In animals, as in humans, there is usually a correlation between numbers and noise. The more individuals that are gathered together, the greater the likelihood that the gathering will be noisy.
My daughter and her visiting boyfriend were pedaling bikes off the beaten path on one of the access roads that leads to cabins and vacation homes on the lake where our family cabin sits. The road penetrates a forest of tall conifer trees, whose branches meet above the roadway to obscure the sun and sky, the diffuse green light giving a sense of seclusion despite the busy lake life here.
In this quiet spot, away from the wash of waves and whine of outboard motors, they heard raspy, squawking calls from high overhead, in volume and direction clearly part of a large chorus, not a solo voice. Despite the noise level, the cause was out of sight. There is an abundance of wild turkeys hereabouts, and they posed that possibility when they paused to talk to a woman they met walking the road.
Hours later, our bikers having long since returned to our cabin, there came a knock on our door. It was the woman they had met on the road, who—when they met—had asked my daughter the whereabouts of our cabin. She had put out a query on a Facebook page maintained for those who have property on this lake. “Great blue heron rookery” was the response that one of the locals had posted. This was what she shared with us when she knocked on our door.
“Great blue heron” shouldn’t have been a surprise. It is one of the birds most frequently sighted on our lake, its slow wing-beat and lumbering flight so remindful of an overburdened cargo plane. We see them in their comings and goings, and sometimes find ourselves wondering why we’ll see one heron winging its way down the shore to the west, and at the same time see it cross paths with another heading down the shore to the east. Does some instinct lead them to expect better hunting for fish, frogs, or crayfish at the end of their flight? Is this the heron version of “the grass is always greener on the other side of the fence?”
We often see herons standing on our dock, or the dock of a neighbor; sometimes on the canvas cover of our boat lift. Does this vantage point help them see prey? Their usual mode of hunting is stalking the shallows on foot, on those long, stilt-like legs. Are the herons that make use of docks and boat lifts adapting to life with us, using humans’ creations to hunt more successfully?
Herons are wary, and not very tolerant of human presence. If I appear in the doorway, or open it just wide enough for a telephoto camera lens, a heron at the end of our dock more than 100 feet away will freeze like a statue, but generally not for long. Usually it will take flight, presumably to land again where it does not feel the weight of human eyes, or hear the click of a camera shutter.
But back to the rookery. With its long, stick-like legs and broad wingspan, a great blue heron does not seem like a bird designed to spend time in trees. It hasn’t the goshawk’s or the grouse’s speed and agility. Even several species of ducks—like the wood duck, merganser and goldeneye, which nest in hollow tree cavities—seem far better suited to an arboreal life.
Some herons do, in fact, build their nests on the round. But the classic great blue heron nesting protocol is the rookery, where dozens to even hundreds of nests may be built in close proximity to one another. The heron is not a bird of the forest under-story. It’s a bird of the canopy, the top tier of a forest where they build a nest mostly of sticks, lined with softer materials like reeds, grasses, or moss.
The term “rookery” comes from a European member of the crow family, the rook, which—like the great blue heron—nests in colonies in the tops of trees. Perhaps because nesting colonies located in treetops are unusual for birds, the term rookery came to be used for most any bird that has this treetop nesting habit.
But why nest so close together in the first place? After all, many birds are highly territorial, and will do battle with others of their kind to secure a choice nesting area. Two of the explanations given by ornithologists—bird scientists—are that birds nest in colonies either for protection from predators, or because they learn about food sources from their neighbors. It’s possible in some birds that this nesting behavior is advantageous on both counts.
But the great blue heron is a solitary feeder. When you see one stalking prey in the shallows of a lake or wetland, it will be alone. So colony nesting to share food source information in all probability was not part of the heron’s evolutionary history.
Protection from predators is likely the explanation for great blue herons. The behavior probably helped them be successful and survive in evolutionary competition. A closely-packed multitude of large birds armed with dagger-like beaks might deter many predators from attempting to rob nests of their eggs or young, or attempt to take on an adult. The nests most vulnerable to predation are likely to be those on the outer fringes of a nesting colony; so the larger the rookery, the greater the percentage of nests that will be in the interior, and therefore less vulnerable.
The world of wildlife art and limited-edition wildlife prints is pretty much dominated by deer, ducks, geese, upland birds like grouse or pheasants and—to a lesser extent—fish. Non-game species are poorly represented. One exception is a piece painted by famous Minnesota artist Les Kouba, entitled The Great Blue Heron Rookery, which memorialized a scene visible and recognizable to thousands of vacationing motorists who over many years drove a county road that encircled Gull Lake in North Central Minnesota.
Storms eventually took down many of the nesting trees there, and this great blue heron rookery is no more. Factors like changing land use, food availability and factors unknown to us may play a part in the creation, demise, or relocation of a great blue heron rookery. But they’re a unique natural phenomenon while they’re thriving, like the one where our lake’s complement of great blue herons apparently nest.