An active rookery is busy because the environment is right for safe nesting. Trees, surrounded by water, Alligators below to ensure nothing attacks from below, and many other factors.
It also means the birds have neighbors, lots of them.
Every bird there is a predator. That means at some point something will happen.
The images above show Great Blue Herons, Great Egrets, and Anhinga. Almost on top of each other. Each rookery tree is the same, precious real estate.
The good news, after a while the surviving young are big enough to protect themselves, and join in the fray.