Living in northern latitudes, I have always wondered why do migratory birds invest so much time and energy to travel twice a year the vast distances between their breeding grounds and wintering areas. The most commonly found explanation, and the one I am grown accustomed to, is the scarcity of resources, resources meaning food and nesting places.
For years I have considered this a quite convenient explanation but recently I came to rethink it.
As you know, many of the longest migrating birds are insectivorous such as European Pied Flycatcher that breeds in much of Europe and western Asia but spends winters in western Africa. While it is easy to see the scarcity of food (insects) in northern breeding areas during winters, it is more difficult for me to see the scarcity of food in southern wintering areas during summers. - Take, for example, ants, that I expect, are abundant in those places all year around. Considering their total biomass, they would seem to hold a potential for a stable food source for masses of insect eaters with minimal food gathering effort required. So why migrate?
Is it that ants (or any other seemingly overly abundant invertebrates such as termites and mosquitoes) are not nutritious enough? Or are they too stingy and poisonous perhaps?
As for nesting places, I am aware that many birds defend their home range (thereby setting an upper limit for population density), but I imagine that such behaviour could be subject to change (or never had emerged in the first place) if there was no need to compete for other resources (mainly for food).
Or are the primary constraining factors disease and predation that would follow from a larger population density?
So in short, my question is, what is (/are) the primary reasons(s) for insectivorous birds to migrate instead of staying in their wintering areas and feeding on (seemingly abundant) local resources such as ants?