3D coordinates of the foraging flights of wild black skimmers

Birds commonly exploit environmental features such as columns of rising
air and vertical windspeed gradients to lower the cost of flight. These
environmental subsidies may be especially important for birds that forage
via continuous flight, as seen in black skimmers. These birds forage
through a unique behavior, called skimming, where they fly above the water
surface with their mandible lowered into the water, catching fish on
contact. Thus, their foraging flight incurs the costs of moving through
both air and water. Prior studies of black skimmer flight behavior have
focused on reductions in flight cost due to ground effect but ignored
potential beneficial interactions with the surrounding air. We
hypothesized a halfpipe skimming strategy for skimmers to reduce the
foraging cost by taking advantage of the wind gradient, where the skimmers
perform a wind gradient energy extraction maneuver at the end of a
skimming bout through a foraging patch. Using video recordings, wind
speed, and wind direction measurements we recorded 70 bird tracks over
four days at two field sites on the North Carolina coast. We found that
while ascending the skimmers flew more upwind and then flew more downwind
when descending, a pattern consistent with harvesting energy from the wind
gradient. The strength of the wind gradient and the flight behavior of the
skimmers indicate that the halfpipe skimming strategy could reduce
foraging costs by up to 2.5%.