In distributed software development synchronized actions are important for completion of complex, interleaved tasks that require the abilities of multiple people. Synchronous development is manifested when file commits by two developers are close together in time and modify the same files. Here we propose quantitative methods for identifying synchronized activities in OSS projects, and use them to relate developer synchronization with effective productivity and communication. In particular, we define co-commit bursts and communication bursts, as intervals of time rich in co-commit and correspondence activities, respectively, and construct from them smoothed time series which can be, subsequently, correlated to discover synchrony. We find that synchronized co-commits between developers are associated with their effective productivity and coordination: during co-commit bursts, vs. at other times, the project size grows faster even though the overall coding effort slows down. We also find strong correlation between synchronized cocommits and communication, that is, for pairs of developers, more co-commit bursts are accompanied with more communication bursts, and their relationship follows closely a linear model. In addition, synchronized co-commits and communication activities occur very close together in time, thus, they can also be thought of as synchronizing each other. This study can help with better understanding collaborative mechanisms in OSS and the role communication plays in distributed software engineering.