I imagine the argument would go something like 'Creatures usually act to preserve themselves; if the bird knew the future actions of the army, it would know about being shot by that Jew; if it knew about being shot, it would not be there (since it wants to preserve itself); the augur's interpretation is true only if the bird knows; it was shot, so it did not know; it did not know, so the augur's interpretation is false.'
There are ways we can rescue this if we want to make excuses for augury and I'm sure you can think of 3 or 4 counter-arguments, but why bother? It's a good story - 'physician, heal thyself!'
The argument was wrong even by the standards of the time. You just misunderstand the concept of divination :-) It doesn't rely on the bird consciously knowing anything. In fact, instead of watching the bird, you can kill it and inspect its entrails. Divination works (or doesn't) because the will of the gods leaks into the pattern of visible things (or doesn't).
Here's the new thread for posting quotes, with the usual rules: