Many birds have evolved as migratory species. This means they spend the majority of their lifespan in flight, traveling long distances when they are not in their breeding period. One such group are Old World swifts which spend up to 6-10 months out of the year airborne. With this knowledge, researchers recently set out to determine if a close relative to Old World swifts, the Black Swift (pictured above), shares similar flight behavior.
Over a period of two years, Hendstrom and colleagues collected data on the flight patterns of seven Black Swifts as they went about their migrations. They found that during the 8 months of the Black Swift’s non-breeding period, they spent over 99% of it airborne. This was coupled with occasional short periods of inactivity, none of which lasted more than 12 hours. This means the Black Swifts must sleep, eat, and notably hunt and forage while airborne. In addition, the Black Swifts flapped their wings at a higher rate during the night compared to the day, indicating that they are likely nocturnal creatures. This presents an issue for the Black Swift as the darkness of night makes it more challenging to spot the small insects they hunt for food, or potential predators.
Hendstrom and colleagues found the Black Swifts adapted a creative solution to this problem. The birds exhibited ascents to high altitudes that correlated with the available light levels of the moon. This means that when the moon shone brightly the birds flew higher in the sky. This increase in altitude reached its peak at the full moon, when the moon is at its brightest. The correlation between altitude and moonlight levels was even further confirmed during a lunar eclipse, when the birds’ flight altitudes mimicked that of daytime. These flight patterns are demonstrated in the graphic below. As shown in this graphic, the birds flew over twice as high when the moon was shining when compared to their flight during daylight. In addition, during a lunar eclipse (farthest panel to the right) the birds’ flight was similar to that of daytime.
Although the researchers could not confirm that the light of the moon specifically was leading to this change, this is the first documentation of moon-based changes in altitude. Henstrom proposes two possible explanations for this behavior. The first is that edible insects are present at this high altitude and only become visible to the swifts when the moon is bright enough. Another explanation is that the swifts may be more visible to nocturnal predators when the moon shines brightly, and thus the swifts fly higher to avoid such predators. Regardless of the reason, this discovery opens the door to a wealth of potential research on Black Swift flight and foraging behavior. As technology continues to evolve, more sophisticated research on flying species can be produced, paving the way for an exciting new future for the field of aerial sensory ecology.
