Development of wing-flapping and flight in normal and flap-deprived domestic chicks.

Lateral flight evoked by dropping appeared 7-9 days after hatching. Drop-evoked bilaterally symmetrical wing extension and slow, low-amplitude wing-flapping were present by Day 1. Flapping rate measured using strobophotography increased up to approximately 13 days. Normal wing-flapping rates and lateral flight distances were achieved by chicks whose wings were immobilized with elastic bandages from Day 1 until immediately before testing at 13 days, indicating that wing movement is not necessary for the postnatal development of basic wing-flapping and flight. The ratio of wing area to body weight, a morphological index of wing-lift efficiency, rapidly increased up to 13 days and slowly declined through 49 days. The peaking of this ratio at 13 days corresponds to the age at which lateral flight is well established and wing-flapping rate is at its maximum. Thus, the development of wing morphology, wing-flapping rate, and flight are strongly and positively correlated.