Impaired flight ability--a cost of reproduction in female blue tits

When prey are attacked by predators, escape ability has an obvious influence on the probability of survival. Laboratory studies have suggested that flight performance of female birds might be affected by egg production. This is the first study of changes in take-off ability, and thus potentially in predation risk, during reproduction in wild birds. We trapped individual male and female blue tits repeatedly during the breeding season. Females were 14% heavier and flew 20% slower (probably as a consequence of a lower ratio of flight muscle to body mass) during the egg-laying period than after the eggs had hatched. However, flight muscle size did not change to compensate for changes in body mass over this period. In contrast, males showed no changes in either body mass, muscle size, or flight ability over the same period. Furthermore, the impairment of flight in females increased with the proportion of the clutch that had been laid, an effect that was independent of body mass and muscle size. This indicates that egg production causes additional physiological changes in the female body that produce impaired locomotor performance. We suggest that courtship feeding of female blue tits by their mates might reduce predation risk during the period when female take-off ability is impaired by reducing the time females have to spend foraging and thus reducing the time they are exposed to increased predation. Copyright 2002.