good introduction to the bird for all aspects. The distribution of the bird can be found here.

5)      Frederick, P. C. 1987. Responses of male white ibises to their mate’s extra-pair copulation behavior.

This paper talks about the male’s response to the extra-pair copulation behavior.

6)      Frederick, P.C. 1985. Intraspecific Food Piracy in the White Ibis –

This paper discusses the behavior of intraspecies piracy amongst both adults and offsprings and how sex and responses towards such a behavior affects the proportions of the types of victims.

7)      Kushlan, James A., 1975. Sexual Dimorphism in the White Ibis

This paper discusses about the sexual dimorphism of the white ibis, which is found to be present in both the young and the adult.

8)      Clutton-Brock, T.H., Albon, S.D. & Guiness, F.E. 1985. Parental investment and sex differences in juvenile mortality in birds and mammals

This article raises the issue of gender-dependent juvenile mortality among animals and discusses the possible factors that might cause such an occurrence.

9)      Frederick, P. & Jayasena, N. 2010. Altered pairing behaviour and reproductive success in white ibises exposed to environmentally relevant concentrations of methylmercury DOI:10.1098/rspb.2010.2189

This article highlights the impact of mercury pollution on the white ibises. The consumption of mercury has caused the male white ibises to exhibit homosexual behavior, reducing reproduction rates.

About why25

My name is Ben. I graduated from Raffles Junior College back in 2007. I am a fan of romantic drama films and island getaways in the Mediterranean region. At work, I have a propensity to throw caution to the wind and do random dance moves while listening to the radio. I like that it makes people smile, and that my spontaneity can lift someone's spirits on a long day. I cherish my friendships dearly and for me, nothing beats a day spent fishing and catching up with my best friend.
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1 Response to The male-female fitness disparity of American White Ibis

  1. Donna M Petersen says:

    Thank you so much for all of that great information on the White Ibis! They are my favorite birds here in Florida… I am fascinated by them. I found your paper highly interesting and informative!!

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