h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

But I guess birds develop a need to certain surroundings, which can hamper their migration potential.

Poor little Grebe.

Many birds here in the tropics and subtropics don't migrate … some are indeed confined to very small ranges, such as forests on a single range of hills. At least no land bird in Africa is flightless...

Oops except for ostriches of course! But they're fairly equipped for travelling far and wide … they used to be all over the continent except the rainforest.

Was that because the rainforest didn't have sand to bury their heads in?

Heh heh and actual rainforest doesn't have much leaf litter for that purpose either. But … I think Africa could do with a large, forest-living ratite...

Would there be something there for them to eat? They're fairly large, and would probably eat a lot. Plus, being able to kick in a lethal way might be good enough enough against some adversaries,chances are it wouldn't keep them safe against the big cats.

Ostriches eat almost anything. And the rainforest in Africa has no cat larger than a leopard, against which ostriches are quite safe. But I don't think they can deal with the humidity, and they're made for fast running over open terrain.

And, somehow, no ostriches apparently moved into a rainforest niche and adapted to it.

But ostrich relatives did! In the Australasian region we have the cassowaries; we used to have the forest-living elephant birds (Aepyornis and Vorombe) in Madagascar, and many of the Moas of New Zealand also likely lived in forests.

Did these go extinct? Or did they move elsewhere?

Three species of cassowary still exist. Elephant birds and moas are all extinct.

Cassowaries are considered the most dangerous birds. I would steer clear of them.