After following the great blue heron (Ardea herodias) rookeries near Baum Lake (Shasta County CA) over several seasons, I noticed consistent behavioral patterns from year to year.
The great blue herons are very noisy and active as they pair up, select nest sites, court and build their nests in February. The herons fly constantly to and from the ponderosa pines where they nest, continually calling in their loud, hoarse “croaks”. When I walked past their rookeries the herons would fly off, calling all the while. Great blue herons hunting prey along the shore of Baum Lake would call loudly and flap away as I wandered by. Very visible and very loud describes the herons’ behavior in February.
In early March it suddenly seems as though the great blue herons have disappeared. Look closely at the rookeries and the herons are still there. But they are totally silent and are much less active. If I watch for a while I see an occasional heron fly to or from their nesting trees – almost like grey ghosts. Even herons that I accidentally jump quietly fly away. Their eggs are incubating and the herons do not want to draw attention to their nests. In about a month I might be lucky enough to see nestlings peeking over nest rims.
The other day I walked under a ponderosa pine where great blue herons are nesting. I could see the parents on the nests or standing on nearby branches. But no herons flew and they were totally silent. When I was perhaps a hundred feet past the tree there was a cacaphony of heron calls. I turned and saw about eighteen great blue herons all flying away from the tree at once and loudly scolding. I could not imagine what was happening. It was an impressive sight – and sound – as the herons flapped about the tree. Slowly the herons settled back into their tree or on nearby trees. Suddenly the birds repeated the cycle of flying, calling and eventually settling down. However this time I saw a red-tailed hawk circling close to the rookery tree. The hawk was disturbing the herons, it was not me bothering them. The hawk flew off and within a few seconds returned to initiate the herons’ defense of their nests for a third time. The hawk finally flew away permanently and the herons resumed their quiet nesting mode.
I did not realize that red-tailed hawks would prey on great blue heron nests. A little research confirmed that indeed red-tails are predators of great blue heron eggs and nestlings. According to the literature, great blue herons will aggressively defend their eggs and young against predators. Again I learned something!
From the appearance of the red-tailed hawk’s tail (not a particularly good picture, but things were happening too fast), the hawk may have previously encountered other protective parents. The great blue heron in the photo was one that flew from the rookery to a nearby tree with an abandoned bald eagle nest.
I have many previous posts on red-tailed hawks and great blue herons. A couple posts with general information about these birds include: Twig Presentation, Great Blue Heron, Feeding Red-tailed Hawk Chicks, Red-tailed Hawk.