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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?

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1 Answer 1

Reason for Migrating Birds:

Migrating for a Meal

Food scarcity: If birds will stay and eat all the bees they find, there will be no bees to feed on for the next generation. So, they fly away to take advantage of these places where there is abundance of food. And when the insects or replenish they come back.

Migrating for Family

Flocks of bird can be easy prey as they are small and are in a huge number. Also for the young ones who can't fly it is very dangerous. (Though they abandon the young one as soon they can fly)

Migrating for better Climate:

Some of the species fly away to sustain the temperature and come back when it is suitable for them. So, Northern birds migrate to tropical places for the winter and come back to north for the summer to lay eggs.


There are too many other theories for bird migration. It had been confirmed that migration is often due to genetics than any other reason.

Further Reading: THEORIES OF BIRD MIGRATION

Sources:

[1] http://birding.about.com/od/birdbehavior/a/Why-Birds-Migrate.htm

[2] http://en.wikipedia.org/wiki/Bird_migration

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