Research on the role of social networks in human migration has mainly relied on single snapshots in time. This paper focuses on the changes in composition and usage of the transnational networks of migrants and why these changes occur. It is based on ethnography and network analysis with forty sub-Saharan African migrants in two transit contexts: Turkey and Greece, over a 17-month period. Findings show that relationship preferences, resources and communication infrastructures constitute an individual opportunity infrastructure affecting how critical events produce network changes. This process is ongoing through the continued experience of critical events, suggesting that the role of networks fluctuates over time. (C) 2017 Elsevier B.V. All...