On an island in the Mississippi River, a stand of cottonwood trees is silhouetted against a gray sky. The bare branches are festooned with big dark lumps, nests made of twigs and branches. Next to the nests, like sentinels at the castle gate, stand great blue herons.
Though they receive far less attention than Minnesota's state bird, the loon, herons are just as good at catching fish and they can soar like eagles. Their intense activity is a reminder of spring's arrival, and a dramatic part of the north's warm season.
At four feet tall, the herons cast an imposing presence, especially in numbers. More than 100 of them have claimed the branches of old cottonwoods just upriver from downtown Minneapolis.
When one takes off and glides away, its six-foot wing span dwarfs the ducks and songbirds sharing the island.
It's hard to tell the males from the females because they're the same blue and gray. But that comes easy to Sharon Stiteler, a part-time naturalist with the National Park Service.
"The males arrive first, and they work out who's going to take which nest," said Stiteler on a recent tour of the rookery. "Where you see one bird standing up, that is most likely a male. He's hanging out there because the other males who are still waiting to attract a female could come by and steal sticks out of his nest to make his nest look better."
On some nests, you can see females already sitting on pale blue eggs the size of small mangoes. But herons are not always good parents.
"If the chick falls out of the nest and lands on the ground, that chick is toast. The parents will not continue feeding it," Stiteler said. "And oftentimes you'll see turkey vultures hanging out at rookeries, and they're waiting for the young to fall and starve, and then they'll have a whole bunch of food."
But at least in a protected spot like this island, there won't be many predators.
The birds were once threatened by humans. Their cousins the egrets were hunted for their beautiful white feathers, and both suffered disastrous population loss until the pesticide DDT was banned.
Now you can see them stalking their prey in streams and lakes all over the southern half of Minnesota. While the loon dives deep to find fish, the heron hunts the shallows. It literally stalks its prey.
Stiteler, who writes a blog called birdchick.com, follows the herons and watches them hunt with beaks shaped like a pair of super-sharp chopsticks.
"They have a lot of patience, and they just stare at one spot for long time, and then they jab down and grab the fish," she said. "Sometimes they catch a huge fish and they have to juggle it around, especially if they have it perpendicular with their beak, they have to jostle it around, and the fish is wiggling, and eventually they get it just right so it's straight in line with the bill, and you can watch this huge thing slide down that long slender neck."
As Stiteler walked through the rookery, the herons were pretty quiet. But her Blackberry is loaded with their sounds, including the prehistoric squawk, squawk, squawk they make when they're startled -- and the strange, almost mechanical chattering the young make when yearning for food.
After the young are raised -- at least the ones that survive -- the herons stay on the river until it freezes over and they can't fish anymore. The birds decamp all at once.
"One day we have great blue herons, and the next day they're gone, and they migrate at night," Stiteler said. "They're a slow flyer, so to an aerial predator, conceivably a bald eagle, it's safer for them to fly at night."
They move south to find food -- to Indiana, Arkansas, even Florida. Tucking back their long necks when they fly, they form an S-shape that hides their true length.