Is there an instinct-driven body language for birds?
In general, animal communication contains both "cultural" aspects and genetic components.

As you suggest, the cultural aspects vary more across geographies and times. The genetic components might vary as well, with genetic differences among different populations, but you'd expect them to be more conservative than the cultural aspects. To make the matter even more complex, if the communication behaviors affect mating; they could in turn have an effect on the genetic components!

This article describes birds raised in captivity that had "maladjusted" songs (which would support both genetic and cultural components), and North Carolina cowbirds raised by Texas parents who grew up with a Texas birdsong dialect (evidence for cultural components). Although the piece is mostly focused on song, I imagine the mechanisms would be similar for body language.