Heron Rookeries – Safety in Numbers
Great Blue Herons are familiar summer residents of inland New Hampshire. As the largest of all North American herons, they can stand four feet tall.
Herons are always conspicuous. Conspicuous in flight, flying with neck folded and legs trailing. Conspicuous in voice: a prehistoric croak or harsh guttural squawk. They’re conspicuous when standing motionless, hunting along edges of freshwater wetlands, rivers, ponds, lakes and saltwater marshes. Unlike most birds, they’re even conspicuous while nesting.
Great blue herons typically nest in colonies called “rookeries.” Herons build large coarse stick nests of in dead trees directly over the water or overlooking wetlands. More rarely, heron nests are located in live trees growing in proximity to productive wetlands which lack standing dead trees.
When nesting together, herons employ a strategy of “safety in numbers.” During synchronous egg-laying, incubation, hatching, brooding and fledging phases of their breeding season, sentries watch for aerial predators including owls by night and hawks by day that might attempt to prey on vulnerable eggs or nestlings.
There is less nest predator danger from below in wetlands due to standing water. Tree-climbing snakes, raccoons or opossums intent on eating vulnerable heron eggs or nestlings would first need to swim before climbing. The larger a colony, the less likely a predator could sneak-in unnoticed. Older nestlings are often left alone while both parents forage. The constant presence of at least one adult in the colony provides security for the entire nursery
Beyond security, there’s efficiency in communal living. In their waterfront condominium community, herons enjoy easy access to wetland food supplies – fish, frogs, tadpoles, crayfish, snakes and snails; even mice! Herons can benefit from observing the hunting success of their neighbors rather than foraging alone in less productive areas.
Heron rookeries attract other birds. In an avian equivalent of the “time-share” concept, Great Horned Owls utilize unoccupied heron nests. Owls incubate eggs and brood their fuzzy owlet families even as the snow falls in February, long before herons return in April. An owl might return to prey on heron nestlings in June. Apparently, there’s no residual gratitude for nest use privileges.
Ospreys often co-locate their huge stick nests in established heron colonies when colonizing new areas. A common attribute of favored heron and osprey nest sites is beaver activity that raises water level to create a specialized habitat of standing dead trees.
In this standing dead forest, ducks including mallards, black ducks and geese build nests. Wood ducks and hooded mergansers nest in man-made duck boxes or natural cavities in dead snags. Flycatchers including great-crested fly catchers, alder flycatchers, olive-sided flycatchers and eastern kingbirds also depend on standing dead trees for feeding perches from which to hawk aquatic insects hatching from standing water.
Most birds exhibit tight habitat affinities during the brief breeding season. Most practice nest site fidelity, returning year after year to the exact same nesting locales. For breeding birds, as with commercial real estate, the three most important things are: location, location, location.
Naturalist Dave Anderson is director of education for the Society for the Protection of New Hampshire Forests. His column appears every other week in the New Hampshire Sunday News. He may be reached at firstname.lastname@example.org or through the Forest Society's Web site: forestsociety.org.