Why are female Harpies bigger than Males?

Male and Female Peregrine falcons (note size difference) http://rfalconcam.com/forum/pix/HowTo/FalconIdentification/ArcherBeautySize.jpg

            After my organism presentation on Harpy Eagles, Emma asked me why there is such a large size difference between male and female Harpy Eagles. Apparently, this phenomenon is common across many species of predatory birds, including hawks, eagles, falcons, owls, and skuas. However, the males of most non-predatory bird species are generally larger than the females. Among predatory birds, the species that exhibit the most sexual dimorphism in size generally feed on birds. Predatory birds with relatively little sexual size dimorphism generally feed on invertebrates or reptiles. Raptors that feed on mammals fall in the middle of the spectrum. There are two main theories why this sexual dimorphism occurs: to avoid inter-gender competition or to maximize the foraging ability of a breeding pair of raptors.

            The theory that raptor dimorphism occurs to minimize competition for prey between males and females of a given species is based on the idea that the males and females would otherwise be competing over a scarce food source. Having males be smaller than females allows the males to pursue smaller or more agile prey than females, while the female can pursue larger organisms. In effect, size differences partition the available food between males and females to maximize the survival of each and the species as a whole.

            The second theory suggests that size differences between males and females allows to forage for a wider variety of prey, which maximizes the amount of food they can provide for their offspring. As with the previous theory, if the male and female are different sizes, they can pursue different prey. Thus, there is a wider variety prey available as food for the pair’s young. This theory explains why predatory birds that prey on smaller birds demonstrate the most sexual dimorphism in size. During the late fledgling period, avian prey populations are at a lower density than mammal, reptile, or invertebrate species, so there is a larger evolutionary pressure on raptors that feed on birds to diversify their foraging abilities.

Sources:

Témeles, Ethan J. "Sexual size dimorphism of bird-eating hawks: the effect of prey vulnerability." American Naturalist (1985): 485-499.