So some presume. But there is a big jump from affirming that the size of a bird's beak can vary to making the claim that the bird turned into a whole other animal. The one has been observed, the other has not.
In every other system, when you make a large number of small changes to an object you end up with an object that is radically different than the object you started with.
This is no "jump", any more than it is surmising that if I push a ball down stairs, the ball will bounce down to the bottom.
It's observable in almost every system.
Actually, the beaks did not even micro-evolve. They got bigger in droughts, but when rain became plentiful, they got smaller again. So there was no positive change either way on beak size, just a temporary change. Because the changes took place, back and forth in less than a dozen years of looking at them. So there was no mutation at all, there was no new genetic information created either way. There was just adaptation through the use of already existing traits in the gene pool of the species. Science recognizes these differences in the gene pool as essential for the survival of a species. That is why when they are trying to bring back a species from near extinction, they try to get as many different animals from as many different places as possible to help them restart the species.