In behavioral biology there is a fair amount of attention to individual quality, which may be determined by genes or parasite load or energy balance, or some interaction among these (and other) factors. Individual quality is honestly indicated by some trait or behavior; a large bright thing hanging of your head, a long bout of complex and energetic dancing, or a very loud complicated song, may be impossible to achieve in an individual with insufficient energy or some sort of disease. Therefore, other individuals looking to choose a mate can observe the traits or behaviors and do what the old guy in the cave said: “Choose wisely.”
Here is one of the nicest demonstrations of the relationship between parasite load and reproduction that I’ve seen in a while. And, as is so often the case, we gain valuable knowledge by closely observing great tits.