The author tells us that a continual replacement of nerve cells is explained by the canary's relatively long life span and requirements of flight, suggesting its brain would be too large for flying if it had to carry all the brain cells needed to process and retain all the information gathered over a lifetime. (D) has no impact on this conclusion because even if take (D) to suggest that canaries that have larger than average repertoires of songs and therefore, have larger brain size, have better-developed muscles for flying, a repertoire developed over one season is still not comparable to a repertoire that could be developed over a lifetime, resulting in the brain size that is "substantially larger and heavier than feasible for flying" (lines 43-45) regardless of how developed the muscles are.
