Montana Best Times: Big Sky Birding Column (November 2005)

Great Blue Heron Nesting: Are the Nesting Colonies Called Rookeries or is There a Better Term?

Great Blue Herons are tall wading birds that breed in colonies across the entire United States and extreme southern Canada, including western inland and coastal areas of British Columbia and southern Alaska. Great Blue Herons, also known as GBH's, feed on a wide variety of prey with their primary diet being small fish oftentimes 1" - 10" in size. As mentioned in the previous MBT column, GBH's typically swallow their prey head first. But they also eat a variety of food items ranging from frogs, salamanders, lizards, snakes, and crawfish, to small birds, rodents and insects. Great Blue Herons are one of the top avian predators in the riparian zones, and represent the mid to upper trophic levels of the food web.

Their feeding habitat is primarily shallow water environments such as estuaries, bays, lake inlets, and slow moving backwater sections of rivers, that act as hatcheries, or refugia for small fishes. But they do occasionally feed on dry land in search of rodents and insects. The daily feeding areas are usually within 20 miles of the nesting colony, and often within 1-5 miles of the colony. Interestingly enough, even in high density coastal areas these birds are found hunting in loose flocks, but are equally and most often successful as solitary hunters. And unbenounced to many people, many herons are just as efficient at securing prey during the crepuscular (dawn and dusk) and nocturnal (nighttime) hours of the day a during the diurnal period. And it is because of that lesser know nocturnal habit, that you do not see GBH's returning to the colony in large flocks every evening.

Nesting and Heronries
Great Blue Herons are also unique in that they are seasonally monogamous, and the seemingly strong pair-bonds diffuse after nesting. In the early spring, adult male herons are the first ones to arrive at the colony and defend a larger nesting territory immediately upon arrival. Nest sites are typically found in tall trees and on occasion in bushes. As more males arrive the defended territory shrinks in size, until it more approaches the size of the nest platform and the immediate adjacent area. As adult females arrive in the colony, the males compete for the available females, with males being larger than females. Herons begin to form pair-bonds, usually in March, after a series of courtship rituals performed by both sexes. It is during this breeding period that GBH's are most vocal. Behaviorally they perform "upright and spread wing" displays, and "bill clapping or bill dueling" displays on the nest that have evolved to accentuate their showy plumes. Males also perform a high "circle flight" over the nest that is quite impressive, and also defend the nest during the day from intruding males stealing sticks. Their plumage becomes more colorful and noticeable during nesting, particularly long black feather plumes extending in back of the head and white feather plumes noticeably protruding out of the lower neck and scapulars. The long black feather plumes or crests above the eye are raised during courtship display and the loral area, between the eyes and the bill, changes to a bright green-blue color.

Once the pair-bond nears completion, the male presents sticks to its mate and the female builds the nest. The tree nest eventually becomes a sloppy flattened platform composed of sticks and twigs. GBH's occasionally nest singly or in small satellite colonies ( 2-20 pairs), but more commonly large colonies numbering twenty to hundreds of nesting pairs. Heron colonies are loosely termed "rookeries". The name comes from a Eurasian member of the crow or corvid family called the Rook (Corvus frugelius). Rooks look like Common Ravens (Corvus corax), only Rooks are entirely black in addition to being 2/3 the size of a raven and having a bare grayish-white face, and a thinner beak, and a peaked head. Rooks are also unique since they nest in conspicuous colonies comprised of a large number of flat-topped nests, hence the term rookery. In North America, a heron colony is technically called a heronry.

Isolated groves of tall trees on islands or close to water and/or disturbance are where heronries are typically located. Initially, the inner section of a stand of trees is utilized, but continual colony use over time may eventually kill the trees in that area, due to the excrement or guano exerted over the years. When the center of the gallery forest dies, the colony moves circularly outward in succeeding years. This strategy creates a "donut" or "bulls-eye" effect when the grove of trees is aerially viewed. Nests are constructed of loose sticks and, if not collapsed by winter weather, may be repaired and used year after year. The nest is lined with reeds, mosses and grasses to help cushion three to seven eggs, typically 4 eggs,that are laid during March and April. The pale green-blue eggs hatch after about 28 days, and both parents incubate eggs and care for the chicks. The young are initially fed a diet of regurgitated food, but eventually eat whole fish dropped into the nest as they mature. Juveniles can start hopping off the nest after 14 days and can fly only short flights from branch to branch. However true fledging takes approximately 60 days. Post breeding movements of herons usually coincides with the timing of declining water levels in lakes and rivers, a most opportunistic time when small fishes are plentiful and vulnerable.

Heronries can be occupied by more than just herons. In some instances, other species of birds such as Bald Eagles, Ospreys, Canada Geese, Red-tailed Hawks, and Great Horned Owls have been known to occupy the flat nests of heronries. These heronries are not without their risks, with a variety of predators getting in on the action. Common Ravens and raccoons are very efficient at taking GBH eggs, while GBH nestlings are vulnerable to predators such as raccoons, Great Horned Owls, Common Ravens, Red-tailed Hawks, Bald Eagles and Golden Eagles. Excessive disturbance by predators/people, food shortages, or adverse weather events can lead to abandonment of these heronries.

Great Blue Herons in Montana can be either migratory and sedentary in their movements, depending on the elevation and the amount of open water and food available in the winter. For wading birds that frequent areas that freeze-up for the winter, GBH's have no choice but to migrate in search of food. How far they migrate is uncertain, but it is speculated high elevation summer residents move to lower elevations and most likely in a westerly/southwesterly direction and spend the winter on the Pacific Coast. The highest mortality of GBH's occurs during the first year of life. On average, GBH's longevity is 12-15 years, with extremes reaching 23 years old.

In the past, mainly egrets and some herons were shot for their showy feather plumes, which were used to adorn hats and garments, and they also provided large, accessible targets for poachers. The slaughter of these wading birds went relatively unchecked until 1900 when the federal government passed the Lacey Act, which prohibits the foreign and interstate commercial trade of feathers. Greater protection was afforded in 1918 with the Migratory Bird Treaty Act, which empowered the federal government to set seasons and bag limits on the hunting of waterfowl and offer complete protection for a broad group of migratory birds including wading birds.

Now that the reader has a better understanding of the nesting ecology of the Great Blue Heron, the next topic scheduled for the January issue of Montana Best Times will be about a raptor wintering in Montana and Wyoming called the Rough-legged Hawk.

Enjoy! And the Best of Big Sky Birding to you!