Minnesota Profile: A Joyful Noise
Heron rookeries are raucous places.
By Paul Gruchow
A single great blue heron is an imposing sight. An adult male stands 4 feet tall, has a wingspan of 6 feet, and wields a half-foot-long beak that looks more like a switchblade than a pair of mandibles. It has fierce yellow eyes with black, unflinching pupils, and a long plume floats out from the back of its head as if from a battle helmet.
The heron is a stalker, a bird that hunts its prey -- fish (mostly not game fish), snakes, frogs, lizards, crayfish, insects, mice, shrews, rats. It relies on stealth and speed. It stands as still as a stone, or prowls in slow motion, or sweeps one of its improbably long toes to gently stir up whatever is lurking at the bottom of a body of water until a suitable meal comes along, and then in a flash seizes the victim in its mandibles or stabs it through, rearranges the catch bellyward, and swallows it whole. When threatened by something bigger or more powerful than it, the great blue heron goes for its attacker's eyes.
The nurseries in which the young are roosted, hatched, raised, and fledged are called rookeries. There are about 230 rookeries in Minnesota, including one on Crane Island in Chisago County's Rush Lake. The island's name reflects the common confusion of cranes with herons. The two kinds of birds are, however, easily distinguished in flight. Cranes fly with their necks extended, herons with their necks tucked in. Depending upon the year, Crane Island supports between 200 and 400 active great blue heron nests (once there were as many as 500), each containing two to four eggs. The herons, in turn, are joined by smaller numbers of nesting great egrets and double-crested cormorants. Its rookery is among the half-dozen largest in the state.
The island itself is slender and small -- about 23 acres. At its highest point it rises just 10 feet above the water. It is surrounded by bulrushes and covered with tall trees -- basswoods and sugar maples, mainly, but also red oaks, silver maples, cottonwoods, ashes, and the skeletons of American elms, which were once the most common nesting sites of the herons. During the nesting season, which runs from mid-April to mid-July, the island is a raucous place.
I went to the island, now designated Rush Lake Island Scientific and Natural Area, by boat one early May day with Steve Kittelson, who works for the Department of Natural Resources Nongame Wildlife Program. Three herons returning from an early morning fishing expedition led the way. The day was overcast and breezy. As we approached the rookery, the nests of the herons came into view: large platforms made of sticks, reeds, and whatever else happened to be available for the scrounging, including garbage and bits of plastic. The nests protruded from the highest and most exposed tree limbs in the forest canopy.
Some critters are remarkable architects, but the great blue herons are not among them. Their nests look haphazard, unkempt, and unsafe. Indeed, Kittelson said, he's seen nests so thin you could spot the eggs from below. Falls from nests are the leading killer of chicks, fewer than 40 percent of which survive their first year. Rookeries, in fact, are pretty smelly places. Young chicks are fed partially digested food, which their parents regurgitate into their mouths. As the young birds mature, the parents simply toss the contents of their stomachs into the nest and let the chicks fend for themselves. I saw the nests writhing and dancing from the relatively light winds. A great blue heron rookery would not be a place for a creature queasy of stomach.
We circled the little island, noting the places where there seemed to be concentrations of nests. We looked, in vain, for evidence of black-crowned night herons, a smaller and much rarer species that sometimes nests among great blues. The black-crowned herons are night feeders, the great blues day feeders, so the association is perfectly compatible.
Even without the comings and goings of the herons, the place bustled with activity. In the bulrush shallows, carp churned and splashed in noisy mating rituals. Flocks of Canada geese honked among the cattails. Along the shore, a plump woodchuck stood gazing out upon the water, looking as content as a Minnesotan with a cup of fresh coffee on the first day of a long holiday at the lake cabin.
Clucking and Skrawking
But it was not until we beached our boat and ventured onto the island (something strictly prohibited during nesting season except with advance permission and good cause -- in this case, a survey of the year's population) that the full extent of the activity in the rookery became obvious. There was, for one thing, the clamor of the herons, both the persistent clucking of the chicks and the sharp skrawking of the adults, a sound, Kittelson said, "that always reminds me of something prehistoric, like the call of an archaeopteryx." There was the abundance of the nests, seemingly one in every place where a nest might have been built. And there was the litter on the ground -- the many broken eggshells, here the bones of a creature consumed, there half a fish dropped before it could be finished, everywhere the broad splashes of white guano and its strong, acrid barnyard odor.
The forest floor at the north end of the island, even so early in the growing season (ground plants in a forest hasten to maturity before the canopy closes in and they find themselves in perpetual shadow), was nearly barren of plant life. It is at this end of the island that the 75-year-old rookery is now concentrated, and the accumulations of guano have substantially altered the soil. Tests on the island in 1980 showed results similar to those at other rookeries. Soils beneath the nests were much more acidic; much higher in concentrations of nitrites, phosphorus, potassium, and other soluble ions; and much less rich in organic matter than soils elsewhere on the island. And vegetation surveys confirmed the strikingly visible consequence: a dramatic drop in species diversity below the nests.
As we headed south, Kittelson picked up the half shell of a heron egg. It was perhaps slightly larger than a jumbo chicken egg and pale blue on the outside, more aquamarine than sky blue. Its interior was coated with a thin, tough off-white membrane. "This membrane is rich in blood vessels, which supply the developing embryo with oxygen," Kittelson said. "In the larger end of the egg, there is an air sac, which supports the bird while it uses its egg tooth to chip its way out of the shell. Just before the birds hatch, you can hear them chirping inside the egg."
The impression of being on the island was a little like that of being contained in just such an egg. The canopy of the trees shrouded us in shade as if in a shell. The substrate at our feet was moist and just barely firm. Toads leapt away from our steps. On higher ground near the center of the island we passed the nests of geese, soft affairs shaped like broad, shallow platters. The barnyard odor of guano grew more intense as we advanced from barren earth toward land where herons historically had nested in greatest concentration. It supported a few flowers -- at this season, violets, Solomon's-seals, jack-in-the-pulpits, and wild leeks. Then came a stand of dead elms and the flash of light they admitted, and the dense thickets of elders flourishing in that light. Then shadows again and muddy ground and clouds of tiny flies. Then sedge marsh. Then the water again.
Even at the edge of the water, beyond the close interior of the island, there was the cacophony of the herons, the shameless begging of the chicks, and the rattling cries of the parents, which reminded Kittelson of the Paleozoic and me of a tropical forest overrun with parrots. Sounds, at any rate, far beyond the human imagination.
"My friends brought him into the kitchen," the poet James Wright said of a small blue heron,
In a waste basket and
Took him out and
Set him down.
I stroked his long throat
On the floor. I was glad to hear him
Croaking with terror.
I was glad, for my own reasons, to hear the din of uncivilized heron noise on Crane Island.
Kittelson set up a spotting scope, and I peered through it at the patrician head of a roosting heron. Its head plume flew wildly in the wind, but otherwise it perched as steady as a soldier at its post. Although I think the bird could not have been looking at me, its big yellow eye seemed to pierce my own. There is no way to describe the intensity of its gaze, the unblinking glare of it, so fierce I imagined it might set fires. To look into the eye of a great blue heron is to know something of the incomprehensible distance dividing herons from humans.
And then we launched ourselves into the water again among the roiling carp, whose carnal activities suddenly seemed quite mundane. A hundred yards from shore nothing was audible but the whine of the engine and the brush of the wind.
Crane Island first came into formal ownership in July 1874. The property changed hands several times over the next century. Before the turn of this century, white oaks were harvested there to be used by a local cooperage in making barrels. It was grazed by pigs and cows in the early 1900s. In the drought of the 1930s, the meadow at the south end of the island was hayed. But the island was never occupied and, except at these brief intervals, rarely disturbed.
The first serious threat that the island might be developed for housing emerged in the late 1960s, just as the state was launching a new program to protect the best and most representative remaining samples of Minnesota's natural features. Heron rookeries, like living organisms, have limited lives, which can end prematurely. One good way to destroy a heron rookery is to subject it to too much human disturbance. The threat to the Crane Island rookery rather quickly came to the attention of the officials organizing the new protection effort, who soon began negotiating to buy the property. The land was acquired in 1971, and in 1974 it became the first of the hundred-some sites officially set aside as SNAs, to be perpetuated forever in a natural state.
On the way back to the boat landing Kittelson told me about another Minnesota rookery that for reasons not understood was recently abandoned. "There is no silence," he said, "quite like the silence that descends during breeding season in a place where a heron colony has been abandoned."
That such a silence has not descended upon Rush Lake is one small gift from those who crafted and have maintained the SNA Program, a gift, one might say of music -- odd, inharmonious, strange music, to be sure, but vital music nevertheless in the overwhelming silence of the great universe.
The DNR Scientific and Natural Areas Program is celebrating 25 years of preserving Minnesota's wild places. To coincide with this event, Worlds Within a World, a collection of Paul Gruchow's essays on SNAs, will be published in September. These essays first appeared as a series in the Volunteer.
Worlds Within a World is available from Minnesota's Bookstore, 800-657-3757 or 651-297-3000, and other bookstores.
Paul Gruchow's books include Boundary Waters: Grace of the Wild and The Necessity of Empty Places, recently republished in a 10th anniversary edition. He lives in Two Harbors.