A new study from Lund University, published in PLOS Biology, has found that the European nightjar's wings are a compromise between efficient long-distance migration and agile insect hunting, and that the bird generates thrust during its upward wingbeat, a mechanism previously observed primarily in bats and insects.

This discovery offers new insights into avian flight mechanics, challenging the idea that long-distance migratory birds are perfectly optimized flying machines. The findings could inform future flying technologies by showing how different demands on flight performance can be balanced within a single design.

Compromise in Wing Design

Researchers at Lund University studied European nightjars (Caprimulgus europaeus), nocturnal migratory birds that travel thousands of kilometers between Europe and southern Africa annually. The team used a wind tunnel to observe nightjars flying at various speeds and measured the airflow behind them. The results indicate that the nightjar's broad wing tips, while beneficial for slow, agile flight needed for hunting insects, reduce the efficiency of lift production at higher speeds. This suggests that the bird's wings are a careful balance of competing demands rather than an optimized design for a single purpose. Christoffer Johansson, an associate professor at Lund University, noted that the nightjar's wings appear to be a compromise between different requirements, allowing the bird to fly slowly and maneuver while hunting, even at the cost of reduced efficiency during faster, long-distance flight.

Upward Thrust Discovery

During their wind tunnel observations, the researchers also made an unexpected discovery: the nightjar generates thrust even during the upward phase of its wingbeat. This active upstroke has previously been predominantly associated with bats and insects, not birds. Johansson remarked on the commonality of this aerodynamic solution across different animal groups, suggesting that the upstroke can actively contribute to thrust generation, particularly when wings cannot fold significantly. The study's findings provide a new detail about how these animals move through the air. The research involved training five wild-caught European Nightjars to fly in a recirculating wind tunnel, with detailed measurements taken from three birds that demonstrated steady flights across speeds ranging from 2.6 to 11 meters per second. High-speed infrared cameras recorded wing movement, while laser-based particle imaging mapped the wake left by each bird.

What we could not confirm

This study did not specify whether other bird species also exhibit active upstroke thrust, or how widespread this wing design compromise is among migratory birds with diverse hunting strategies.