birds are determined, even angry, because they are a locked into an
existential struggle for the future of their species. Sadly, this will
ultimately prove futile.
Many people believe that Angry Birds is
simply an amusing time-waster game, whereas in fact it is a magnificent
historical reconstruction of the tragic end of the dodos. This occurred
because dutch sailors brought pigs to the island of Mauritius, which
plundered the dodos' nests and ate their eggs and ultimately drove the
birds to extinction. (It is a misconception that the sailors ate the
dodos themselves - they rarely did so as the meat tastes bad.)
The clinching evidence for this interpretation is that dodos were flightless birds, so of course they would have needed to use catapults.
To give credit where it's due, I had not recognised the documentary nature of this game until it was pointed out to me.
How true this theory is, only the makers at Rovio can tell, but it seems to fit the game? Why else would the birds be SO ANGRY?