The Isle of Korybos

The Storyweaver

You and your companions find yourselves upon the sable beaches of Korybos Island, at its Southernmost point. You've heard a number of strange tales about this place which warrant investigation as possible sources of shard magic.
One man, heavily in his cups at the tavern, told you that there is a cove in the Northeast where the crabs have begun to construct a crude but functioning civilisation. Another, an aging naturalist, swore that she had seen a mating pair of two-headed birds roosting near a rock-face to the West. Finally, a young child showed you a necklace which his father had found on the island. Its beads were the shape of shark's teeth, yet its form was that of a pure, sky-blue opal. The child said it was no jeweller's work (and the piece was idiosyncratic enough that you believe him). Rather, his dad had found it in a cave just East of your current position.
Realistically, you have only allotted sufficient supplies to investigate one of the phenomena. Which lead seems the most promising?
A crab civilization? Surely the ramblings of a drunken fool. But then again, what if it were true?
Two-headed birds? The source seemed reputable. Could the tale be true?
[LINKS BROKEN] You saw the strange blue shark teeth yourself. The child had no reason to lie and was clear his father had come to Korybos. This is the most solid lead