- She looks above; the bird is coming straight towards her.
Is the use of semicolon valid here?
Yes, but it's not the only punctuation mark that could be used.
The litmus test for checking the validity of a semi-colon is whether or not both sides of the punctuation mark form complete sentences on their own. In this case, they do.
This means, by default, a writer has a choice: to use a semi-colon, or to break it into two sentences. Moreover, in cases like this one, a dash could also be used:
She looks above – the bird is coming straight toward her.
According to the Purdue OWL, an author would use a semi-colon when both parts of the sentence should have equal emphasis. When a dash is used, that indicates there is additional emphasis on the part following the dash. That's why I'd prefer a dash in this case – the part about the bird zooming toward the woman seems more important and significant to the woman glancing above her head.
By the way, there are additional usages of a semi-colon; however, the one I've discussed here is the usage that applies to this sentence.