Great Blue Herons (Ardea herodias) hold a special place amongst my extensive list of favorite creatures. They were one of the first birds I was able to name when most birds seemed unidentifiable. Consequently, whenever I spy that striking blue-gray bird, I feel as though the world is alright at that moment.
So, imagine my surprise this morning when I spotted a GBH landing on the crown of the White Ash tree (Fraxinus americana) directly outside of my front window. This tree is old, singular, and quite a distance from the lake. I could not fathom why the bird chose to land there.
I’ve never seen a GBH sitting in a tree that wasn’t part of its rookery. My favorite rookery was in a wetland surrounded by an industrial area in northern Indiana, near the polluted Calumet River. (It’s gone now. It was active for 60 years before the herons moved on.)
I’m pretty sure that the trees hosting the nests at that location were Black Ash (Fraxinus nigra). This was a relatively small rookery of about 25 nests. Large colonies can have more than 100 nests. Regardless of the size, the scene is impressive.
Made of sticks, the nest is about 4 feet across and about 2 feet deep. The nest can be situated as low as 20 feet or as high as 100 feet above the ground or water. The birds lay three to seven eggs sometime in April or early May and 60 days later the babies fledge.
Imagine researchers’ surprise to learn that GBHs intentionally build nests near Bald Eagles (Haliaeetus leucocephalus), their feared predators. The rationale for this is counterintuitive. Eagles will prey on GBH babies, but the eagles are themselves territorial and will chase away others within the vicinity of their nest. This defensive behavior by the resident eagles results in some losses for the herons. However, the losses are fewer than if many eagles could swoop in unchallenged because it is unclaimed territory. This odd nest placement strategy by the GBH is called the predator-protection hypothesis and is seen in a few other species pairs as well.
In this part of Maine, we do have a GBH rookery close by. We also have at least two pairs of nesting eagles whose territories appear to overlap the rookery’s airspace.
Perhaps what I witnessed was a relatively young GBH hiding from a nearby eagle. Or maybe he/she just needed a quick rest before heading back out to fish the lake. In any case, the sighting was magical.
More info about Eagle-GBH interaction: https://www.theatlantic.com/science/archive/2021/01/great-blue-herons-bald-eagle-prey-predator/617806/