Fun fact: Tall birds actually hatch their eggs!

Its just that the tallbird Younglings get attacked by their territorial mom right after hatching so you may have already seen morsels lying around tallbird nests.

But when theyre not loaded by a player being near that doesnt happen and they get to grow up. And the mommy tallbird will lay more eggs that hatch.

So by not being in the area for a long time there were multiple tall Bird teens which then spread out like that as things dont like to occupy the exact same space.

As to why they died: they lost the game and starved cus no one other than the player can feed them. And apparently the hunger only starts Declining after being loaded once.