  A method to calculate hadron momentum spectra after feed down from resonance
decays in the context of ultra-relativistic heavy ion collisions described by
relativistic fluid dynamics is presented. The conceptual setup uses the
Cooper-Frye freeze-out integration together with an integral operator
describing resonance decays. We provide explicit expressions for the
integration over the freeze-out surface for a smooth and symmetric background
solution, as well as for linearized perturbations around it. A major advantage
of our method is that many integrals can be precomputed independently of a
concrete hydrodynamic simulation. Additionally, we examine the influence of
adding heavier resonances to the decay chain on the spectrum of pions and show
how to include a phase with partial chemical equilibrium in order to separate
the chemical from the kinetic freeze-out.
