Birds migrate in order to optimize the balance between their energy intake and expenditure, finds a paper published online this week in Nature Ecology & Evolution. This rule also applies to non-migratory species, and provides a general explanation for the global distribution of all birds.

Around 15% of the world's bird species migrate between breeding and non-breeding habitats, allowing them, for example, to escape food shortages and unfavourable weather during winter months. However, identifying driving factors that are common to the movement of all migratory and non-migratory species has not been possible until now.

Marius Somveille and colleagues use the movement patterns of the world’s migratory bird species to calculate the energetic costs of moving between two different habitats, based on theory about the metabolic costs of movement, reproduction and thermoregulation. They then populate a 'virtual world' simulation with all of the planet’s birds based on the amount of energy available in a region (estimated from the amount of vegetation, or primary production). Compared to a world in which energy is not considered important, they show that only a model incorporating energy efficiency is able to reproduce the true seasonal distribution patterns of birds around the world. The authors also suggest that the model is general enough to be applicable to other highly mobile animals such as dolphins, fish and whales.