Final answer:
To discern whether male birds' long tail feathers function in intersexual or intrasexual selection, one should observe mating behaviors and competitions. The handicap principle can explain female choice in traits that may hinder survival but indicate male fitness. Courtship behaviors, like bowerbird structure building, also reflect mechanisms of female mating preference.
Step-by-step explanation:
When observing a male bird with exaggerated secondary sexual characteristics, such as really long tail feathers, clues that can distinguish whether these traits function in intersexual or intrasexual selection are the behaviors displayed by the bird and the responses of other birds. For intersexual selection, you might observe females appearing attracted to the trait, such as when female peacocks choose a male with the brightest plumage. On the other hand, intrasexual selection may be indicated by male to male competitions or displays of dominance.
In terms of female choice influencing these traits, different mechanisms can play a role. The handicap principle suggests that females might prefer males with exaggerated traits because they indicate the ability to thrive despite survival disadvantages. This might be at work if the male displays traits that seem to hinder its survival – yet it still appears healthy or dominant. Alternatively, the male could be engaging in a form of courtship behavior, like the male bowerbird that builds elaborate structures, as a direct way to appeal to female mating preferences.