Rookery in the Grasslands

This is a rookery in an old Oak Grove where hundreds of egrets, herons, cormorants, and other birds were

resting. The leaves of the tree were covered in white droppping and all manners of squeeks and squawks were

signaling some strange gathering of species into a city of birds. I wonder if these rookeries change location

and if so when and why? Here I felt like I was in the Amazon and not the Valley.