1 INTRODUCTION
Uncertainty and complexity are some of the key defining features of human migration - one of the main global challenges of today’s world. In particular, this holds true for the formation or change of migration routes, or responses of flows to the underlying drivers, which are characterised by very high volatility, as witnessed during the 2015–16 Syrian asylum crisis (Kingsley 2016). One reason behind this complexity, and behind the inefficiency of attempts to control migration, is the agency of various actors involved – migrants, institutions, intermediaries, and so on (Castles 2004). This agency also remains one fundamental reason why migration typically eludes attempts at its theoretical description, explanation and prediction, the efforts undertaken having historically remained scattered across various disciplines (Arango 2000).