SummaryThe paper is concerned with the relative importance of environmental changes and social stimulation in bringing about synchronized social breeding inPloceus cucullatusandMelanopteryx (Ploceus) nigerrimus. Field work was carried out in central Uganda.Observations on a single colony of each species extending over a complete breeding period showed that breeding started abruptly with the arrival of a large number of males and the construction of new nests. The majority of females arrived later within a relatively short space of time. Subsequently the number of birds and the amount of activity declined gradually. Nests were repeatedly destroyed and reconstructed in the same positions. InP. cucullatusan entrance funnel was generally added only after occupation of the nest by a female.Studies extending over a large number of colonies showed that main breeding seasons correspond approximately with the two rainy seasons each year. Isolated colonies may be active between seasons, however.Nest‐building and breeding are highly synchronized within colonies. Uniform responses to rain may partly account for this synchrony, but fluctuations of activity in neighbouring colonies are relatively unsynchronized, suggesting that social stimulation is also an important factor. This is supported by direct observations of social interactions and by the fact that small colonies are relatively less successful than larger ones, apparently because the pair‐formation process may fail in the absence of sufficient social stimulat