  The result of Siegel that the Tamagawa number of $SL_r$ over a function field
is 1 has an expression purely in terms of vector bundles on a curve, which is
known as the Siegel formula. We prove an analogous formula for vector bundles
with quasi-parabolic structures. This formula can be used to calculate the
Betti numbers of the moduli of parabolic vector bundles using the Weil
conjucture.
