Natural barriers: waterfall transit by small flying animals

Area of Science:

• Biomechanics
• Animal Flight
• Fluid Dynamics

Background:

• Waterfalls are complex geomorphological features with multiphase flows.
• Larger birds like swifts can navigate waterfalls, but smaller fliers face unknown barriers.
• The physical constraints and kinematic responses of volant animals transiting waterfalls are not well understood.

Purpose of the Study:

• To investigate the flight behavior of hummingbirds and insects passing through an artificial sheet waterfall.
• To determine the physical constraints and kinematic adaptations required for waterfall penetration by smaller volant animals.
• To assess the role of momentum in overcoming water surface tension and flow dynamics.

Main Methods:

• Observed flight behavior of hummingbirds and insect taxa through a sheet waterfall.
• Launched plastic balls at varying speeds to analyze inertial dependence of penetration.
• Measured kinematic responses including speed, stroke amplitude, body orientation, and tail adjustments.

Main Results:

• Hummingbirds successfully penetrated the waterfall, reducing speed and stroke amplitude, increasing body tilt, rotations, and tail spread.
• Smaller plastic balls and some insects (flies > 1.6 m/s) penetrated the waterfall by overcoming surface tension and water momentum.
• Objects with lower momentum were impeded by the water sheet and fell with the flow.

Conclusions:

• Waterfalls act as significant physical barriers for small, slow-moving flying animals.
• Specific kinematic adjustments enable hummingbirds and certain insects to traverse waterfalls.
• Waterfalls may function as ecological barriers, excluding predators and parasites from avian nests.