Birds don't die because they squawk when attacked. They die perhaps because they have faulty ears and can't hear a predator behind them.
I personally don't believe there is much merit to the argument that squawking attracts more predators and therefore, death, as opposed to survival. Let's say a bird does not have the reflex to make a sound, and is able to escape from his single enemy, I'd say it is very likely that;
A. He will die shortly after from his wounds.
B. His wounds will heal but I.E., make him limp, and an easy target to predators.
C. He will heal, with some damage, and be unable to find a mate for his I.E. lack of feathers and sickly appearance. So his genes will not be spread anyway.
D. His response of not making a noise when attacked, may also carry into a lack of overall communication when alerting other birds of a predator in the vicinity. He may be excluded from the flock, or die from lack of awareness. This actually may be a disadvantageous trait in the big picture.
I think the squawking is completely irrelevant to natural selection, but rather, what got the bird attacked in the first place is what's key. Therefore, it is important to remember that we have vestigial organs. If a trait is not particularly disadvantageous, it may take evolution an extremely long time to weed it out.