Once an individual is unkind to a pigeon, their face is never forgotten.
Even chasing pigeons once can be shown to cause them to recall your face (Crows are known to react in a similar way, except they tend to form a gang and attack any human against whom they hold such a grudge).
Today’s invention is an anti-terrorist system, based on the above.
An accurate 3-D mask would be made of a known terrorist’s face, based on surveillance and press pictures.
This would then be worn by eg a security guard who would then scare away a set of say five birds. These birds would then be allowed to walk/fly around within eg airport buildings.
Each pigeon would be equipped with a small camera which would take a picture of anyone from whom the bird suddenly shied away and send this back for scrutiny by security staff.