Migration drivers

Migration is a global phenomenon; from mass migration of human refugees, to annual migrations of birds and whales.  How often do we stop to think about what drives migration?  Annual avian migrations are driven by climate and access to plentiful food supplies. For whales and salmon migration is associated with the breeding cycle as salmon travel upriver to spawn and die, while whales travel south to give birth in warm lagoons.  Migration of humans on the other hand is more of a disruptive phenomenon, whether migration over time from rural to urban areas in search of higher paid work, or triggered by conflict, as evidenced by the tidal wave of Syrian refugees spreading across Europe. In "Crows' Feat" the birds are driven by overpopulation, scarcity of resources, and environmental threats (disease and the encroachment of humans) to embark upon a more disruptive migration of their own.