How did migration evolve?

We can't know for sure how migration arose, as it leaves no fossil evidence behind. Migration is probably as old as the first motile organisms, and may have arisen on many independent occasions in different species groups over the course of evolutionary history. It may have evolved de novo in birds, but it's also quite possible that they inherited the tendency to migrate from their dinosaur ancestors.

Even though we don't know how migration evolved, it's important to come up with an explanation of how it could have arisen by natural selection. However, evolutionary accounts of migration either skirt around the issue, or offer explanations that lack plausibility, so a different approach is required.

First, we need to define our terms. What do we actually mean by migration?

Migration n. A periodic cyclical movement of the centre of activity, the path of which is to some extent heritable [Latin: Migrare].

In other words, bird migration as usually understood involves a 'return' journey in which the outward and return leg (and any 'hops' that occur in between) follow an inherited programme with respect to compass direction and distance.

We also need to be clear about what isn't migration. For a part of each year, birds are attached to their nests by a piece of invisible elastic, but for the rest of the time, they can go pretty much where they please. This means that on average they end up in different places inside and outside the breeding season. Wherever they end up outside the breeding season, it pays to go back to where they bred successfully before, which means that all birds undergo a 'return' journey over the annual cycle. Does this mean all birds are migrants? The answer is no, because not all of these journeys have an inherited directional component.

When a non-migrant abandons its nest site at the end of the breeding season, it might move in any direction. If it sees a patch of fruiting shrubs to the north, it might go north. If it sees a comfortable roost hole to the east, it might go east. Because non-migrants move in random directions, their average position stays the same, and the space occupied by the population does not shift it's centre of gravity.

Subpages (1): How did migration evolve? Part 2

How did migration evolve?

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