Just an interesting question a friend posed to me the other day, and I would love to hear your collective opinions on it:

Do you think birds would have evolved flight in the absence of trees?

Wow, what a question! :) Cutting a long and complex story rather short, I'd say probably not becuase the vertebrates that have evolved flight (be it gliding, or powered flight, and so including things like bats, pterosaurs, flying squirrels, sugar gliders, flying frogs etc.) seem to have done so in trees. This makes a lot of sense - it's a very quick way to get from one tall structure to another, without the time, energy and dnager of going all the way down, along, and the back up again. Take away trees, and that environment and evolutionary selective pressure no longer exists, and rocks or cliffs are not really the same kind of thing and probably wouldn't produce the same results.

Fair enough, but could it not be argued the other way that they evolved flight to give them access to food/habitat/niche that the other non-flightless animals were unable to reach? If yes then even in the absence of trees that same pressure to access foods in higher and difficult to reach places would still be there.

I see your point David but I very much doubt it, it can still be efficient to climb up to things (look at monkeys), the real issue tends to be transport between high points, and so you need to have plenty of them around in a single area to make it effective as a mod of transport and we really don't have rock formations that act anything like trees, and even if so, they would not be very productive for plants and by extension things like insects or fruit, so there wouldn't be any resoruces to access.