At 4 feet tall, with a wingspan of up to 6 feet, and weighing about 6½ pounds, the great blue heron is one of the largest — and most impressive — birds in Massachusetts.
Whether standing statuesque at the edge of a pond while waiting to grab a fish or frog with its large, dagger-like bill, or flapping across the sky with its long neck tucked in and its equally long legs trailing behind, this blue-gray colored bird is an awe-inspiring sight. Flying overhead it even looks a bit pterodactylish, and one can almost imagine the extinct flying reptile soaring through prehistoric skies.
“Herons have a primitive look,” said Wayne Petersen, director of Mass Audubon’s Important Bird Areas Program. “People notice them. They get attention. Great blue herons are the largest heron species in North America.”
In addition to being found across Massachusetts and the rest of New England, great blue herons range from southeastern Alaska and central Canada, south through Florida, Mexico, and the Caribbean, and are even found in the Galapagos Islands in the Pacific, according to Mass Audubon.
Not that long ago, great blue herons were a rare sight in the Northeast due to hunting pressure and pollution, said Mass Audubon, but they’ve made an impressive comeback in the past few decades. In addition to legal protection of herons and the wetlands they utilize, another factor has helped boost the numbers of great blue herons in Massachusetts and elsewhere in New England: beavers.
“Great blue herons are the beneficiaries of beaver activity,” said Petersen. “Beavers have rebounded because they’re protected, and beavers create habitat that other wildlife uses.”
When beavers dam streams, they create ponds and swamps, and the trees that are standing in those newly created wetlands eventually die from the flooding, Petersen explained. Those big dead trees create ideal nesting sites for great blue herons, which build large, flat nests made of sticks. The herons tend to nest communally in colonies called rookeries or heronries, which sometimes contain just a few nests, with larger colonies containing up to 50 or so nests.
Andrew Vitz, state ornithologist at MassWildlife, said the increase in the number of nesting herons and rookeries throughout the state over the last several decades has been dramatic.
“Similar to raptors, herons certainly experienced steep population declines from DDT poisoning,” said Vitz. “The ban on DDT and increases in nesting habitat from a growing beaver population set the stage for the recovery of this species.”
The habitat is a big factor, Petersen explained.
“Great blue herons don’t like to nest in trees that are not in water,” said Petersen. “They prefer the moat-like habitat that beaver ponds and swamps provide.”
Just like moats filled with water used to protect ancient castles, the water surrounding the trees that herons nest in help to provide protection from potential nest predators, like raccoons and foxes, Petersen explained.
Great horned owls will sometimes nest in heron rookeries, said Petersen. The owls begin breeding in late January and in February, about a month before the great blue herons return in March, and the owls will move into an empty heron nest from the previous year.
“Sometimes a great horned owl will take a heron chick,” said Petersen. “But they can coexist, and the owls generally don’t bother the herons.”
Bald eagles, on the other hand, apparently do bother herons.
“We are now finding that a pair of eagles sometimes take over a heron nest, and the entire colony may have to relocate,” said Vitz. “Eagles are opportunistic predators, and I imagine go after herons from time to time. I think their presence is enough to cause herons to think about relocating.”
Great blues mate and nest in the early spring. The males collect sticks, said Petersen, which the females arrange into a nest.
Mass Audubon said an average of four eggs are laid, which hatch in about a month. Initially, chicks are fed regurgitated food by the parents, but when the chicks get bigger the parents just leave small fish in the nest. The young are able to fly in about 60 days, but typically only two chicks per brood survive due to starvation, falling out of nests, predators, and people disturbing the rookeries.
“Historically, people would sometimes go into nesting colonies and shoot the herons,” said Petersen. “Nowadays, in some cases, photographers and bird watchers may also unknowingly disturb them.”
When the dead trees eventually fall, the herons will abandon the nesting colony. There’s always movement of great blue heron colonies, Petersen explained.
The average lifespan for a great blue heron in the wild is around 15 years, according to iNaturalist.
Great blue herons make a hoarse, loud croaking sound when they fly, said Petersen, and you can sometimes hear them making noise at night.
Threats to great blue herons include nest predators, pesticides in fish, fishing line entanglement, and catching fish with hooks or lures in them, said Petersen.
Although great blue herons feed mainly on fish, their diet also includes crabs, insects, frogs, snakes, salamanders, and small rodents, according to Mass Audubon. Occasionally, they’ll even eat baby birds such as ducklings, Petersen said.
Great blue herons have sometimes been known to try to eat prey that’s too big to swallow, and choke to death in the process. The scientific literature contains accounts of great blues choking while attempting to swallow large frogs, carp, bullheads, snakes, and lampreys.
Most herons in our region migrate to the Southeast in the fall, but a small number overwinter, said Petersen, mostly along the coast, such as on the Cape, where they can find open water in salt marshes and other coastal environments, and continue to catch fish. But a cold snap can kill overwintering herons.
“Ice is the big thing,” said Petersen. “If the water freezes, they can’t get food.”
Mass Audubon said conflicts between people and herons occasionally arise when herons feed on expensive, exotic fish stocked in peoples’ private fishponds. In this situation, it is recommended to try to scare the heron away by shouting at it, which will usually keep them from coming back.
“The bottom line is great blue herons are winners,” said Petersen. “Great blues have made a huge increase in numbers in the last 30-plus years!”
Don Lyman is a biologist, freelance science journalist, and hospital pharmacist who lives north of Boston. Send your questions about nature and wildlife in the suburbs to firstname.lastname@example.org.