Birds. Why did they do this?

All zombies, not just birds, essentially navigate the world based on what they perceive to be the path of least resistance.

There's a bunch of details, but the TL;DR is that it's about block HP and time taken. If Path B has a combined vs, they'll take path b.

Ultimately it's all about time taken; Block HP only matters in that it contributes to time taken.

To pose an example, You make a wall that has 4000 HP, but there's an opening in the wall big enough for a zombie to walk through. If you stand on the opposite side of the wall, they'll see the wall, with 4000 HP and conclude it would take, for example, 60 seconds to break through it; Meanwhile there's an open path to you that only takes 5 seconds. In this case they take the open path, because it's the path that takes the least time... If the wall was only 200 HP and broken in 5 seconds, and the path around would take 20 seconds, they'd beat through the wall instead.


In your case, You've got a hefty concrete 'wall'(floor), and then a bunch of weak easily broken terrain blocks surrounding it. It would take minutes to break through the concrete, seconds to break through the dirt; so they navigate to the dirt and dig down.

The solution is, essentially, to increase the size of the surface platform to the point that the pathing AI no longer has the range to consider the alternate path of just digging into the dirt; You'd need to go like 15-30 blocks from the edges of the place you can stand in the underground location. Ish.