Obligate Versus Facultative Migration in Birds

Everyone knows birds migrate, but the root cause behind this migratory behaviour can fall into one of two categories. Obligate migration is what one usually thinks of when it comes to bird migration; the seasonal flights made between predictable breeding and non-breeding grounds at specific times of the year that seem to be hard-wired into the birds that follow this behavioural pattern. The other form of migration called facultative migration happens at unpredictable times of the year and is in response to a change in resource availability or an environmental shift. These migrations are not associated with mating and happen seemingly randomly and do not have a set destination. There are situational advantages and disadvantages to both of these forms of migration and they have evolved separately multiple times across all orders of birds.