I first observed the colony of Great Blue Herons in the spring of 2011. There were only a half dozen nests on the North Raccoon River in a dense area of the flood plain between Dawson and Perry. Last year the colony doubled in size.
This year, on April 5, when I canoed past the nesting area I was amazed how the rookery had grown. I tried to count the nests; there were more than forty in various stages of completion. The herons had chosen an isolated spot, a wooded area high on the flood plain that had many tall trees.
The birds started to give their squawking alarm calls as my friends and I paddled past.
Many of the birds were flying from the nest- perhaps going to feeding areas or to obtain more sticks for nest building. A few birds were flying into nests with large sticks in their bills.
The male herons forage for sticks and bring them to the nest site; at times they will steal sticks from another nest if it is not guarded. The female does the nest construction, weaving the sticks to form a thick platform structure about three feet in diameter. Sometimes the mated pair will refurbish an old empty nest instead of building a new one and on occasions they will start to build a nest but stop and begin building a nest in a different tree.
A colony of nesting Blue Herons is called a "rookery". The term comes from the "rook", an English crow that is also a colonial nester. It is estimated that 13 percent of all birds nest in this manner, such as swallows, grackles, blackbirds, finches, gulls, and many species of sea birds.
There is a rookery of Blue Herons on the Mississippi River that has an estimated 1,500 birds.
Ornithologists believe that the main reason for birds behaving this way is due to a scarcity of nesting areas that are isolated, near an abundant food supply, and contains tall trees. Also, by living in large groups there is cooperative protection of their young from predators. If a hawk or raccoon is seen stalking a baby heron several adults in the colony will physically attack the predator. This is called "mobbing" and it is very effective; several of the long dagger-like bills of a heron can easily kill most predators. Also, individual herons may learn where to find productive feeding areas from each other.
However, there is downside for birds that live in rookeries. The colony may attract predators- it is easier for a soaring hawk to spot a large number of baby birds. The hawk could swoop in and grab one that is not near an adult. There can be an increase in disease and parasites such as mites and lice because the birds live in close proximity to each other.
Great Blue Herons have been known to use rookeries for decades. However, they will relocate their colony in response to predation, a decline in their food supply, destruction, and human disturbance. Many years ago there was a large Blue Heron colony on Beaver Creek near Highway 141 between Granger and Grimes. The birds probably abandoned this area because of housing developments and the building of the Beaver Creek Golf Course. Another large colony on the Des Moines River near the 4-H Youth Camp was washed away in the flood of 1993.
I have enjoyed seeing this new colony of Blue Herons develop and grow the past three years. Seeing the rookery is always a highlight of my canoe trip when I float by it. I hope that it is around for a long time and will provide a large number of these spectacular birds for the people of the Raccoon River Watershed to enjoy.