What does "The bird is on the wing" mean in "Mr. Flood's Party"?

"The bird is on the wing" is a quotation from the "Rubáiyát of Omar Khayyám," an 1859 poem by Edward FitGerald that is a loose translation of 12th century Persian poet, Omar Khayyám. The bird represents time, and hence the mortality that must come for all living creatures. When "the bird" of time is "on the wing," time is in flight, meaning that death is approaching.
In the context of "Mr. Flood's Party," the quotation from the Rubáiyát reinforces the melancholy tone of Robinson's poem. The appearance of the Harvest Moon signifies the figurative onset of the "winter" of Eben Flood's life. The overall mood is portentous and grim; the bird that is the symbol of time is also a portent of Flood's impending death. Thus, it is ironic that the title of this mordant little poem contains the word "party."