November 2009


HERONS AND EGRETS

This month we will discuss seven species of herons and egrets in south Florida. All of these wading bird species feed in the daytime, may occur in the Refuge, and are in the family Ardeidae. Bitterns and night herons are in the same family but they are either nocturnal or secretive feeders so they are not included in the discussion.

 Great blue heron, great egret, and snowy egret looking for fish
Great blue heron, great egret, and snowy egret looking for fish

We will start with the important similarities among the great blue heron, great egret, little blue heron, reddish egret, tricolored heron, snowy egret, and green heron. These similarities are mainly anatomy, way of feeding, and early life history.

Then we will discuss how differences might decrease competition between two species of Ardea and among four species of Egretta. The focus will be on habitats ("addresses") and feeding niches ("professions").

CLASSIFICATION OF SPECIES
Disobeying Refuge signs 
Disobeying Refuge signs:
little blue heron
and great blue heron

Kingdom ANIMALIA
Phylum CHORDATA
Class AVES
Order CICONIIFORMES
Family ARDEIDAE
Ardea herodias (83 ounces)
great blue heron
Ardea alba (25 ounces)
great egret
Egretta caerulea (12 ounces)
little blue heron
Egretta rufescens (15 ounces)
reddish egret
Egretta thula (13 ounces)
snowy egret
Egretta tricolor (13 ounces)
tricolored heron
Butorides virescens (7 ounces)
green heron

 

SIMILARITIES OF HERONS AND EGRETS

Egrets And Herons Are Not Really Different

The discerning reader will have noticed that some of these species are called herons and others egrets. Only two of the three species of "egrets" have the striking breeding plumes called "aigrettes." The other "egret" and all the species of "herons" have fancy breeding plumes on the head, breast, or back. All were killed for their breeding season feathers at the turn of the last century and as a result many became endangered species.

Great egret displaying "aigrettes" Aigrette hat: A single egret sadly sat,
its mate became a feathered hat
Great egret displaying ‘aigrettes’ Aigrette hat: A single egret sadly sat,



And both "herons" and "egrets" get brighter colored beaks, lores (bare skin in front of the eye), eye iris, and legs when courting.

Head of breeding great egret 

Head of breeding tricolored heron with red  eye and fancy plumes

Head of breeding great egret  Head of breeding tricolored heron with red  

Similarity in Anatomy and Mode of Feeding

Despite the range in size from the great blue heron (46 inches long) to the green heron (18 inches long) all have many similarities. They all have sharp long beaks, long necks, long legs, and all catch live prey in their beaks from just under, on, or just above the marsh water surface.

To catch prey all Ardeid herons and egrets uncurl their S-shaped neck as they strike. This takes less than a second! They either grab small prey at the tip of the bill or spear larger prey.

Sequence of striking at and capturing prey Diagram of S-shaped heron neck, neck
bones and esophagus 
Sequence of striking at and capturing prey  Diagram of S-shaped heron neck, neck bones and esophagus 

  Great blue heron skull

The skull of herons and egrets is almost all beak! The beak is an awesome hunting weapon.

Great blue heron skull
   
Head on view of snowy egret

Herons and egrets have a channel along the side of the head so that their huge eyes can see forward with an overlapping field of vision. This allows binocular vision with depth perception. Depth perception is necessary for them to accurately strike living prey.

Head on view of
snowy egret
 

 

Similarity in Early Life History

Herons and egrets are also similar in their early life history in that they have altricial hatchlings. These are nearly blind, featherless, and helpless. In contrast ducks and moorhens have precocial hatchlings. These hatchlings see well, have down feathers, and can swim and feed on their own.

Altricial herons must be fed by parents Precocial moorhens swim on their own, feed themselves, but cannot fly yet
Altricial herons must be fed by parents Precocial moorhens swim on their own, can feed themselves, but cannot fly yet. 

The largest, oldest, and loudest great egret nestling get the most foodHeron and egret eggs hatch asynchronously; so the 2-4 altricial nestlings may differ from each other by a day in age and size. If food is scarce only the first one to hatch -- the largest, loudest, and strongest -- will get enough food from its parents. This provides an insurance that at least one chick will survive if food is scarce. If food is abundant all nestlings will survive to become independent, i.e. to fledge.

The largest, oldest, and loudest
great egret nestling gets the most food

 

DIFFERENCES AMONG HERONS AND EGRETS

Feeding Niches

Based on beak size and leg length it is reasonable to hypothesize that the great blue heron would catch the largest prey and wade in the deepest water. And, the green heron would catch the smallest prey and wade in the shallowest water. Observations of prey caught confirm the prey size prediction and the next two figures support the feeding depth prediction.

Silhouettes of great blue heron, great egret, an Egretta species (all four species are the same size), and green heron. The leg length determines maximum wading depth. 

 
   
 

Tricolored herons catch fish up to a depth of 16 inches, just up to its belly feathers

 

 

A corollary of beak and body size is the kind of prey that a heron or egret will try to catch. The very large great blue heron will not waste time striking at tiny prey. Rather it stands, watches, and waits for a large prey to come close. But the very small green heron cannot catch large prey and it should and does try for any small prey. It will get plenty of calories overall since small prey like mosquito fish and killifish are so much more abundant than large prey like bass and pickerel.

 Feeding Problems of the Four R's

Tricolored heron trying to deal with the problem of reflection 

Herons and egrets all catch most of their live prey under the water surface and this poses the problems of the four R's. Can you think of what these problems are? On a sunny, windy, or rainy day what would make it difficult to see a fish under the water? I have already given one of the R's away and that is that rain-drops disturb the water surface. A related problem on a windy day is ripples. Reflection is a problem on a sunny day.

   Tricolored heron trying
to deal with the
problem of reflection 

And even with no reflection, refraction is a problem. To see why refraction is a problem put a straight wire into a bowl of water and look at it from above and from the side. It looks like the wire bends at the water surface. We really do not know how birds compensate for refraction when striking at a live prey under the water.

 

The green heron avoids the problems of the four R's. It feeds in the shade along the shore in very shallow water. Or it feeds where duckweed completely covers the water and strikes when a fish disturbs the plant cover. Or it may use bait, such as a bit of dirt or a piece of leaf, that it drops on the water surface to attract small fish! I keep hoping to see this wonderful behavior in person. It is apparently learned from other green herons.

 A green heron watches for prey to disturb a covering of duckweed 
   A green heron watches for prey
to disturb a covering of duckweed.

 

The four medium sized wading birds in the genus Egretta solve the problems of the four R's in similar ways though each emphasizes one or two modes.

 A snowy egret jiggles its yellow foot to
lure prey to the water surface.

 
 

The tricolor heron and reddish egret both use their wings to create a shade canopy to reduce reflection and both run actively while foraging. Here, a reddish egret runs to stir up prey and uses its wings to shade the water and so reduce reflection.

Reddish egret   

 

The little blue heron feeds methodically in the shade and catches prey on the duckweed and off plants at the water's edge.

A little blue heron watching for prey

AVOIDING COMPETITION
AMONG SPECIES IN THE SAME GENUS

Herons and Egrets in the Genus Ardea

Both the great blue heron and great egret are in the same genus, Ardea, but they probably do not compete much for food. They differ three-fold in body weight and bill size and so the great blue heron can catch and eat much bigger prey.

Great egret with a frog  Frog resists being swallowed; never give up  
Great egret with a frog  Frog resists being swallowed; never give up  
 
 

All the four species of Egretta are almost identical in size, wing-span, leg length, and beak length and can and do catch and eat the same kinds and sizes of prey, especially mosquito fish, killifish, and insect larvae.

Tricolored heron with an insect larva 
  Tricolored heron with an insect larva 

 

To avoid competition the Egretta species tend to feed in different habitats. Here are some examples of habitat separation that I have seen in south Florida

 

REVIEW QUESTIONS

a. All egrets have spectacular breeding feathers called aigrettes.

b. All egrets and herons develop brighter colors on their non-feathered parts, like beaks and face skin, in the breeding season.

c. All egrets and herons have a long S-shaped neck that "uncoils" as they strike at live prey with their long sharp beaks.

d. To avoid strong competition snowy & reddish egrets and little blue & tricolored herons eat very different sizes and kinds of live prey.