Does that title capture any attention? Well, I've been looking at ways to sex adult Red-tailed Hawks by plumage since the late 1980's, and guess what....YOU CAN'T! I checked thousands of adult Red-tails from the East and West and came up with some differences, but they don't hold enough water to make even a half-definitive statement about sexing -- did I waste all that time or what? What I found I will share anyway for fun. One thing to remember, when you have a bird in-hand, size is evident, so if tail patterns match with the smallest or largest individuals, it lacks great significance anway, but if it's a bird in flight or a bird in a photo, it could be fun to notice tail patterns. 

I looked at the width of the sub-terminal tail band on adult Red-tails as well as the darkness to the red color and found that the smallest Red-tails (ones that take 7A bands and weighed under 900 gms healthy) tended to have broader sub-terminal bands and darker reddish tails than "large" Red-tails (weighing over 1200 gms and requiring 7D bands). The real issue (besides overlap in these traits) is that most Red-tails fall between these measurements and show a sub-terminal band intermediate in width! Another issue arises when comparing light-morph to dark-morph birds, birds from different geographic locations, or factors that could effect tail color (fading, etc.).

By the way, there is overlap in the sub-terminal tail band width of adult male and female Swainson's Hawks as well, so do not use that trait on its own to sex Swainson's Hawks, especially when sexing dark-morph birds! I should also mention that there is a paper on sexing Western Red-tailed Hawks using a combination of measurements authored by Kara Donohue that you can download here:

http://onlinelibrary.wiley.com/doi/10.1111/j.1557-9263.2006.00003.x/abstract

 Presumed male Eastern

 Presumed male Western

 Presumed female Eastern

  Presumed female Eastern