Wall-crossing of universal Brill-Noether classes

We give an explicit graph formula, in terms of decorated boundary strata classes, for the wall-crossing of universal Brill-Noether classes. More precisely, fix n>0 and d<g , and two stability conditions φ^-, φ^+ for degree d compactified universal (over the moduli space of stable n-pointed curves of genus g) Jacobians that lie on opposite sides of a stability hyperplane. Our main result is a formula for the difference between the Brill-Noether classes, compared via the pullback along the (rational) identity map. The calculation involves constructing a resolution of the identity map by means of subsequent blow-ups.