In this paper, we derive the averaged bit and symbol error probabilities as approximated closed-forms for M-ary quadrature amplitude modulation (MQAM) burst transmission of adaptive decode-and-forward (ADF) relay systems over quasi-static Rayleigh fading channels. Within a burst, there are pilot symbols and data symbols. Pilot symbols are used for channel estimation schemes and each relay node's transmission mode selection schemes. The approximated error rate expression is verified based on the number of relays, pilot, and data symbols. Its accuracy is confirmed by comparing with simulation results.