I'm no expert with memory analyzing, but there appears to be some kind of leak with the crows and their navigation data storage:

This may be a cause for memory usage building up as players explore new areas, progressively getting laggier until it reaches a point where the server simply crashes at 100% memory use.
Additionally, here's a heap summary provided by Spark: https://spark.lucko.me/iPUhjtUr5h
That summary was taken with memory usage sitting around 5.6GB, as new chunks had been explored. Restarting the server would reset that to around 1.6GB, which it stays at while in already generated areas.

I've been travelling for half an hour with crows disabled, memory usage remains about the same (goes up to around 5GB before garbage collection brings it back down to 3.3GB or so, about average with a few players in modded Minecraft). There's definitely a memory leak in their AI.