Owls Aren't Wise and Bats Aren't Blind (20 page)
Even larger than the great blue, the whooping crane towers nearly five feet in height and is the tallest bird in North America. With a wingspan of seven and a half feet, whoopers in flight are truly majestic, one of the most stirring sights in all of our continent’s rich wildlife heritage.
Nowhere are the differences between great blue herons and whooping cranes more distinct than in their nesting habits. Great blues nest in colonies, known variously as rookeries or heronries. There they build huge nests out of sticks, sometimes locating them one hundred or more feet above the ground, in the tallest suitable trees available. So closely are nests crowded into a rookery that a single tree may bear a virtual village of the crude but impressive nests! Some heronries are extremely large and may contain as many as two thousand adult great blues, to say nothing of their numerous offspring. However, heron rookeries come in all sizes, and some may harbor a mere handful of nests. In rare cases, great blues may even be solitary nesters.
Scientists are still debating the reasons why great blues nest in colonies. Some theorize that the birds and their nests are more secure from danger in such numbers. Others believe that since the herons don’t mate for life, a large colonial gathering facilitates the annual task of selecting a mate. Still others feel that colonies permit great blues to share information about prime feeding sites. Perhaps a combination of all these advantages is responsible for the heron’s colonial nesting habits.
A heron rookery happened to be the indirect cause of an incident that our family treasures—an episode in which the misidentification of the great blue heron took a new, unexpected, and humorous twist. At the time there was a small rookery of great blues, visible from the highway but several hundred yards distant in a huge dead elm. Numerous spectators pulled their cars off the highway at this strategic spot in order to train binoculars on the nesting birds.
Our family was there one day, observing the herons, when a couple with two or three children joined the group of watchers. They happened to be standing next to us when one of the children inquired what the birds were. “Herrings,” their father replied. “Blue herrings.” All five of us heard this very distinctly, and we nearly choked with suppressed laughter. However, because we didn’t want to diminish the father in the eyes of his family, we bit our tongues and refrained from any comment. Back in our car and safely out of hearing, however, we lapsed into paroxysms of mirth!
After mating, the female great blue lays three to six or seven eggs, with an average of about four. These hatch in four weeks, and the parents are thence-forward committed to feeding the voracious appetites of their rapidly growing brood. Considering that the heronry may be located miles from good food sources, the adults are often taxed to the limit to provide nourishment for the young herons. Back at the nest from a fishing expedition, an adult great blue feeds its nestlings by regurgitating partially digested food into their eager beaks.
About two months after hatching, the young herons begin to leave the nest and fly about. Soon they scatter and begin to hunt for food on their own. Now they must quickly learn the many critical lessons about feeding and foraging, or else perish—as many do. However, great blues that survive to full adulthood may live as much as twenty years in the wild.
In direct contrast to the great blue heron’s tree colonies, whooping cranes are solitary ground nesters in marshy areas. There they construct mounds of bulrushes, cattails, and other marsh vegetation, with their nests in a hollow on top. Whereas the herons prefer trees, both for nesting and roosting, whooping cranes demand wide-open spaces where, one might assume, they can see predators approaching from a considerable distance. Indeed, trees are nothing but a hindrance to whoopers, because the construction of their feet is such that they can’t even grasp a limb, let alone roost for any length of time. Where the great blue has an opposable back toe that enables it to grip a branch in combination with the front toes, the whooper’s back toe is nonfunctional and almost vestigial.
As a sidelight, this anatomical feature of the whooper, which it shares with other species of cranes, may strike connoisseurs of Japanese art as a bit puzzling. Drawings and paintings from Japan frequently depict cranes roosting in trees. The answer to this little artistic conundrum is that Japanese cranes can’t roost, either; the Japanese simply liked the effect gained by portraying them thus, and opted for artistic effect over scientific accuracy.
One of the most notable features of whooping crane behavior is their dramatic and highly ritualized mating dance. Male and female first face each other, then leap high off the ground with their legs and feet pointed at each other. Following this display, the pair bow toward each other, then repeat the entire performance over and over, often interspersing their movements with croaking calls.
After mating, the female whooper lays only two eggs. This number alone would represent a low reproductive rate, but that’s only half of the crane’s problem. Assuming that both eggs hatch, only the larger, stronger chick survives. Either the parents feed the larger one nearly to the exclusion of its smaller sibling, or the larger one pecks the smaller one to death or drives it out of the nest. Once outside the protection of the nest, of course, the chick quickly becomes prey for one predator or another.
This minuscule reproductive rate, combined with relentless hunting for its feathers and even more deadly habitat loss, caused the whooping crane population to dwindle steadily. Whoopers’ numbers were probably never very high, but by 1941 only fifteen whooping cranes remained alive in the wild—all that stood between their species and extinction! Incidentally, some sources list the wild population of whooping cranes in 1941 as either fourteen or sixteen, but the official count at the Aransas National Wildlife Refuge in Texas, where the whoopers winter, was fifteen.
Efforts to save the whooping crane began at that point—efforts fraught with all manner of difficulties, not the least of which is the whooper’s extraordinarily low reproductive rate. No one knew where the cranes nested, so until 1954 little could be done except give the remaining whoopers total protection and hope that they might eventually rebuild their shattered population, albeit at an excruciatingly slow pace.
Then serendipity struck. In that year, a pilot who was flying over Wood Buffalo National Park in Canada’s Northwest Territories saw a pair of whooping cranes and what he believed might be a chick. He was right, and scientists now held the key to accelerated whooping crane reproduction.
After much study and debate, biologists decided in 1975 to remove one egg from some of the whoopers’ nests. This had no adverse effect on whooping crane reproduction, since only one chick survives anyway. The pilfered eggs were then placed in the nests of the whooper’s slightly smaller relative, the sandhill crane. This technique seemed to work well at first, as the sandhill cranes successfully hatched and raised the young whoopers. Then a major flaw became apparent.
Much of what whooping cranes do appears to be learned, rather than instinctive, behavior. Unfortunately, this happens to include mating behavior. Raised by sandhill cranes, the whoopers simply wouldn’t mate and raise chicks when they reached maturity. Thus it was back to the drawing board for the scientists overseeing the whoopers’ recovery.
Whooper eggs could readily be incubated artificially, but a major obstacle arose after the chicks hatched: How could they be raised without becoming imprinted on humans, which they would then regard as their parents? As biologists knew all too well from the sandhill crane experiment, cranes raised in this fashion would fail to reproduce.
The solution was to feed the whooper chicks using hand/arm puppets that resemble a crane’s head and neck. Workers also donned crane costumes when caring for the chicks or carrying out other activities with them. The chicks and young adults are never allowed to see a human except under unpleasant circumstances, such as having a veterinarian catch and examine them. These techniques have been much more successful than the experiment with the foster-parent sandhill cranes.
As the number of whooping cranes in captivity rose steadily, in 1992 scientists began to release them into the wild on Florida’s Kissimmee Prairie. Their goal was to create a nonmigratory flock containing 100 to 125 whoopers a year or more old.
Early releases were devastated by bobcats, which initially killed nearly two-thirds of the birds. In 1995, however, recovery experts changed tactics by switching from permanent to portable holding pens placed in safer habitat— low grass and freshwater marsh, where it’s much more difficult for bobcats to stalk the cranes unseen until they’re close enough to pounce. Once the cranes are properly acclimated, the pens are removed, leaving the birds on their own. With this change, bobcat predation on new crane releases has been reduced to a much more tolerable 30 percent.
The goal of one hundred whoopers age one and up in the Kissimmee flock now seems very feasible, for there are currently seventy-five of them, with the number steadily increasing as more are released. So far, none of those cranes has reproduced, but that all-important step seems to draw closer each year. Last year there were nests, but no eggs were laid. This year two pairs of whoopers nested, and each pair laid two eggs. Those eggs didn’t hatch—possibly they weren’t fertilized—but biologists are by no means discouraged. Whooping cranes, which mate for life and are long-lived, often take several years to settle into a routine of successful reproduction.
There are now 366 whooping cranes, 104 of them in captivity. In addition to the seventy-five wild whoopers in the Kissimmee flock, there are now 183 in the migratory Aransas flock—the original flock that was once reduced to only fifteen birds. This flock is growing at the rate of about 4 percent a year. There are also four remaining whoopers from the unsuccessful experiment using sandhill crane parents.
Biologists would also like to start another migratory flock—an extremely difficult task, since migration is a learned behavior in whooping cranes. Experiments are under way with both whooping and sandhill cranes to use ultralight aircraft as a means of leading young cranes to migrate.
With their present large population well distributed throughout North America, great blue herons appear to be thriving in most areas. Still, biologists note that there are at least three things that could reduce heron numbers in the future.
The first, ironically, is the return of the bald eagle, itself only recently taken off the endangered species list. Although the eagles only occasionally kill adult great blues, they prey on their chicks. More serious is the fact that they often frighten parents off the nests before their eggs hatch; as soon as a heron’s nest is left unguarded, hordes of crows and other predators swoop in to seize the eggs. Although eagles thus take a direct and indirect toll of herons, it seems doubtful that the big raptors, which require a large territory, will become so numerous that they’ll do serious harm to the herons. After all, they previously coexisted successfully for countless millennia.
The second concern stems from the great blue’s ability to catch fish. As aquaculture has expanded dramatically in the past few years, great blues have discovered that these fish-filled ponds are pure largesse for a hungry heron. Naturally, owners of these commercial fish-raising ventures aren’t thrilled at the sight of their profits vanishing down those elongated gullets, and so they’ve sought and received depredation permits to shoot as many as four thousand of the offending herons annually (great blue herons are otherwise strictly illegal to kill). While this number is by no means sufficient to harm the heron population, scientists are keeping a wary eye on the upward trend in depredation permits in case it becomes a problem.
The third concern—and by far the most worrisome in the long run—is increasing human intrusion near and in heron rookeries. Suburban sprawl, timber cutting, wetland drainage, development, and other human disturbances are slowly but surely nibbling away at this critical nesting habitat, which rarely has any legal protection.
Still, there is one encouraging sign in this regard: On Bloodsworth Island, an active U.S. Navy bombing range in Chesapeake Bay, great blues are happily nesting on wooden towers studded with nesting platforms. If great blues elsewhere display this sort of adaptability, it bodes well for the species.
Herons have been around for a long time—at least 14 million years, according to the fossil record—and the great blue heron, or a remarkably similar counterpart, existed nearly 2 million years ago. Despite the concerns already noted, the great blue seems destined to remain abundant.
Although the whooping crane’s prospects grow brighter year by year, this magnificent bird is by no means out of danger. As a race, cranes are very ancient, and the earliest crane fossils date back roughly 50 million years to the Eocene epoch. By way of comparison, that’s only about 15 million years after the extinction of the dinosaurs. Clearly, cranes have evolved in a way that’s made them survivors over an immense span of time, but humans have recently added a level of stress never before experienced by these great birds.
Now, after nearly sixty years of painstaking work by dedicated biologists, the restoration of the whooping crane to its rightful place seems likely. Still, until at least another two flocks of successfully reproducing whoopers are established, it’s too early to declare victory in the struggle to save this splendid crane.
