Department of Biology, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA.
J Exp Biol. 2024 Aug 15;227(16). doi: 10.1242/jeb.246855. Epub 2024 Aug 22.
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 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 4 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 flight behavior of the skimmers indicate that the halfpipe skimming strategy could reduce foraging cost by up to 2.5%.
鸟类通常利用环境特征，如上升气流和垂直风速梯度，来降低飞行成本。这些环境补贴对于那些通过连续飞行觅食的鸟类可能尤为重要，例如黑燕鸥。这些鸟类通过一种独特的行为觅食，称为掠食，它们在水面上方飞行，将下颚降低到水中，触水捕鱼。因此，它们的觅食飞行在空气中和水中都会产生成本。先前对黑燕鸥飞行行为的研究主要集中在由于地面效应而降低飞行成本上，但忽略了与周围空气的潜在有益相互作用。我们假设黑燕鸥采用半管式掠食策略，通过利用风速梯度来降低觅食成本，即在掠食过程结束时，黑燕鸥在觅食斑块处进行风速梯度能量提取机动。我们使用视频记录、风速和风向测量，在北卡罗来纳州海岸的两个野外地点记录了 70 只鸟的轨迹，为期 4 天。我们发现，在上升过程中，燕鸥会更多地迎风飞行，然后在下降时更多地顺风飞行，这一模式与从风速梯度中收获能量一致。风速梯度的强度和燕鸥的飞行行为表明，半管式掠食策略可以降低 2.5%的觅食成本。
