Some long and indirect migration routes can most plausibly be attributed to birds following ancestral routes of post-glacial range expansion. Some species continue to follow these routes, even though, for birds breeding at the end of the colonisation route, apparently suitable alternative wintering areas are available much closer. Their failure to switch to new routes may be due to the difficulty of making in one step the necessary huge changes in directional preference and fattening regime. In effect, the birds are locked into an existing genetically controlled system, in which small incremental modifications are possible, but not the big single-step change necessary to bring a fitness benefit.
From current migration patterns, possible intermediate stages can be envisaged that might have occurred in the evolution of disjunct migration patterns, in which breeding and wintering areas are well separated, even though such migration might now involve long sea or desert crossings. The same is true for chain and leapfrog migration patterns, both of which can be explained historically. Similarly, migratory divides may mark the meeting points of two populations that recolonised northern areas after the last glaciation, with each population spreading from a different refuge and then following on migration its ancestral colonisation route. However, such patterns are likely to persist only if they serve adequately in present conditions. Populations that were isolated during glacial or other climatic extremes often have distinct DNA (and sometimes also distinct taxonomic status), as well as separate breeding and wintering areas.
Loop migration, in which birds take different routes in spring and autumn, probably occurs in response to seasonal differences in conditions, notably weather (especially wind) and food supply.
In many species, migration is likely to have developed hand in hand with range expansion, as birds spread by normal dispersal into higher latitude areas where they could breed, but from which it was necessary for them to return to lower latitudes to winter. This was probably the prevailing pattern as birds re-colonised high-latitude areas following each glacial retreat. It can be seen in process of development today in some species that are spreading northward within Europe or North America, developing migration secondarily as they reach more seasonal environments. Conversely, other species, introduced to high latitudes by human action, have changed from resident to migratory, utilising lower latitudes in winter. The same probably happened at the start of each glaciation, and as advancing cold changed resident to migratory populations, before eventually obliterating the higher latitude ones altogether.
Whereas roughly one-third of the Nearctic/Neotropical and eastern Palaearctic/ Indomalayan migrants winter in tropical forest, almost none of the European/ African migrants do so, wintering instead in scrub. This is attributable to the fact that, throughout the glacial periods, forest occurred continuously from tropical to boreal latitudes in the first two regions, but the European-African forests were broken by a broad band of scrub-desert, limiting the movements of forest species, but favouring the movements of scrub-dwelling ones.
When breeding populations expand and become more migratory as they spread from lower to higher latitudes, competition is likely to occur in common wintering areas. This competition could result in: (1) a constraint to further growth in numbers and expansion of breeding range; (2) development of chain migration; (3) development of leapfrog migration; or (4) elimination of breeders from the original range, depending on circumstances, particularly the competitive relationships between breeders from different latitudes and the availability of suitable wintering habitat beyond the original breeding range.
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