North Woods Rookeries
On a misty dawn morning, the sight of a great blue heron emerging from the fog over a lake, wings slowly beating, can stir the heart and mind as powerfully as any wildlife experience. The majesty of large and graceful birds is difficult to surpass and seldom fails to draw lengthy oohs and aahs from wildlife-watchers. Yet the pinnacle of heron-watching may not be in sighting the individual but rather in watching the nesting rookeries that resemble great airports as the adults wheel in and out with food for gaping juvenile mouths.
Many novice birders are surprised to learn that most herons nest in substantial colonies, building large stick nests usually in a cluster of dead trees. These rookeries can be quite small, but large rookeries may include many hundreds of nests and several heron species. In the Upper Midwest, the heron family includes the herons great blue herons, black-crowned night herons, green-backed herons, and yellow-crowned night herons (rare in the area); the egrets (though egrets really are herons that were given the name for the long plumes they wear during breeding) cattle egrets and great egrets; and the bitterns American bitterns and least bitterns. Of these birds, only the bitterns and green-backed herons are not colonial nesters, instead nesting individually near the water surface in marshes and bogs. In large rookeries, several of the other species often come together to live in tightly packed neighborhoods and are frequently joined by double-crested cormorants.
Many nests are often placed on limbs of the same tree, and given the substances that might fall out of the nests (herons regurgitate indigestible food in pellet form, for instance), the lower nests, I would suppose, constitute the"low rent" housing for the neighborhood. Recently dead trees caused by beaver flowages and areas of flooding are choice sites for building rookeries, though colonies may also be built in live trees. One colony I know of on the Turtle-Flambeau Flowage in northern Wisconsin is on an island in thriving red pine trees. Another rookery, Cathedral Pines, is in the top of old-growth white pines.
Rookeries come and go as the dead trees topple due to wind, decay, and ice shear, so traditional sites may not have long histories or stable futures. They may also be abandoned for reasons unknown, then rebuilt in another area, or forgotten altogether.
Herons eat mostly small fishes, only a small portion of which are game fish. Eating small fish has the effect of culling a population of fish so numerous that growth is stunted. Other foods include the delectable wetland smorgasbord of crayfish, frogs, salamanders, snakes, aquatic insects, and rodents.
Herons return to their nesting colonies early in the spring, and the incubation of the eggs takes less than a month. Young fledge in about one and a half to two months. During this period it is imperative that human watchers do not intrude upon the colony but remain a respectful binocular range away. In July the young are of good size, and it's a full-time job for the adults to keep them fed, so a constant progression of birds is on the wing back and forth. By August the young fledge and the colonies are usually abandoned until the next spring.
Rookeries should under no circumstances be entered while birds are present for the obvious reasons that this will disturb feeding and care of the young. Adults will abandon entire colonies if there is too much disturbance, so stay well offshore from island colonies or concealed by trees near inland colonies. If birds appear to be flushing due to your presence, get away from the area.
Details mentioned in this article were accurate at the time of publication