In these days of ever-present GPS and easy access to route-planning apps, taking the long way can feel like a waste of time. Efficiency, in terms of both time and energy expenditure, especially on long trips, is often a high priority. It is therefore somewhat surprising that when bar-headed geese (Anser indicus) make their remarkable twice-yearly migration across the Himalayan mountain range, they take the long route – ascending and descending along mountain valleys instead of flying in a straight path at high elevations. This somewhat counterintuitive route was reported back in 2013 by Lucy Hawkes and colleagues (Hawkes et al., 2013, Proc. R. Soc. B 280, 20122114). At the time, the researchers hypothesized that the geese kept close to the ground because the costs of continuous flight at high elevations, where low oxygen levels increases the effort needed to maintain movement, were too high.
This hypothesis was recently put to the test by a large international team of scientists, led by Pat Butler of the University of Birmingham and Charles Bishop of Bangor University, UK. These researchers have been studying the energetics and biomechanics of bar-headed geese for a number of years in the hope of shedding light on how the birds accomplish their remarkable migration. In the past, the team has used laboratory-based experiments on the bar-headed geese and other birds to be able to obtain reliable estimates for metabolic rate (an approximation of energy expenditure calculated using heart rate) and metabolic power (how much work is needed to perform a task estimated using wingbeat frequency). Thus, the team had access to most of the information needed to estimate the costs of the migration; all that was needed were direct measurements of these values from the geese during migration.
Using sophisticated data loggers – that provided not only location and elevation but also core body temperature, heart rate, pressure, acceleration and wingbeat frequency – the team successfully obtained data from seven birds over 391 h of flight during migration. Similar to the previous study, the tagged birds took the long route, up and down along mountain valleys, and generally remained close (within ∼60 m) to the ground throughout the migration. This path added nearly 112 km to the total distance travelled, when compared with a straight line flight at high elevation, but was by far the less costly of the two routes. From their previous work, the team knew that the lower density air at high elevations causes an increase in metabolic power, similar to what humans experience when exercising at high altitude. They had also determined that for continuous flight at a single elevation there is a high correlation between metabolic power and metabolic rate. Therefore, remaining at high elevation throughout migration would have cost the birds more than ascending the odd mountain peak. Thus, the birds’ ‘roller coaster’ path up and down the peaks and valleys of the Himalayas was actually the path of least resistance. Perhaps even more surprising was that, overall, the heart rate of the birds remained at relatively low levels, showing just how well adapted these birds are to flight under what for most species would be challenging conditions.
