Figure 1.

The relationship between condition (e.g., mass) and fitness is generally assumed to be positive and linear (a). However, in cases of highly monogamous species with low rates of extra-pair paternity (b) and polygynous species (c), the relationship may be positive, but non-linear. In some circumstances, the relationship between condition and fitness could be more complicated than in a–c. For example, the trade-off between mass and predation risk may make it costly for birds to gain mass because it increases their chances of being caught by predators (d). The dotted line indicates the optimal fitness yield for condition.