Flight muscle atrophy and predation risk in breeding birds

1. The speed with which small birds can get airborne is critical to the effectiveness of their escape response when attacked by a predator. However, take-off ability is likely to be affected by physiological changes occurring as a result of egg formation. 2. To investigate whether reduced take-off velocity is a cost of reproduction, the physiological costs of egg production in the Zebra Finch (Taeniopygia guttata) were experimentally manipulated by varying both the number of eggs a female laid and the quality of her prelaying diet. The effect of changes in postlaying flight muscle condition and body mass upon alarmed flight take-off velocity (a measure of escape-ability in birds) was subsequently measured. 3. Changes in muscle condition were found to correlate positively with changes in various measures of flight velocity: treatments that caused the greatest declines in muscle condition during egg-laying were associated with the greatest declines in flight performance over this period. In contrast, breeding attempts that caused the smallest declines in muscle condition were associated with improvements in flight performance (i.e. birds flew faster at the end of laying than at the start). 4. These effects were independent of changes in body mass, and occurred postlaying, suggesting that the cost of egg production lies primarily in the formation of the eggs, rather than in carrying them as other studies had suggested. The observed trade-off between muscle loss resulting from egg production and escape ability could have important implications for the evolution of optimal clutch size in birds.