The snowy egret- looks can be deceiving

Author: Christopher Rizk

It was a Sunday morning at the beautiful Brazos Bend state park, when I heard a deep croaking sound.  Maybe because of the harsh sound and the dreary weather, I sought out the source of the sound expecting to lay eyes upon a hippo or some other large mammal.  Instead, I laid eyes upon a snowy egret, Egretta thula, a crystal white, majestic looking egret with black legs and yellow feet.  In disbelief, I listened as it proceeded to croak again and thought to myself “looks really can be deceiving.”

As pursued the Internet I realized that I wasn’t the only one who was awed by the beauty of the snowy egret.  As the snowy egrets approach their breeding season in March, they form a magnificent plume.  The snowy egret’s beauty actually earned itself a place on the 37 cent stamp, issued by the U.S. Postal service.  Unfortunately, not everyone was content with harmlessly admiring the snowy egret in the wilderness or on a stamp.  In the 19th century the fashion gods decided that the snowy egret’s plume would make the perfect addition to women’s hats.  So perfect in fact, that the plumes were more valuable then gold (by weight)!!  As you can probably imagine this almost led to the extinction of the snowy egret.  Thank God the Audubon society instituted laws to protect the great and snowy egrets, thus stopping this from happening.

Now these beautiful plumes that almost led to the snowy egrets demise had to be good for something, right?  According to Clay Green, these plumes may give the snowy egret a slight camouflage edge over the competition when foraging.  Green compared the effects of plume color between the snowy egret (white plume) and the little blue heron (dark plume) by quantifying how long the prey lingered in front of each bird and how much biomass each bird captured.  It appears that the white plume of the snowy egret did in fact confer an advantage when foraging in open water, most likely because it did not contrast with the background as the dark plume of the little blue heron did.  However the snowy egret did not have a significant advantage when foraging with a vegetative background (trees/forest) because regardless of the plume color, the plumes just blend in with the background (Graph Below).  Advantage or not, I still had no idea how the snowy egret captured prey in the first place.

As I watched several snowy egrets’ behavior throughout the day, I was a little confused.  Initially I saw one snowy egret standing around, not really moving just pecking at the water and looking around really confused, just like a kid who lost his mother in the middle of a park.  A little later I saw another snowy egret doing what looked like a bad rendition of Michael Jackson doing the moon walk, around a rather stationary ibis.  Later on I saw a group of four snowy egrets soar through the air and land near me.  They then proceeded to slowly walk around and peck at the water, in what looked like a coordinated fashion.

Thanks to Donald Kent’s paper on snowy egret’s feeding behavior, I was able to make sense of what I saw at Brazos Bend.  To my great surprise I found out that snowy egrets are actually not Michael Jackson fans- what I saw was probably typical foraging behavior.  Apparently snowy egrets have multiple feeding strategies including: standing, walking around slowly, and stirring the area with their feet.  I still wondered what the differences between these strategies were and which was the most popular.  Since snowy egrets consume many different types of food, they have several foraging strategies.  Although they can use any “strategy” to catch any type of food, there is a preferential strategy for each type of food.  When catching fish the snowy egret usually stands fairly still and goes in for the kill when it sees a desirable fish.  When they are in the mood for worms they usually adopt the “walking slowly” strategy.  In order to catch prawns they mix-up or stir the area with their feet.  Overall, the snowy egret is usually seen employing the walking slowly or foot stirring strategies.

Although my Michael Jackson hypothesis was foiled, I still had many questions: why was one egret foraging with an ibis, one foraging with another egret, and the third going at it solo?  Michael Erwin answered my questions and even more in his paper discussing the habitats of nesting birds.  Snowy egrets will forage for food alone or in small or large groups.  Most of the snowy egrets Michael Erwin observed were alone or in small groups.  What was even more interesting was that when foraging in a group, most of the time the group consisted of multiple species of related birds.  The average group consisted of about 10 snowy egrets, however the snowy egrets only comprised 39% to 55% of the group (the exact breakdown in the chart below).  The snowy egrets flexibility in group size and consistency parallels its many foraging strategies.   For example, when foraging in groups, the snowy egret was shown to most commonly forage with the glossy ibis.  Everything was finally starting to make sense: however, why the ibis?  Apparently, the ibis and snowy egret form a “Beater-follower” relationship in which both benefit through the team effort.  This comradeship isn’t limited to foraging either; the snowy egret is known to forage, fly, and form colonies with similar species.

Although women’s hat fashion may have taken a hit since the protection of the snowy egret, I must say it was well worth it.  What a tragedy it would have been to lose this bird for hats!  In case you want to read more about the snowy egret, the information I used in this blog can be found in the articles I have cited below.

Brzorad, J.N., Maccarone, D., Conley, K. 2004. Foraging energetics of Great Egrets and Snowy Egrets. Journal of Field Ornithology. 75: 266-280.

This paper has information detailing foraging behavior and what it costs the snowy egret energetically.

Erwin, R. M. 1983. Feeding habitats of nesting wading birds: spatial use and social influences. Auk 100:960-970.

This paper contains a multitude of information on snowy egret foraging behavior and social interaction.

Green, M. C., P. L. Leberg. 2005. Influence of plumage colouration on prey response: does habitat alter heron crypsis to prey? Animal Behaviour 70: 1203-1208.

This paper talks about the effects of the plume color on ability to capture prey.

Kelly, J. P., H. M. Pratt and P. L. Greene. 1993. The distribution, reproductive success, and habitat characteristics of heron and egret breeding colonies in the San Francisco Bay area. Colonial Waterbirds 16: 18-27.

This paper discusses the colonies formed by the snowy egret and other related species.

Kent, D. M. 1986a. Behavior, habitat use, and food of three egrets in a marine habitat. Colonial Waterbirds. 9: 25-30.

This paper tells all about the foraging behavior that the snowy egrets and other related species employ.  Also discusses habitat utilization.

Ohio Department of Natural Resources. Snowy Egret. , Web. 5 Feb 2011. <http://www.dnr.state.oh.us/Home/species_a_to_z/SpeciesGuideIndex/snowyegret/tabid/6761/Default.aspx&gt;.

Website containing a sound clip of the snowy egret and a few fun facts.

Photo of a snowy egret, Egretta thula, showing yellow feet and yellow upper bill area (not in breeding season). Taken by Johnath- http://www.flickr.com/photos/johnath

Photo of a snowy egret, Egretta thula, showing plume and red upper bill area (in breeding season). Taken by Len Blumin- http://www.flickr.com/people/9062441@N02

The above graph is taken from the 2005 Green and Leberg paper in Animal Bahaviour. Based on the three types of prey listed above, it appears that the snowy egret has an advantage due to its white plume color in open water (a). Although it may appear that the snowy egret also has an advantage in vegetation (b), it is not statistically significant. The advantages were based on the total biomass caught.

The above chart was taken from the 1983 Michael Erwin paper in Auk. This chart shows that the snowy egret (SNEG) will forage in groups ranging in size from 3 to more than 30 birds. It also shows that the majority of groups are mixed- meaning they contain multiple bird species with around 39%-55% of the group being snowy egrets. This chart highlights the cooperative foraging nature of the snowy egret.

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