FIGURE 2.

The metabolic cost of high-altitude flight can be substantial, as reflected by heart rates logged during the natural migration

A: altitudes and heart rates of an individual bar-headed goose flying across the Tibetan plateau, showing the changes in heart rate during descent and ascent. B: the average heart rates exhibited during flight increase with elevation. C: environmental assistance (e.g., uplifting winds) can lessen the heart rates and presumably the metabolic costs of climbing flight when available. An individual goose is shown as an example, with blue lines indicating sequential data points (numbered in minutes) for an event of assisted lift that lies outside the typical relationship between ascent rate and heart rate (red color intensity reflects the overall density of observations). Image modified and reproduced from Ref. 4 with permission.