We propose a new MAC protocol for IoT applications, called Bird-MAC, which is highly energy efficient in the applications where IoT sensors report status in a quasi-periodic manner, as in structural health monitoring and environmental monitoring. Two key design ideas of Bird-MAC are: (a) no need of early-wake-up of transmitters and (b) taking the right balance between synchronization and coordination costs. The idea (a) is possible by allowing a node (whether it is a transmitter or receiver) to wake up just with its given wake-up schedule, and letting a late bird (which wakes up later) notify its wake-up status to its corresponding early bird (which wakes up earlier), where the early bird just infrequently waits for the late bird's wake-up signal. The idea (b) is realized by designing Bird-MAC to be placed in a scheme between purely synchronous and asynchronous schemes. We provide rigorous mathematical analysis that is used to choose the right protocol parameters of Bird-MAC. We demonstrate the performance of Bird-MAC through extensive simulations, and real experiments. The experiments with sensors deployed at our office building and Yeongjong Grand Bridge in Korea, shows that energy consumption is reduced by about up to 45% over existing sensor MAC protocols.