Flapping flight is very energy intensive. A wind tunnel test on an 89 g starling showed that the bird used 12 W of metabolic power to fly at 11 m/s.
A migrating starling flies steadily at 11 m/s for 1.0h, using energy from its fat stores. How many grams of fat does it burn? Consider that metabolizing 1 g of fat releases approximately 37 kJ of energy.