The face of the Merlin is less strongly patterned (incl. faint dark malar stripe) than in most other falcons. 
Females are usually heavier and bigger than males. 
Such sexual dimorphism is common among raptors; it allows males and females to hunt different prey animals and decreases the territory size needed to feed a mated pair.

El Dorado Lodge area, Sierra Nevada de Santa Marta, Colombia, 31 October 2014