The evolution of a long pulse (pulse length much greater than the slippage distance) in a tapered wiggler free electron laser oscillator is studied by numerical solution of the one dimensional theoretical model for a realistic set of magnet, electron beam, and optical resonator parameter values. Single pass gain curves are calculated for low and high light intensities. It is found that an initial, low amplitude, incoherent pulse grows into a coherent pulse whose growth rate agrees with the calculated small signal gain curve. The transient evolution of coherent pulses is calculated for several different cavity length detunings, and a quasi-steady-state desynchronism curve is obtained. Various pulse features for two points along the desynchronism curve are given. The frequency changing behavior (chirping) of the optical pulse during transient evolution is examined
