The Great Egret and Habitat Selection

 

The role of adaptability to a species' survival

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This is a pictorial essay on the role of habitat in the life history of the Great Egret. The egret is classified as a waterbird but is also found in grasslands, wetlands, agricultural fields, desert, wooded swamps, and even the ocean. This series of pictures demonstrates the adaptability of the egret by showing the different types of environment that it can successfully exist in.

Perhaps no other taxonomic group has, and presumably exercises, the potential for habitat selection that birds do. Birds are extremely mobile and wide ranging, and of the range of habitats they pass through or over, only specified ones are used for breeding or foraging or wintering. The uniqueness of birds with respect to habitat choice was discussed by Hilden (1965), who distinguished between and summarized the ultimate and proximate factors involved in the choice. The evolution of habitat preferences is determined by, and determines, the bird's morphological structure and behavioral functions, its ability to obtain food and shelter successfully in the habitat. The proximate stimuli for the choice of habitat might be structural features of the landscape, foraging or nesting opportunities, or the presence of other species. Such factors might operate independently, hierarchically as a system of sequential decisions or overrides, or synergistically in a complex fashion or "gestalt".

Habitat Selection in Birds, Martin L. Cody

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 A habitat is used for breeding, foraging, and wintering. When a species does all three activities in the same area then it is a permanent resident. The Great Egret in most areas is not a permanent resident. It breeds as a colony in suitable trees near water sources, and then disperses.

The Great Egret nests with many other Great Egrets, generally in an evergreen forest near a convenient source of water. This site is the Audubon Canyon Ranch in Bolinas California, a sanctuary set aside for nesting egrets. There are about thirteen birds visible just left of center in this picture. The lagoon and ocean can be seen in the background.

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Kelp
Wooded swamp
Wetlands
From the shore of the wetlands
In the water at the wetlands
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Breeding
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 By nesting in the trees the egrets are protected from many of their enemies but they are still vulnerable to Gt. Horned Owls from the air and racoons coming up the tree to take the eggs from the nest.

Here there are at least four pair of egrets at nest sites and up at the upper right is a Great Blue Heron that is sitting on its eggs.

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Desert
Kelp
Wooded swamp
Wetlands
From the shore of the wetlands
In the water at the wetlands
In the reeds of the wetlands
Breeding
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Two birds are mating at their nest site. The egrets will gather together to breed and will feed near the colony but after the young have fledged they will disperse from the breeding area.

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Kelp
Wooded swamp
Wetlands
From the shore of the wetlands
In the water at the wetlands
In the reeds of the wetlands
Breeding
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The Great Egret is classified as a water bird. It has long legs that allow it to wade in the water to obtain food. But its long legs and long beak make it possible for this predatory bird to also feed in a variety of other habitats. Its ability to feed in different habitats means that there are more types of animals that it can eat, and that makes it more successful as a species.

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Wooded swamp
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From the shore of the wetlands
In the water at the wetlands
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Breeding
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 There are many types of grasslands such as this grassland in Los Banos, in the Central Valley of California. There is water in the background but the birds are feeding in the grass.

In addition to the Great Egrets there are White-faced Ibis, Snowy Egrets and Cattle Egrets. There is very little competition between the four species. The White-faced Ibis has a slightly decurved beak which requires that it feed on different prey items than the Great Egret. The Snowy Egret and Cattle Egret are smaller than the Great Egret, and faster and also feed on different prey.

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Wooded swamp
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From the shore of the wetlands
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Breeding
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Seeing a Great Egret in the California desert is a strange site. This bird was photographed in Salton Sea State Park in California.

Great Egrets have a wide range of food that they can eat. It includes all the vertebrates (fish, amphibian, reptile, bird, and mammals) and many invertebrates such as insects. In this environment it is probably looking for insects and other invertebrates.

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From the shore of the wetlands
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Habitat provides the opportunity to find appropriate food and the opportunity to feed in a secure manner. Basically, each species wants to feed in a manner that allows it to not compete, for a prolonged period of time, with other species. Habitats can successfully facilitate species being able to feed in a non-competitive way.

Many species are specific to one or two types of habitat, or even one or two parts of a single habitat. Other species are very adaptable to a variety of habitats. The White-tailed Kite in California is easily found in most wetlands, sometimes sitting near a Great Egret. But it feeds almost exclusively on Meadow Voles (Microtus microtus). This is why it is classified as a Microtene Obligate. Its choice of habitat is determined by the presence of the vole. (For a discussion on the White-tailed Kite go to the White-tailed Kite Curriculum).

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The Great Egret, as with other long-legged waders, has a variety of hunting styles. This bird is feeding on land and utilizes its long legs, long neck, and sharp beak.

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From the shore of the wetlands
In the water at the wetlands
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Breeding
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Generally, egrets feed by themselves. It seems that on this day some kind of signal went out that there was a special source of food that was very available. Perhaps the egrets noticed that the White Pelicans had gathered in this area to feed and followed them.

There are about 30 Great Egrets, and almost the same number of Snowy Egrets all competing for food with a group of White Pelicans (in the middle). This waterway was part of a much larger wetland (Baylands) in Palo Alto, California.

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Wooded swamp
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From the shore of the wetlands
In the water at the wetlands
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Breeding
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This egret is flying over a wetlands found in California, that is made up mostly of pickle weed. This is a tidal area so the egret will fluctuate its feeding in response to the tides. A high tide will eliminate much of the real-estate that the mammals depend on and make them more accessible to the egret during that time.

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Desert
Kelp
Wooded swamp
Wetlands
From the shore of the wetlands
In the water at the wetlands
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Breeding
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A water site has three areas that an egret can feed in. It can feed on the shore of the area, as this bird is doing, looking for small vertebrates and invertebrates. It can then move into the water and feed within the vegetation close to shore. Eventually it can move into the deeper water to feed, as the next picture demonstrates.

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Desert
Kelp
Wooded swamp
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From the shore of the wetlands
In the water at the wetlands
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Breeding
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One of the feeding techniques of a Great Egret is to stir things up to find food. It does this by using its feet to disturb the bottom of the wetlands. It drags its foot along the bottom and this will dislodge anything that has sought refuge on the bottom. If the animal swims up to the surface, then the egret can grab it with its sharp beak.

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Desert
Kelp
Wooded swamp
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From the shore of the wetlands
In the water at the wetlands
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Breeding
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The egret is capable of feeding at many different levels of a particular habitat. This egret is as deep in the water as it can go, as the water level is almost up to its feathers. The feathers are not waterproof, so the Egret needs to avoid getting them wet. When the feathers soak up water the bird gets heavier and thus it is harder to fly.

Grassland
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Kelp
Wooded swamp
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From the shore of the wetlands
In the water at the wetlands
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Breeding
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Egrets are generally solitary feeders, but here a Great Egret is feeding with a Snowy Egret in the middle and a Great Blue Heron on the right.

The food must be consistently available here. If you look at the rocks behind the birds you can see droppings from these birds or others as proof that this is a frequent feeding area for large birds.

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This bird is feeding in an area that is protected by a pier so it is not vulnerable to waves. From afar it looks as if the egret is standing on the water, but actually is standing on a kelp bed which can support the bird and its hunting. A kelp bed attracts a wide variety of vertebrates, so the Egret can hunt for food from this position. Despite its size the egret does not weigh very much and can easily balance its weight on the bed of kelp.

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In the Corkscrew Swamp National Park in Florida the egret is surrounded by mostly land. It can use its long beak to grab small vertebrates and invertebrates that it finds on the ground or along a tree limb. The water that is available is in the form of small waterways that move through the swamp.

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One way to appreciate the role of habitat in a species' success is to look at distribution maps. In Sibley's Birds of North America, multicolored maps give a fair report of each species distribution.

When we look at the Reddish Egret we realize that it is only found along the coastal waters of the southeast; mainly between and including Texas and Florida. The Great Egret, which in many ways is morphologically similar to the Reddish Egret, is found in 45 of the 48 contiguous states. The Great Egret is not found consistently in Montana, Wyoming, and Colorado. The relative of the Great Egret, the Great Blue Heron, is found in all 48 contiguous states.

Why is the Great Blue Heron found in more states than the Great Egret? Can we learn something about the role of habitat in the ecology of waterbirds by comparing the distribution of the Great Blue Heron and the Great Egret?

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 Appendices

A - Food Items of Gt. Blue Heron, Reddish Egret, and Snowy Egret

B - Body size Comparison of Gt. Blue Heron, Reddish Egret, and Snowy Egret

Appendix A Top  

Food Items of Gt. Blue Heron, Great Egret, Reddish Egret , Snowy Egret and White-faced Ibis (Notes taken from A. C. Bent's Life Histories of North American Birds

Great Blue Heron Food Items (From Bent's Life Histories of North American Birds)

Food: The principal food of the great blue heron is fishes of various kinds and it seems to be willing to accept whatever kind of fish is most easily available. Ora W. Knight (1908) says:

Frogs, eels, horn-pouts, pickerel occasionally, suckers, shiners, chubs, black bass, herrings, water puppies, salamanders, and tadpoles, are the items I have discovered among their rations. They do not frequent as feeding grounds the spots where trout usually congregate, and I have very strong doubts that they eat trout, except very rarely, let alone consuming them in the vast quantities certain persons have affirmed.

It fishes by night as well as by day and employs two very different methods, still hunting and stalking. The former is the best known and probably the commonest method. Standing as still as a graven image in shallow water, where fish are moving about, it waits patiently until one comes within reach, when a swift and unerring stroke of its well trained bill either kills or secures the fish. Usually the fish is seized crosswise between the mandibles; if it is a small one, it is tossed in the air and swallowed head first, so that it will slip down easily; but if the fish is a large one, the heron may walk ashore with it and beat it on the ground to kill it or may kill it by striking it in the water. I have never had the patience to watch a heron long enough to learn how long it would stand and wait for a fish to come to it. I have found it more interesting to watch it stalking its prey, a more active operation. Slowly and carefully, with stately tread, it walks along in water knee deep, its long neck stretched upward and forward; its keen eyes are scanning the surface and an occasional quick turn of the head indicates a glimpse of a fish; suddenly it stops, as if it had seen a fish, but it moves on again; at last comes its chance, as in a crouching attitude the long neck darts downward, quick as a flash; the stroke is not always successful, but sooner or later the heron secures a meal. Sometimes, in its eagerness, the heron may step beyond its depth and lose its balance, but a few flaps of its wings restores its equilibrium and its dignity.

Audubon (1840) says:

The principal food of the great blue heron is fish of all kinds; but it also devours frogs, lizards, snakes, and birds, as well as small quadrupeds, such as shrews, meadow mice, and young rats, all of which I have found in its stomach. Aquatic insects are equally welcome to it, and it is an expert flycatcher, striking at moths, butterflies, and libellulae, whether on the wing or when alighted. It destroys a great number of young marsh-hens, rails, and other birds; but I never saw one catch a fiddler or a crab; and the only seeds that I have found in its stomach were those of the great water lily of the Southern States. It always strikes its prey through the body, and as near the head as possible. Now and then it strikes at a fish so large and strong as to endanger its own life; and I once saw one on the Florida coast, that, after striking a fish, when standing in the water to the full length of its legs, was dragged along for several yards, now on the surface and again beneath. When, after a severe struggle, the heron disengaged itself, it appeared quite overcome, and stood still near the shore, his head turned from the sea, as if afraid to try another such experiment.

 

Wilson (1832) includes in its food grasshoppers, dragon-flies and the seeds of splatter docks. Mr. Hastings says that it eats great quantities of insects and mice. When the grasshoppers have been thick he has seen it feeding in the open meadow on these insects entirely, often for two hours at a time; it does not chase them but stands very still, allowing the insects to come within reach of its quick beak. Arthur H. Howell (1911) adds crustaceans to the list. Bartlett E. Bassett wrote me that a bird he shot for me was carrying a large black snake in its bill. Altogether the food habits of this species are decidedly beneficial. It may occasionally take a few trout, but it does not ordinarily frequent the streams where trout are found.

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Great Egret Food Items (From Bent's Life Histories of North American Birds)

Food: Egrets obtain their food in the marshes and rice fields and around the marshy shores of lakes and ponds where their tall, graceful figures tower above the low vegetation or are reflected in the smooth waters as beautiful silhouettes in white. Their movements are stately and the strokes of their rapierlike bills are quick and sure. Their food consists only partially of small fishes and it includes frogs, lizards, small snakes, mice, moles, fiddlers, snails, grasshoppers, and other insects, as well as some vegetable matter. Oscar E. Baynard (1912) says:

Food of 50 young egrets that was disgorged by them at the nest immediately after being fed, running over a period of four weeks. The total of the 50 meals follows: 297 small frogs, 49 small snakes, mostly the water moccasin, 61 young fish, suckers, not edible, 176 crayfish.

Dr. Alexander Wetmore (1916) says of a bird taken in Porto Rico:

The single stomach available for examination contained 4 per cent of vegetable rubbish taken as extraneous matter with the animal food. Remains of one mole cricket (Scapteriscus didactylus) and seven entire grasshoppers, with fragments of many more, were found, as well as a moth and three large dragon flies. A small goby and seven entire frogs (Leptodactylus albilabris) with fragments of others, made up 69 per cent of the contents. Orthoptera amounted to 15 per cent, a surprising fact and one that should be given due weight in considering the status of this species.

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Reddish Egret Food Items (From Bent's Life Histories of North American Birds)

Food: Being a bird of the seacoast the reddish egret probably obtains most, if not all, of its food in salt water. Large numbers of these birds maybe seen at times standing in the shallow waters around their breeding grounds, or way off on the mud banks or sand shoals in the lagoons, where they stand motionless watching for their prey or walk about slowly in search of it, until the rising tide forces them to leave. Mr. Cahn (1923), however, writes:

Just where the old birds went for food is a question. On a quiet evening hundreds of them would be seen standing in the shallow water that surrounds their island, but the birds remained almost motionless in the red glow of the setting sun, and there was little evidence that they caught their food so near home. On the contrary, with the approach of evening and the lessening of the intensity of the sun, the birds usually took wing and disappeared in small groups to the southwest, in which direction undoubtedly lay their feeding ground. The food consists of a small fish and frogs, tadpoles, and an occasional crustacean, which are probably caught in the marshes of the mainland coast. Before dark the birds were all back and at the nest, and there was relatively little night activity. With the daylight the birds would fly away once more to the feeding grounds, returning again before the heat of the sun was sufficiently intense to endanger their precious eggs or babies. Then followed another period of inactivity during which the birds remained close to the nest, preening their wonderful feathers or playing at repelling intruders.

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Snowy Egret Food Items (From Bent's Life Histories of North American Birds)

Food: Audubon (1840) has described the feeding habits of the snowy egret so well that I can not do better than to quote his words, as follows:

The snowy heron, while in the Carolinas, in the month of April, resorts to the borders of the salt-water marshes and feeds principally on shrimps. Many individuals which I opened there contained nothing else in their stomachs. On the Mississippi, at the time when the shrimps are ascending the stream, these birds are frequently seen standing on floating logs, busily engaged in picking them up; and on such occasions their pure white color renders them conspicuous and highly pleasing to the eye. At a later period, they feed on small fry, fiddlers, snails, aquatic insects, occasionally small lizards, and young frogs. Their motions are generally quick and elegant, and, while pursuing small fishes, they run swiftly through the shallows, throwing up their wings. Twenty or 30 seen at once along the margins of a marsh or a river, while engaged in procuring their food, form a most agreeable sight. In autumn and early spring they are fond of resorting to the ditches of the rice fields, not unfrequently in company with the blue herons.

Wilson (1832) adds: "It also feeds on the seeds of some species of nymphae, and of several other aquatic plants." Oscar E. Baynard (1912) found that 50 meals of young snowy egrets consisted of 120 small suckers, 762 grasshoppers, 91 cut-worms, 2 small lizards, 29 small crayfish, and 7 small mocasins, a most interesting collection, which proves that this species is decidedly beneficial. Dr. Alexander Wetmore (1916) says of its feeding habits in Porto Rico:

Frequently the snowy egret feeds in lowland cane fields, especially when these are wet or partly flooded. Often in flocks of three or four they feed in the dry upland pastures. Two stomachs were available for examination, both of birds which had been feeding in mangrove swamps. The main content of these is animal matter, vegetable remains occurring only as rubbish secured with other food and amounting to but 1 per cent. One bird taken near Rio Piedras had eaten two dragon-fly nymphs, a small crab, a lizard, and a small frog. The stomach of the other, secured, near Mameycs, was nearly filled with bones of small gobies, the remainder of the animal food consisting of fragments of flies of the family Dolichopodidae and bits of a grasshopper. In their excursions to drier fields the birds must secure other insects. They feed to a large extent upon fish, but the fishes taken are of no great importance and the birds are not abundant enough to become noxious.

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White-faced Ibis Food Items (From Bent's Life Histories of North American Birds)

Food: Like the white ibis, this species often makes long flights to its favorite feeding grounds, along the banks of rivers and on the shallow margins of muddy pools, ponds, and marshes. On the hog-wallow prairies of the coastal plains of Texas are many such pools, where we often saw this species feeding, walking about gracefully and probing in the mud; the crops of birds we shot here were crammed full of ordinary earthworms. Its food also consists largely of crawfish, various small mollusks, insects and their larvae, small fish and frogs, newts, leeches, and various other forms of low animal life. Probably a certain amount of aquatic vegetation is also eaten.

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Appendix B

Comparative body sizes of certain Ciconiformes

Species
Length
Wing span
Weight
Great Blue Heron
46"
72"
5.3 lbs.
Great Egret
39"
51"
1.9 lbs
Reddish Egret
30"
46"
1 lb.
Snowy Egret
24"
41"
.75 lbs

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