The other day I spent a long time observing the Baum Lake great blue heron (Ardea herodias) rookery. Many birds were flying onto and away from the two large ponderosa pines that form the core of the rookery. I noticed that several of the nests from last year had a single bird perched on them. There was none of the calling I remember from last spring nor were any birds carrying twigs, which was a prevalent activity last February. In addition many more birds were landing on nearby ponderosas rather than in the two nesting trees. That made me curious about great blue heron nesting behavior.
According to Kelsall and Simpson who studied great blue herons in British Columbia in the 1970s, colonies are dynamic. The colonies join to form larger groups or split into smaller units. They change their location or will spread into nearby trees. There is also an interchange of birds between colonies. This might explain why the birds did not appear to be settled and were moving between the two old rookery trees and other nearby trees.
Great blue herons are monogamous throughout one breeding season but choose new partners each year. In early February males settle on nest sites and perch on them to court females who arrive a few days later. Courtship includes territorial defense, presentation of twigs to the female, displays of plumes and bill “clapping” (the tapping together of the tips of the bills of the courting couple). Two things determine the spacing of nests in a tree – the structure of the tree (how the branches are arranged) and the spacing of the trees. Also territorial defense keeps nests as far apart as adults can extend their necks and bills. I can count over thirteen old nests from last season in each of the ponderosas.
The female does the nest building, or repair of an old nest, using twigs and sticks that the male brings to her. The nest takes from two days to over a week to complete before the light-blue eggs can be laid. The eggs will be another post.
I think my recent observations were made at the very beginning of the great blue heron breeding season. Some of the males had decided on their nest site and were waiting for the females. Others were still choosing their nesting location.
I am anxious to go back and observe the rookery. I expect to see more birds settled on nests. I wonder if they will be in adjacent trees or concentrated in the original two rookery ponderosas. As happened last spring males should be carrying twigs and there should be much more calling. What fun to watch the breeding season again.
An article by Helen M. Pruitt, again in the 1970s, also provided me with information about great blue heron nesting.