A9 Collision

Topic

A bird in flight can collide with a solid surface—such as a glass window, a wind turbine, a power line, an aircraft, or an urban structure—and among the scenarios considered by this set of models, this type of collision is the most significant for applied conservation: collisions with windows alone cause hundreds of millions of bird deaths annually in North America. The physics governing this event involves impulse and linear momentum: the momentum the bird possesses just before impact is what ultimately determines the extent of the damage caused by the collision.

The total impulse transferred from the solid surface to the bird during the collision depends directly on the bird's mass and its flight speed at the moment of impact; a heavier or faster-flying bird carries more momentum that must be abruptly halted, and thus it experiences a greater total impulse when that momentum is stopped against the solid surface.

However, that same total impulse can translate into vastly different peak forces depending on the duration of contact between the bird and the solid surface: for a given total impulse, the shorter the contact duration, the higher the peak force the bird experiences, because the momentum is brought to a halt in less time. Upon impact with glass, the contact duration is very brief and the resulting peak force is very high; conversely, upon impact with vegetation, the contact duration is longer, and the momentum is decelerated more gradually, resulting in a much lower peak force. That peak force determines the mechanical stress experienced by the bird's skull and skeleton at the moment of collision: the higher the peak force, the greater the mechanical stress the bone must withstand. A collision with a surface involving a very short contact duration—such as glass—concentrates mechanical stress on the skull and skeleton in a way that a collision involving the same momentum but a longer contact duration never would.

In addition to total impulse and peak force, the collision involves kinetic energy—derived from the bird's mass and flight speed—which must be completely dissipated at the moment of impact. The fraction of this kinetic energy dissipated as energy within the bird's tissue (rather than through the deformation of the solid surface it strikes) determines the actual tissue damage sustained, over and above the immediate mechanical stress experienced by the skull or skeleton.

Based on the energy dissipated in the tissue and the mechanical stress on the skull and skeleton, the model calculates the probability of mortality for a given collision as a function of the bird's kinetic energy and mass. It allows for the comparison of risk across different species—based on their mass and typical flight speed—and across different types of structures—based on the contact duration each imposes. This transforms the collision event—described simply through the physics of impulse, linear momentum, and kinetic energy—into a direct tool for determining which structures pose the greatest actual risk to specific species.

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