Data aggregation mechanism is employed to reduce the redundant messages and energy consumption by collecting and aggregating messages from different sensor nodes in wireless sensor networks. The authors present an energy efficient data aggregation protocol for selection of aggregation head on the basis of its residual energy and nearness to the base station as well as comparative threshold energy when an event is detected. The efficiency of this aggregation protocol is shown through simulation results in terms of network lifetime by estimating number of dead and idle sensor nodes after fixed number of rounds as well as number of packets transmitted to base station as compared with the existing approaches.