Birds face high energy demands during their flight. Some species alternate between flapping and gliding, which should allow them to save energy. We equipped hand-raised Northern Bald Ibises (Geronticus eremita) with data loggers during human-guided migration. We monitored the position of the birds, wingbeats, overall dynamic body acceleration (ODBA), and heart rates as a proxy for energy expenditure. The energy expenditure was significantly affected by the length of flapping and gliding bouts. At a gliding proportion of about 20%, we measured a maximum of 11% saving based on heart rate measurement. This study provides empirical evidence that intermittent flight is energetically beneficial and can reduce the high costs of flights.

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