Great blue herons have been reusing their nests year after year—long before recycling became fashionable. A wonderful example clearly visible to all is the heronry along Route 2, near Acton. (Photo by Mary Holland)

Just as some people are more at ease living in apartment houses, comforted by the proximity of fellow humans, some birds prefer to nest in groups (of single pairs), or colonies, surrounded by other birds. Approximately 13 percent of all birds are colonial nesters, including herons, swallows and most seabirds. The great blue heron, the largest and most widespread heron in North America is, because of its size (3 feet tall with a 6-foot wingspread) and the corresponding size of its nest, one of the most visible examples of this form of nesting in Massachusetts.

In March, the male great blue herons arrive back at the heronry where they nested the previous year and settle on the existing nests (not necessarily the exact nest they used the previous year). These platform nests are usually built in trees, up to 100 feet above the ground. The trees are often located in lowland swamps, on islands or near lakes and ponds surrounded by woods. The exact location of a heronry is usually determined by the proximity of feeding grounds, as well as the relative safety of the site from predators such as raccoons and black bears. Herons will return to the same heronry year after year, with the result being that some colonies have existed in the same location for more than 70 years.

The nests of great blue herons, like those of eagles, ospreys, and other large platform-nesting birds, are composed of strong building material, for, unlike those of most songbirds, which are only used for one year, the nests of great blue herons are reused year after year. The males collect fairly stout sticks from the ground, nearby trees, or the unguarded nests of other herons. The sticks are placed on the nest by female herons. This behavior continues throughout the nesting period. Within a few days to a couple of weeks, a platform nest is constructed. Its diameter can be anywhere from 1½ to nearly 4 feet. Every year it is repaired and added to by its inhabitant, which necessarily increases its size as it ages. The nest is shaped like a saucer and the interior is lined with pine needles, moss, dry grass, or small twigs.

The size of a heronry varies greatly, consisting of just a handful of nests up to 500 or so. There must be advantages to this nesting arrangement, and the most obvious appears to be increased protection from predators. Usually the nests are located in hard-to-reach spots, often in the middle of beaver ponds, in the tops of snags. The presence of many birds means that there is often an impressive number of individuals defending the area. Ornithologists have found that breeding is often synchronized in nesting colonies, with all the young hatching around the same time, thus providing more potential food for predators than could be consumed.

There are also obvious disadvantages to colonial nesting. Great blue herons feed primarily on fish, and it takes a lot of fish to support a large heronry. The feeding grounds of most birds are usually nearby, and the competition for them is inevitably increased. Also, one can’t help but think that the noise and physical structures produced by colonial nesters must bring a certain amount of attention to the heronry, including that of predators.

Regardless, the advantages must outweigh the disadvantages, at least for the species that continue to utilize it. One hazard for colonial nesters, at least, has diminished. In the past, they have been used by humans as a source of food in the form of eggs and meat, as well as down for bedding and feathers for quill pens and hat decorations. This led to the extinction of at least one large seabird, the Great Auk, as well as the decline of many other species. Fortunately, these vulnerable birds are now protected.