Seen from a distance, lovely and still at water’s edge, or on the wing, seemingly in slow motion, as if held aloft by cosmic strings, herons are easy to romanticize. Herons are, in fact, formidable predators, and, as is usually the case with top predators, they lead mostly solitary lives. (Herons often gather in groups, roosting and feeding in proximity to each other, but at the same time maintain a strict and definite zone of personal territory: Woe to the neighbor who breaches this space.) The courting rituals of herons may hold for us the allure of elegant and stately displays, but for the birds these rites are a process through which they drop their natural aggression as territorial predators to perform the essential task of producing offspring.
There are six heron species in the San Francisco Bay Area: great blue heron, great egret, snowy egret, cattle egret, green-backed heron, and black-crowned night heron. (Egrets are also herons; their name derives from “aigrette,” the gorgeous nuptial plumes for which they were hunted almost to extinction in the early 20th century.) Great blues sometimes mate as solitary pairs, but generally all these herons breed in colonies. The advantages of colonial breeding are several: With many possible partners, the duration of courtship is shortened, and there is more sexual stimulation and a better selection of mates. Also, a colony provides collective defense against other predators (which was probably more important in the past, when large predators were more common). In some areas, colonies may consist of only one species, but around the San Francisco Bay, several species are present in each heronry.
The nesting season begins between late February and April, with the males arriving at the colony (or a solitary nest) and claiming an existing nest or a new nesting site. Tree nesting is common among large herons like the great blue and great egret, while the smaller snowy egret usually nests in a bush or tree five to ten feet above the ground. The great blue nests toward the tops of trees, the great egrets below them, and the snowies in the lowest branches. The nests can be fairly close together, especially among the snowies, the nest density increasing in direct proportion to population density.
The defended territory around the nest shrinks as more males arrive and occupy nearby sites. As females reach the colony, usually a few days after the males, courting begins. Both male and female herons have grown their nuptial plumage, gracing the head, throat, and back, and they have undergone color changes of the legs, bill, and lores (bare skin between the bill and eyes). In the great blue the lores changes from blue to green, the legs and bill yellow to pink. The great egret’s lores go yellow to green, and the bill yellow to red. The snowies’ lores and feet change from yellow to pink. The subtle qualities of these feather and color changes, and their intimation of strength and health, are probably quite evident to the courting birds. The males begin courtship with the Circle Flight, flying in a wide loop around the colony with an exaggeratedly slow wing beat, neck fully extended instead of the usual curve placing the head back over the shoulders. The powerful wing strokes make an audible “whomp.”
Having gained the attention of females, the male now stands at the nest site doing the Stretch display. Plumes bristling, he gives a call as his neck and bill reach the vertical position, legs flexing as the neck bends back; in some species, he snaps his bill at the display’s end. (Once a pair forms, an abbreviated Stretch becomes their Greeting Ceremony, employed each time one joins the other at the nest.) The great egret uses a variation called the Backward Stretch, where it shakes its plumes while undulating its neck. Another display used while the birds are still feeling aggression toward each other is the Forward, in which the bird spreads its wings, draws its head back and pecks at the other. Herons the world over engage in stereotypical displays, but there are broad and subtle variations in these behaviors between species, even between individuals within a species.
A female’s initial attempts to join the male at the nest are sometimes accepted by the male, but are often rebuffed, the male driving her away in a gesture known as the Supplant. The duration of such rejection ranges up to a couple of days, as the male’s aggressiveness calms enough to accept the female’s presence. Courtship displays continue, including the Snap, in which the head wags atop the fully extended neck, with all the nuptial plumage displayed. This pose can progress into Bill-Clappering, where both birds fluff their feathers and repeatedly snap their bills. This behavior can culminate in the female grasping the male’s bill in her own, a pivotal gesture marking his waning aggressiveness toward her. The male may then present the female with a stick for the nest, a domestic gesture that symbolizes his acceptance of her as his mate. The female uses this stick to begin a new nest, or add to an old one. The nest, initially a flimsy platform, can become quite massive over the years. Even after creation of a pair bond, the displays of ritualized aggression continue, with the actual hostility progressively diminishing, just as the courting colors of the birds’ bare body parts fade while they settle into breeding behavior. Greeting one another with the abbreviated version of the Stretch, the pair will also Bow to each other and preen one another’s feathers.
The male continues to bring nesting materials to the female until the nest is complete. Copulation takes place at the nest, the female storing the sperm in her oviduct to fertilize her eggs. She produces an egg every other day until there are three to five eggs. They share the incubation and raising of the young, until the day arrives, after one to three months (depending on the species), when the youngsters must fend for themselves. Nest by nest the colony disperses in every direction, some to travel hundreds of miles. The adults return to the mostly solitary life of a top predator, and the youngsters attempt to learn their trade well enough to be among the 30 percent who survive their first year, and perhaps even arrive at that third spring when they fly to a nest site and stand, hormones coursing, resplendent in the breeze wafting off the water.