Cottonwoods and herons nesting in sync

The cottonwoods along Denver's South Platte River are unfurling their leaves in earnest now, each triangular blade catching and releasing the morning light. The canopy thickens daily, transforming bare winter branches into dense green shelter. If you're walking the river trail this morning, you can hear the rustle of new growth overhead and the deeper sound beneath it: the guttural croaks and wing beats of great blue herons settling into their nesting colonies.

This timing is no accident. Eastern cottonwoods (Populus deltoides) reach early leaf expansion precisely when great blue herons (Ardea herodias) and black-crowned night herons (Nycticorax nycticorax) establish their rookeries. The herons need what the cottonwoods provide: a canopy dense enough to hide their stick nests from predators and thick enough to buffer their young from wind and weather. The cottonwoods deliver this cover just as the herons begin their most vulnerable season. Great blue herons build their platform nests sixty to eighty feet up in the cottonwood crowns, while black-crowned night herons prefer the middle story, twenty to forty feet above the water. Both species return to the same grove year after year, their nesting success tied directly to the health and timing of these riverside trees.

The relationship runs deeper than simple shelter. Cottonwoods leaf out in response to soil temperature and day length, biological cues that also trigger the herons' return from wintering grounds. The trees' rapid spring growth creates not just cover but habitat complexity. As the canopy fills in, it generates microclimates that insects depend on, and insects feed the fish that herons hunt in the shallows below. The cottonwoods' extensive root systems stabilize the riverbank, maintaining the clear, slow backwaters where herons wade and strike. When the trees release their cottony seeds in early summer, they provide nesting material for smaller birds whose presence creates the ecological richness that supports the entire riparian food web. The herons, in turn, bring nutrients from distant feeding areas back to the grove through their droppings, fertilizing the soil that feeds the cottonwoods.

Listen now to the sounds layering above you: the soft percussion of new cottonwood leaves in the breeze, the deeper whoosh of heron wings as they settle onto hidden nests, the occasional harsh call that carries across the water. The light filtering through the expanding canopy shifts and dances, creating the dappled shade that will soon shelter eggs and then nestlings. This is the sound of a relationship measured not in moments but in seasons, where timing means survival and the river binds everything together.