An interesting one

You eagerly wait to hear Rancor’s theory about the reason behind the harpies’ endless mating season and increased ferocity. Your master gave you this assignment to make you more experienced and you also think this is a test to see how you differ from the average goblins, so you are going to do everything to impress your master and prove that you are far superior to the others of your kind. You are not going to walk into the harpies territory without proper information and Rancor seems to have some.

Of course, you will use your own judgment and intelligence while deciding a course of action, but it never hurts to have as much information as possible and Rancor seems to know what's he talking about, so he may be onto something.

“I think it’s one of two things” he continues “but before getting to that, you must understand something about magic, it is an incredibly powerful thing and not everyone can tolerate extended exposure to it and remain unharmed in some ways. Exposure to weak magic is not a problem, but extended exposure to powerful magic can alter you in some way, it can be both positive and negative, it can give you things or it can take things away from you, but it will happen. Now, the barrier your master put up is in itself a feat of tremendous magical power and that coupled with the ancient magic that is entrenched in this forest and the mountains make that kind of a ****”

He pauses to let you absorb what he said. “And that takes us to the question why the harpies are the way they are? I think either the extended exposure to the magic enhanced the harpies’ mating urge and gradually made it insatiable or it made the men around that place less virile than they used to and eventually made them impotent, so the harpies can’t get pregnant and their mating urge does not subside. The region was already scarcely populated before, after the war, it became even less so and due to the shortage of virile men the harpies are desperately searching for mates who can impregnate them” He concludes his thought on the matter.

He makes very good points, this can very well be the actual case,you should ask him about possible solutions.