Interesting how she moves from the “note” of a “Bird” being worth more than a “Million” words, to the deadly image of a single “sword” being all that’s required to make a point. You wouldn’t typically equate gentle nature with violence, especially considering the old saying of the pen being mightier than the sword (which Edward Bulwer-Lytton coined in 1839), so perhaps she means something else?
Emily wrote this poem more than a decade after the end of the US Civil War, however the memory of the rhetoric found in the newspapers would have remained and tensions in rebuilding America would still have been high – much like how tensions in the US are still high nearly twenty years (at the time I’m writing this) after 9/11. So perhaps she isn’t necessarily equating the “note” of a “Bird” with violence, but rather she is making a distinction by saying that the “note” of a “Bird” is what we should be listening to because even a single “word” is like a “sword” ready to be drawn from its “scabbard”. A word can kill, and when those words are printed (such as in the newspapers) they can lead to violence.
And there is a slanted reference here to printing in her use of the word “scabbard” which up until 1787 was the term used for the “thin board used … by printers in making register” which is the tool used to justify text on a page, now known as the scale-board. This she might be alluding to how words can be used to justify violence – the raising of a “sword” – and this seems plausible since “word” and “sword” share all but a single letter.