The Great Egret: greater than you might think

By Andrew Zicker

It was a beautiful morning in Tallahassee, Florida, and I was walking out of my apartment complex to head off to my classes at Florida State University. As I was passing by the community pond, I noticed several birds were present, including a Great Egret (Ardea alba) on the side of the pond closest to where I was passing by. It was just standing in the shallow water in a corner, completely still, looking in the distance. All of a sudden, it looked down at the water in front of it, as if something had caught its eye. The egret pulled it neck back slowly, in its unique “S” shape, and then…wham! The bird shot its head into the water like a slingshot, making quite the splash, as the nearby Canada Geese continued swimming around so peacefully as if nothing had happened. When the egret pulled its head out of the water, I noticed that it had a tiny fish in its bill, which it soon devoured. I would have loved to stay and watch it some more, but I was short on time and needed to get to class. That same day, in my ornithology class with Dr. Emily H. DuVal, my classmates and I were informed of an assignment in which we needed to observe a species of bird and write a blog about it. I knew instantly that I wanted to watch the Great Egret for my bird blog, thus I signed up for it.

When I decided to go watch my bird, I first went down to another pond in my apartment complex to see if I could find a Great Egret there, which sure enough, there was. After I climbed over the fence to get a little closer, I noticed that a very close cousin of the Great Egret, the Great Blue Heron (Ardea herodias), was also at the pond. This was intriguing, since I learned from my ornithology class that a white morph of the Great Blue Heron, as compared to their normal “blue” morph, very closely resembles the Great Egret. Both have white plumage and a yellow bill, but the difference is that the Great Blue Heron has yellow legs, while the Great Egret has black legs. Anyways, back to my story. The Great Egret was across the pond from me, but as I got closer, I must have startled it, so it flew off in the direction of the other pond where I first found an egret. As I saw it flying, I noticed the difference between flight of egrets and that of cranes. Egrets fly with their necks retracted, in an “S” shape, whereas cranes fly with their necks outstretched. Just another nifty birding tip I learned. I went over to the other pond and found the egret, with some other avian neighbors, in the form of an American Coot, a flock of Canada Geese, a female Hooded Merganser, and a Pied-billed Grebe. The Great Egret did not exactly interact with these birds, but seemed to mostly be focused on one thing – finding food. During the majority of the hour that I watched this bird, it stared across the pond while standing still, like the first time I found it. The bird did occasionally walk in the shallows, but not more than ten feet in either direction of its original location, something I found very interesting. Whenever the bird found something, it tilted its head as if it was trying to look at the object in the water with just one eye, before shooting its head in to grab the tasty prey. I observed it feed on a few cricket frogs and some small fish. This intrigued me, so I delved into the primary literature to find out more about foraging behavior and prey choice in Great Egrets.

I found an interesting paper on the different types of prey species that Great Egrets feed upon. A study by Leopolodo Miranda and Jaime A. Collazo in Puerto Rico on the diets of various wading birds found that Great Egrets feed on fish, crustaceans, lizards, and insects. In this study of four wading bird species, only Great Egrets and Snowy Egrets were found to consume grasshoppers, although these were not very important to the birds’ diets, as well as anole lizards. Of all the wading birds studied (Great Egrets, Snowy Egrets, Tricolored Herons, and Little Blue Herons) crustaceans and fish made up 74.1% and 24.8% of the birds’ diet, respectively. What I thought was very interesting was that only Great Egrets were found to consume crabs and shrimp, the latter being the most important prey species for the Great Egret. Until reading this, I was under the impression that almost all wading birds were opportunistic feeders, and that crustaceans were fair game. Moreover, this study found fish was a more important group of prey for the Great Egret, with their diet consisting of seven families of fish, of which smaller fish like guppies, were most important. This would make sense, since the fish I saw my individual feed on were roughly guppy size, although I did not know specifically what fish it was. I also saw my individual feeding on frogs, which could just be a regional difference between Florida and Puerto Rico.

What really interested me about watching my bird more than its foraging behavior was why the bird “tolerated” this pond, so to speak, with all the pollution and littering that I noticed. If I had noticed the sort of filth that was in the pond in my apartment, I might be able to walk quickly past it. However, if I had to fish my food out of it every day, I would probably be looking to terminate my lease and find a new place to live. This is an apartment complex for college students, so there were empty beer cans, Styrofoam boxes, and plastic bags in the pond, to say the least. Wondering how the egret was apparently unscathed by the pollution, I again looked into this in the primary literature, via Web of Science, accessed through the FSU library. I found an interesting study by Gaea E. Crozier and Dale E. Gawlik that was done here in Florida, specifically in the Everglades. It’s known that over the last few decades, the Everglades have become more enriched with nutrients due to various forms of human pollution. This spells disaster for many marine systems, causing sea grass to overgrow and causing blooms of toxic algae. In this study of the Everglades, the authors found that wading bird abundance, including Great Egrets, tends to be highest in nutrient enriched areas. In contrast, non-wading bird abundance is lowest in nutrient enriched areas. Nutrients damage ecosystems in the Everglades like in the ocean, so with this knowledge, Great Egret abundance could potentially be a bioindicator of the health of the ecosystem! How pleasantly surprising and intriguing is that? I for one, as a fan of conservation, am pleased with this finding. However, this does raise some very interesting questions, specifically about interspecific (between species) competition between Great Egrets and other birds. Perhaps this suggests that other birds move out in areas where Great Egrets are present. I’m not sure that’s all there is to it, since I noticed a great diversity and abundance of other birds at the same ponds I found my Great Egret, but it is something that I would like to see studied further.

In addition to findings about nutrient enrichment, I also found two studies about mercury contamination in the habitat of Great Egrets. Mercury contamination in aquatic systems has been a hot topic and a rallying point for the environmental movement in recent decades. Primarily, this is because mercury is an example of an element that undergoes bioaccumulation, meaning that if mercury is consumed by an organism at a low trophic level (low in the food chain), which then passes on to higher trophic levels (higher in the food chain), it will accumulate and result in a large amount in higher trophic level animals. The effects of mercury poisoning have become prevalent in Japan, where dolphin meat is consumed, resulting in newborn babies with severe birth defects. It has also been known that the amount of mercury in the Everglades ecosystem has increased over the decades. A study by Stephanie E. Duvall and Mace G. Barron found that piscivorous (fish-eating), animals in the Everglades were at the greatest risk of mercury contamination, and those that consumed larger, and thus, higher trophic level fish, were at the greatest risk of all. Great Egrets were one of the sample species, which were found to be at greater risk than raccoons, but less than alligators. Furthermore, juvenile Great Egrets were at a greater risk than adults. I was saddened to hear this, but also pleased since we now know more about what humans must do to protect these great birds. The elevated risk in juveniles leads me to another important study that I found on the effects of mercury on Great Egrets. Gawlik, along with Garth Herring and Darren G. Rumbold, published a study on the physiological effects and mercury concentrations of nestlings of Great Egrets and White Ibises. They found that feather concentrations of mercury were higher in 2006 than 2007, with egrets having higher concentrations than ibises. They did also find that fecal corticosterone and stress protein 60, both important regulators in many animals for a variety of processes, were lower in 2006 than in 2007, however the authors believe this is not due to mercury levels, instead due to variations in habitats between the two years. I disagree, since I viscerally believe that mercury is terrible to the environment and to human health. I also believe that the almost direct correlation of high mercury concentrations to low production of stress proteins is not a coincidence, but instead mercury is directly causing this physiological suppression. However, this topic will need to be studied further in the future to learn about the direct physiological effects of mercury on nestlings. Regardless, it confirms the previous study that I mentioned that Great Egrets are at great risk of mercury contamination in the Everglades. This study notes that Great Egrets are at greater risk than White Ibises because they are higher trophic level piscivorous birds.

Speaking of Great Egret nestlings, I noticed something peculiar about the individual I was watching. As I was watching the egret while it was still through my binoculars, I noticed that this individual had green lores (the skin near the eyes), indicative of an individual that had reached sexual maturity. However, this individual did not have long, elegant semiplumes on its back and wings, which are indicative of breeding condition. This is not too surprising, since most breeding seasons for birds happen after the winter. Regardless, the wind that came by at that moment still made me jealous of the bird’s plumage and made me wish I had brought a sweatshirt. Again, I decided to look more into the primary literature for more information about breeding and nesting in Great Egrets. One study by Bonnie J. Ploger and Matthew J. Medeiros studied the unequal food distribution in Great Egret nestlings. The authors found that parents often feed their young an average of 5.4 meals a day, of which each contains an average of 3.9 boluses, or lumps of food. During each of these bolus deliveries, nestlings often fight for access to the food. They were arranged into three groups: those that fought and won, those that fought and lost, and those that did not fight. The researchers found that parents deliver more meals when nestlings are separated, accomplished in this study via Plexiglas, and that nestlings still attempt to fight even while they are separated. However, what intrigued me the most is that parents tend to feed the young who are dominant in fights more than the others. The researchers believe that this reinforces subordination in inferior young, defined by being smaller due to later hatching, and helps make up for the cost of fighting in the superior young. I thought this whole study was very fascinating, and it adds to the mystique of bird cognition and social behavior. This, along with the aforementioned dietary niches and inherent ability to indicate environmental health have increased my fascination with birds, and inspired me to undertake the new hobby of birding, which I hope everybody who reads this will be as well.

References

Miranda, L. and Collazo, J.A. 1997. Food habits of 4 species of wading birds (Ardeidae) in a tropical mangrove swamp. Colonial Waterbirds 20:413-418. doi:10.2307/1521591

This article was about the prey choice of Great Egrets and other wading birds.

Crozier, G.E. and Gawlik, D.E. 2002. Avian response to nutrient enrichment in an oligotrophic wetland, the Florida Everglades. The Condor 104:631-642. doi:10.1650/0010-5422(2002)104[0631:ARTNEI]2.0.CO;2

This article was about the abundance of birds in nutrient enriched areas in the Florida Everglades.

Duvall, S.E. and Barron, M.G. 2000. A screening level probabilistic risk assessment of mercury in Florida Everglades food webs. Ecotoxicology and Environmental Safety 47:298-305. doi:10.1006/eesa.2000.1949 

This article was about the difference in mercury levels among raccoons, Great Egrets, and alligators.

Herring, G.; Gawlik, D.E.; Rumbold, D.G. 2009. Feather mercury concentrations and physiological condition of great egret and white ibis nestlings in the Florida Everglades. Science of the Total Environment 407:2641-2649. doi:10.1016/j.scitotenv.2008.12.043

This article was about differences in mercury levels between Great Egrets and White Ibises and their physiological conditions.

Ploger, B.J. and Medeiros, M.J. 2004. Unequal food distribution among great egret Ardea alba nestlings: parental choice or sibling aggression?. Journal of Avian Biology 35:399-404. doi:10.1111/j.0908-8857.2004.03253.x

This article was about fighting amongst siblings for food and how parents distribute food among young.

Second pond where I found Great Egret

This photo is of the pond where I found the Great Egret that I watched for one hour (the second overall pond where I found it).

Observed individual here

This photo is of the pond where I first found the Great Egret, where I followed my individual.

Great Egret I observed

This is the Great Egret that I observed, surrounded by a flock of Canada Geese.

Closer look at a Great Egret

A closer look at a Great Egret. Note the green lores and long semiplumes of the breeding condition. Note the yellow bill and black legs. This photo was taken from a Google Images search.

White morph Great Blue Heron

A closer look at a white morph Great Blue Heron. Note the yellow legs that distinguish it from the Great Egret. This photo was taken from All About Birds.

Variation in bird abundance

These graphs came from the article by Crozier and Gawlik (2002). They show the abundance of various birds sampled in different nutrient levels in the Everglades, showing that abundance varies among species.

Mercury levels in Great Egrets and White Ibises

This graph came from the article by Herring, Gawlik, and Rumbold (2009). It compares the levels of mercury in the feathers of Great Egrets and White Ibises in 2006 and 2007, showing that Great Egrets had an overall greater concentration of mercury than White Ibises in their feathers, and that concentrations in both species were higher in 2006 than in 2007.

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2 thoughts on “The Great Egret: greater than you might think

  1. Pingback: The Great Egret

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