Costs of migration in free-flying songbirds

Billions of songbirds migrate between continents twice each year, but the energy costs of this feat have never been measured for free-flying individuals. Here we follow New World Catharus thrushes during their nocturnal migratory flights1,2, recapturing individuals after journeys of up to 600 km and determining their energy expenditure by using doubly labelled water3,4. Although flight itself is costly, our results confirm the counterintuitive prediction that songbirds expend double the amount of energy during stopovers that they spend on flight over their entire migration5,6.