An iconic sight, during the “dog days” of late summer in central Texas, is a statuesque Great Egret poised motionless at the edge of a stock tank. Flocks of Cattle Egrets speckle pastures, accompanying herds of cattle. Elegant Snowy Egrets stalk wetlands.
Ever wonder why egrets are white? One would think that being so bright would make them easy targets for predators, and natural selection would lead toward more muted colors. Evolution does seem to have played a role, but predation has apparently not been the deciding factor. Egrets are large enough to ward off attacks by most avian predators. They also roost communally at night, a strategy offering protection from Great Horned Owls. And they inhabit open environments – lakeshores, mudflats, marshes, and grasslands – where they can spot an approaching terrestrial predator from afar.
For egrets, visibility appears to be an advantage. Just as we can see bright white birds from long distances, so can other egrets. Some biologists think the white plumage of egrets is a way of signaling to other wading birds that food is present. A large assemblage of white birds is like a flashing neon fast-food sign to a bird flying overhead. But why would a bird want to signal to others that food is present? Would it not be better to keep the bonanza to itself? Not necessarily. Even when food is present, it is not always easy to catch.
Egrets and other herons are strictly carnivorous, feeding primarily upon fish, amphibians, aquatic invertebrates, and – especially in the case of Cattle Egrets – grasshoppers and other large terrestrial insects. The most common method for finding their food is to stalk slowly, or to stand statue-like and wait for prey to come within range. Since fish must keep water moving over their gills to obtain oxygen, they usually move around a good bit, so the herons’ method is often successful. Still, the prey don’t want to be caught, and can be quite secretive. It helps to have something flush them out of their hiding places.
A large animal, such as an alligator, moving through the water will send fish fleeing – often into striking distance of an egret’s sharp bill. The alligator is then unwittingly serving as a “beater” (as in “beating the bushes”) for the egret, a commensal relationship in which the alligator neither gains nor risks anything. Of course, following an alligator too closely can be hazardous. Ibises and spoonbills, which feed by probing in bottom sediments or sweeping their bills through the water, are also good beaters, and herons often follow these safer companions.
A neighboring heron can serve quite nicely, too. When herons congregate in a pond or marsh, the movements of one will often spook fish and other aquatic animals toward its neighbor. Observers have noticed that a heron will catch more food in the same length of time when foraging in a group than when feeding alone. Unlike the commensal relation between alligators and egrets, both parties may benefit from the association, so it is more properly termed a symbiotic relationship.
So why is it that all herons are not white? For many, there may be an advantage to darker or cryptically patterned plumage. Bitterns and Green Herons, for instance, stalk through dense marsh vegetation or along wooded streams. The night-herons hunt at dawn and dusk. White plumage would handicap these birds by making them more conspicuous to their prey.
Great Blue Herons will also sometimes hunt in more confined areas, such as along rivers and smaller ponds. In these areas, competition from other herons may outweigh the benefits of a beater. Intriguingly, the wide-open marshes of the Everglades have produced an all-white race of the Great Blue Heron – called the “Great White Heron.”
There was a time when a new predator made the white plumage of Great and Snowy Egrets a tremendous liability. In the late nineteenth century, human plume-hunters decimated populations of these birds all along the east coast of the United States, to sate the demand for egret plumes for fashionable ladies’ hats. Fortunately, public outcry against the slaughter, instigated in part by fledgling Audubon Societies, eventually made the wearing of these “aigrettes”— as the egrets’ spectacular nuptial plumes are called – as unpopular as wearing fur coats today. Legislation in the early twentieth century banned the sale of feathers, the market dried up, and egret populations rebounded – an early example of successful environmental activism. And the dazzling, big white birds are back again, now drawing human admirers as well as other herons.