I suspect the Macbeth reference is an implied metaphor. A falcon “swoops” on its prey (it also “stoops” as it dives), and the lines suggest that the children and their mother – the chickens and their dam – were all carried away in one deadly attack.
Here’s the link to the Mailbag item: What is the origin of “one fell swoop”
I suspect you’re right, Bradster, and that the juxtaposition of the term “fell swoop” (whether Shakespeare invented it or only used it) with “chickens” is meant to convey exactly that imagery. Yet another indication of the brilliance of Shakespeare’s writing.
And some more info:
Science fiction writer David Gerrold once used the phrase “one swell foop.” (He wrote the “Tribbles” episode of STAR TREK.)
Those who do not learn from the past are condemned to relive it. Georges Santayana