Body Size

• Male roadrunners tend to be larger than females, so it is likely that the larger bird in any breeding pair you observe will be the male. Sources such as the Animal Diversity Web and Birding Information.com agree there are no obvious exterior markings on the birds to distinguish the sexes. Some birds show a bright patch of red and blue skin behind their eye, but this does not indicate a bird's sex. Males and females both develop these patches as they mature.
  
  

Courtship

• Male roadrunners pursue the females during courtship, to offer gifts of food such as lizards, spiders or small snakes. Other courtship behaviors that distinguish the male include the bird bowing, as well as wagging its erect tail from side to side. The male may also jump into the air and "clumsily hover," according to the Smithsonian's Zoogoer website.
  
  

Calls

• During the mating season of early spring, the male roadrunner produces a distinctive "whirring" call, quite different from the "cooing" and "clacking" noises both male and female birds make at other times.
  
  

Nesting

• Male and female roadrunners both take part in nest-building. The Smithsonian website notes the male spends more time collecting nest-building materials while the female occupies herself more with the actual construction. These behaviors can help observers establish a bird's sex. However, both males and females take turns sitting on the nest to incubate the eggs, so brooding behavior is not a reliable indicator of a bird's sex.