Final answer:
In chapter 15 of 'Hatchet,' the foolbirds' unpredictability and noisiness frustrated Brian as they were hard to catch for sustenance.
Step-by-step explanation:
In Hatchet by Gary Paulsen, specifically in chapter 15, the aspect of foolbirds that drove Brian crazy was their unpredictability and noisiness, which made them difficult to catch for food. These birds, known as foolbirds due to their seemingly foolish behavior, eluded Brian despite his numerous attempts to hunt them. Their presence served as a reminder of the challenges he faced in the wilderness, where securing food was a relentless task.