Galahs

When Europeans arrived in Australia, galahs were typically inland birds, quite sparsely distributed. Now they range from coast to coast, and are common. Why did this change occur? Why didn't it occur earlier? I argue that because galahs feed on the ground they found Australia's dominant inland grasses too tall to get at the seed, so relied on an agency to shorten them: Aboriginal grain cropping before contact, introduced stock after it.