  The open source development model has become a paradigm shift from
traditional in-house/closed-source software development model, with many
successes. Traditionally, open source projects were characterized essentially
by their individual volunteer developers. Such tradition has changed
significantly with the participation of many organizations in particular.
However, there exists a knowledge gap concerning how open source developer
communities evolve. In this paper, we present some observations on open source
developer communities. In particular, we analyze git repositories of 20
well-known open source projects, with over 3 million commit activities in
total. The analysis has been carried out in three respects, productivity,
diversity and growth using the Spearman's rank correlation coefficient,
diversity index and the Gompertz/logistic curves, respectively. We find out
that (a) the Spearman's rank correlation coefficient between active
contributors and commit activities reveals how changes in the size of the
developer community impacts the productivity of the community; (b) the
diversity index of an open source developer community reveals the structure of
the community; and (c) the growth of open source developer communities can be
described using different phases of growth curves as in many organic matters.
