c. Costs of migration

The benefits of migration come at a cost - the cost of a journey that may cover tens of thousands of miles and cross the Earth's most challenging environments. From a 1600km crossing of the Sahara, to flying over the Himalayas, to crossing the Atlantic: there are no geographical barriers that are not regularly crossed by migrants³. Neither is distance is an object: Arctic terns (Sterna paradisaea) may cover up to 50,000km per year, a distance greater than the length of the equator (about 40,000km). With a life expectancy of at least 25 years, these terns could theoretically reach a lifetime mileage of over 1,000,000 km - about three times the distance from Earth to the moon⁵!

Flying such long distances requires a huge amount of energy: a bar-tailed godwit on its annual migration to Africa will use around 48% of the total energy it needs in a whole year. Godwits wintering in Europe use only 22% on their migration journey². However, as we have seen in the last section, some of this cost is recouped: the energy expended by bar-tailed godwits wintering in Africa is half that expended by those wintering in Europe.

The extreme costs of migration and the very fact that such awe-inspiring journeys are undertaken are perhaps the best proof of its benefits. Migration could not have evolved if the benefits did not at least balance the costs. However, the costs and benefits differ between individuals and populations: birds of the same species but different geographic populations are likely to migrate to different places, whilst birds of the same species but of different sex, age or origin may adopt equally varying migratory strategies. For instance, some populations of Blackcap (Sylvia atricapilla) migrate annually, some are predominantly resident, whilst of the others, some individuals migrate and some remain.

See the next section, HOW, to discover how birds minimize the costs of migration.