Last updated: January 31, 2018
Many of our friends in the animal kingdom display something called "sexual dimorphism," which is simply a difference in size or physical appearance between the male and female. Birds are a great representation of that: for a lot of species, males tend to be vibrantly colored for attracting females. For the peregrine falcon, and most raptors, the physical difference is overall size. This is especially pronounced in the peregrine, as the female of the species is roughly 1/3 larger than the male. Because of this, the male is often referred to as the "tiercel," while the females are referred to as falcons.
It can be a little difficult to discern the falcon from the tiercel by size difference alone, if only one peregrine is in view. Luckily, because of the size difference, the male and female have different wingbeat styles. The female takes longer, graceful wingbeats; while the male must beat his wings a bit faster. It still takes somewhat of a trained eye to notice this difference.
So you can get an idea of the size difference, this is a photo of our peregrine pair from 2015.
NPS Photo/Don Endicott,Patricia Simpson - Two peregrines, left is female with larger wingspan, right is male with shorter wingspan.