Sexual dimorphism is the term that is used to describe differences between males and females of the same species. Sexual dimorphism can manifest itself in several different ways. There can be differences in size, color, shape behavior or more. Reverse sexual dimorphism is when a species exhibits differences that are opposite of what we as humans consider the norm.
In the world of raptors the female of the species is larger then her male counterpart. Since in our world it is the male that is typically larger this is an example of reverse sexual dimorphism. It is hard to tell the difference in size from the pictures above but the female in the lower picture is larger. The merlin falcon, pictured above, is one of a small number of raptors that shows a form of sexual dimorphism. In the case of most raptors males and females have the same coloration. However in some of the smaller species the male is much more colorful then the female. This is not an uncommon trait with many different passerines (song birds) and ducks. The reason for this is because the female needs to have some camouflage when she is sitting on the nest incubating eggs. Since the males job is not to incubate eggs but bring the female food he does not need the same camouflage. In larger raptors it is not necessary because there are few things that would mess with the nest of an eagle, red tail, great horned owl, or other larger raptor, but these smaller raptor, like the merlin, need to take care so that they do not become a meal for one of the larger raptors.