Here's how...their overall plumage is variably barred or speckled with thin, black, horizontal bars or spots. Females and juveniles are more heavily marked than males - adult males may be almost pure white, although they have up to three tail bands. Adult females are distinctly barred throughout, and have from four to six tail bands. Immatures are very heavily barred throughout, and dark spotting may codominate or dominate the overall plumage. Intensity of dark spotting varies with the sex of the immatures, females being the darkest. Juveniles are uniformly brown with scattered white tips of down.
So, do you have it figured out? OK, I'll help you. Female on left, male on right.
Photo Credits: Cindy Palmatier & Dave Dorsey / Bird TLC