Great Blue Herons, First of 2013

What: Great Blue Herons, the first I've seen in 2013
Where: Lowry Bridge in North Minneapolis. First, I see one individual in flight. If flies about 1/2 mile up the Mississippi River and lands in a rookery. I count about 30 nests and at least 4 other individuals.
When: 5:30 pm Saturday, March 23
Observers: Abbie and Scott, out on a 10+ mile walk. About 15 minutes later, I see one more individual in flight and Scott asks me how I can identify it from such a distance and from a ventral vantage point, no less. I tell Scott I recognize it by size and the character of its flight. But how would I articulate what I'm seeing? Once home, I read what Pete Dunne has to say about GBH flight:

"Steady wingbeats seem slow and ponderous. . . . Great Blue Heron flies on severely down-bowed wings (seeming to cup the air), and the wings don't appear to rise above the body. The bird flies as if dipping its wingtips into an invisible cauldron, testing the air."

Conditions: Overcast and about 40 degrees. Once home, I do a bit more research. At the top of my search results is this article which leads me to believe I'm looking at a new rookery, started in May/June 2011 after the May 22 tornado destroyed more established rookery further upriver. Next, I turn to e-Bird for a little more information (click image to enlarge).

In early March, GBH appears on about 1% of checklists. Then frequency steadily rises until by mid-April, GBH appears on about 28% of checklists. (This data includes Hennepin County checklists from 1900 to 2013.)