Theory

Yesterday (3/30) Susan Stillwell, who lives on Hays, told me that she saw "more than 20 but less than 50" herons circling in the air some days ago. I had just returned from the end of the road where one can see/where I did see ten herons engaged in nest-building activity (herons passing sticks to one another and inserting them), but also some unoccupied nests, whereas when I'd been there on 3/27 all visible nests were occupied.

Presence of an eagle might have resulted in the birds' being in the air. It might also explain the drop in numbers/nest occupancy that I've been wondering about.