Willie saw the bird. Frank heard the noise.
There is nothing to suggest that the two things are related. For all we know, the noise may have nothing at all to do with the bird.
Willie saw the bird; Frank heard the noise.
Now it's clear: the two things are related. The noise Frank heard might have been the noise of the bird, or the noise which scared the bird which then flew off to be seen by Willie. This is the equivalent of the following:
Willie saw the bird, and Frank heard the noise.
...and that, I am certain, you would not mark as incorrect.