How birds hear

The ears are connected by an air-filled chamber below the brain case. The most sensitive region of the birds’ hearing is in the 1 to 3 kilohertz range — also the peak acoustic noise output of a modern jet engine. The intense jet noise may overstimulate the bird’s inner ear sensory receptors, interfering with a bird's ability to hear, and leaving them unaware they may be flying directly into an aircraft.