Electrocardiogram recordings in free-ranging gannets reveal minimum difference in heart rate during flapping versus gliding flight

Gliding flight is one of the major features that allows flying animals to cover extensive distances while minimising their energy expenditures. This has been supported by studies recording heart rate as a proxy for energy expended, but the exact amount of flapping and gliding during flight is often not taken into account, making a genuine assessment of the heart rate evolution with flight modes problematic. We used miniature accelerometers and electrocardiogram recorders attached externally to free-ranging Cape gannets Morus capensis to examine how heart rate varies when birds use gliding or flapping flight. Flapping phases (in beats per minute; 255.5 bpm) showed consistently higher heart rates than gliding phases (217.2 bpm), with the changes in heart rate at the onset of a new phase (flapping or gliding) being almost instantaneous, irrespective of the duration of the subsequent phase. Surprisingly though, the difference between the heart rates measured during flapping and gliding flights only amounted to about 20%. Such a small difference does not accord with the fact that gannets are known to have elevated flight costs. This discrepancy suggests that heart rate and metabolic rate are not correlated linearly in M. capensis. Cardio-vascular adjustments, such as a variable stroke volume (following Fick's law), might have evolved because local wind conditions and gannet foraging strategies are not always compatible with gliding flight.