Ontogeny of flight capacity and pectoralis function in a precocial ground bird (alectoris chukar)

Flight is the defining characteristic of birds, yet the mechanisms through which flight ability develops are only beginning to be understood. Wing-assisted incline running (WAIR) and controlled flapping descent (CFD) are behaviors that may offer significant adaptive benefits to developing birds. Recent research into these forms of locomotion has focused on species with precocial development, with a particularly rich data set from chukar partridge (Alectoris chukar). Here we briefly review the kinematics and aerodynamics of flight development in this species. We then present novel measurements of the development of pectoralis contractile behavior during the ontogenetic transition toward powered flight. To obtain these new empirical data, we used indwelling electromyography (EMG) and sonomicrometry and tested WAIR and CFD in seven age classes of chukar (2-4 birds per age) from 5 days post hatching (dph) to adult (300dph). For each age class, we measured muscle activity during maximal performance, which was WAIR at 65 in birds 5 dph, CFD in birds 9 dph, WAIR at 80 in birds 14 dph, level flight in birds 25-61 dph, and ascending flight in adults. We also measured muscle activity during sub-maximal performance in all age classes. Flapping chukar chicks use near-continuous activation of their pectoralis at relatively low electromyography amplitudes for the first 8 days and progress to stereotypic higher-amplitude activation bursts by Day 12. The pectoralis undergoes increasing strain at higher strain rates with age, and length trajectory becomes more asymmetrical with greater variation in contractile velocity within the shortening phase of individual contractions. At 20-25 days (12-15% adult chukar mass), pectoralis activity and locomotor performance approaches that of adults, although strain rate exhibits a temporary decrease at 61 dph concurrent with using newly-replaced primary feathers. To better understand how these patterns relate to the evolution of life-history strategy and locomotion, we encourage future efforts to explore these behaviors in altricial and semialtricial bird species.