The interplay of cross-polarization cross-phase modulation, higher-order dispersion effect, shock effect, Raman response, and coherent erbium amplification of a femtosecond pulse in a birefringent erbium-doped fiber amplifier is numerically studied. Under amplification, cross-phase modulation becomes stronger, and hence the period of the peak position oscillation of the two polarization components around their center of mass decreases. One of the polarization components may gain more energy depending on the spectral positions of the two components under cross-phase modulation. In the situations considered, the amplification generates a red shift in spectrum and delays the pulse because the gain on the long-wavelength side is higher.