Dead
sprig upon its breast
The funeral custom of placing flowers on a corpse. Dickinson's specificity—not flowers, but a 'sprig'—suggests something small, plain, probably homegrown.
simple gravity
Gravity as both seriousness and physical weight. The corpse has literal weight (gravity pulls it down) and metaphorical weight (the seriousness of death).
prone to periphrasis
Periphrasis = talking around something instead of naming it directly. The educated speakers can't say 'dead' so they say 'birds have fled' (a euphemism for the soul departing).