A survey of the science:
Decades of studies with frosted lenses, magnetic coils or scent deprivation show they use pretty much every clue available. The most difficult one for us to comprehend may be the earth’s magnetic field.
Birds see it, but what it looks like to them, nobody knows. Work by Roswitha and Wolfgang Wiltschko in Germany, among others, suggests that this sense relies on quantum mechanics—that is, birds detect something happening in the eye at a subatomic level. Light striking the retina seems to stimulate chemical reactions that produce pairs of molecules with electrons that are "entangled," meaning they share certain quantum properties. One of those properties, called "spin," is affected by a magnetic field. That effect could tell the bird which way is north.
[E]ach dove carries on its body a permission document that allows a Saudi woman to travel. Notwithstanding their circumstances, all Saudi women are required to have this document, issued by their appointed male guardian.