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Social systems of great blue herons vary dramatically, from solitary nests to large heronries, and some heronries include nests of great egrets and cattle egrets

Crane Hollow Road, one mile southwest of Hygiene, provides a view of about 50 nests in a great blue heron rookery, or more precisely, heronry. Massive nests made of sticks, up to 4 feet in diameter and 3.5 feet deep, can be seen high in the cottonwoods along Saint Vrain Creek. Great blue herons (Ardea herodias) up to 4.5 feet tall with wingspans of 6.6 feet, can be seen standing beside or sitting in their nests. 

A heronry is an entertaining site, for the herons are constantly active. Birds leave to feed or gather nesting materials and soon return. A female on the nest becomes attentive as her mate glides in and lands softly. They may mate, or go through an elaborate welcome ceremony that involves mutual displays of affection such as standing with chests appressed, necks embracing one another, erecting their head and chest plumes and clappering their bill tips. 

In addition to dark plumes on the head, neck and back, great blue heron adults have down feathers that grow continuously from the chest. These specialized feathers fray constantly but never molt, so the tips turn to powder down that the heron combs from these feathers and applies to other feathers to keep them dry and free of slime. 

Living in a heronry involves many close neighbors. Photo by Jeff Mitton.

Early settlers called herons cranes, suggesting that the heronry was occupied in the 1800s when Crane Hollow Road was named. It has not been occupied continuously since then, but it is mentioned by bird ecologists working in several decades, suggesting it has been occupied much of the time.  In a study in 2014, CU Professor Alexander Cruz (deceased) and his graduate students identified 51 heronries along rivers in Colorado. But that number changes frequently as new heronries are established and others are abandoned.

Social systems of great blue herons vary dramatically, from solitary nests to large heronries, and some heronries include nests of great egrets and cattle egrets. This range of variation drives curious minds to wonder if there are advantages to placing nests in dense colonies. 

A large study of great blue heronries along the Pacific coast in British Columbia tested the hypothesis that a colony of nests serves as an information center that helps the residents find food. That study showed that birds left the heronry in groups that foraged in different sites every day. As the herons left the colony, they would stop to join other heron feeding flocks and even joined feeding flocks of gulls. Finally, they showed that herons in feeding groups caught fish more quickly and efficiently than solitary herons. This study concluded that heronries provided advantages for fishing when prey were locally abundant but moving unpredictably in space and time--birds benefitted substantially when many others helped search for prey. Conversely, herons nested solitarily and defended feeding territories when food was reliably available in that territory.

While the study in British Columbia showed the advantage of nesting in dense groups when birds had to search for food each day, it does not explain why the birds in other heronries fly to private fishing territories that they defended against other herons. Why are some colonially nesting herons solitary fishers?

My local experiences with great blue herons are with birds nesting in heronries that forage and fish alone. I usually see a great blue heron standing motionless at the edge of the water, waiting for fish to approach. Herons may also walk slowly in shallow water, occasionally flicking their wings repeatedly out and back. In the water, the resulting flickering shadows may cause fish to panic and flee.  

This is not a criticism of the BC study explaining advantages of nesting colonies and group hunting. But that study was conducted in cold coastal rain forest and it would not be astounding to note that different strategies were needed to adapt the same species to a shortgrass prairie with an arid, temperate climate. We certainly have much to learn about great blue herons.

How long might the heronry in Crane Hollow be entertaining naturalists this year? When I visited last week, some nests were under construction, but other nests had an adult sitting serenely, perhaps incubating eggs. Females lay one egg about every other day to produce a clutch of 2 to 6 eggs. Adults begin incubation when the last egg has been added, and eggs will begin hatching in 27 to 29 days. Nestlings need 49 to 81 days to grow feathers, increase in size and learn to fly. So, the nests will be active and this magnificent spectacle will continue for at least another 2.5 months.

Living in a heronry involves many close neighbors. Photo by Jeff Mitton.