These redtails can become large and live quite long, tbh. The grey belly can be tricky to use as a reference and so does the size. For not all specimens will grow up the same max size wether that concerns a female or male. And when an adult male becomes excessive in size, the black on his belly will become grey as well. and when both sexes will become excessive large, the black on their whole body will become less black and becomes more a faded black.Females appear a bit larger than males and females can have a more grey belly rather than a more black colored belly.