How did the skeleton change during bird evolution?

(answer with just 1 or 2 sentences please)

reduction in birds skeleton

Explanation:

The reduction in a bird's skeletal weight is largely a result of hollow construction of the skeleton. The fusion or loss of individual bones reduces the number of movable joints in the bird's skeleton, making it rigid and better adapted as the internal framework of a flying machine.

what statement best describes the difference between the sympathetic and parasympathetic nervous systems?

the sympathetic nervous system prepares the body for intense physical activity and is often referred to as the fight-or-flight response. the parasympathetic nervous system has almost the exact opposite effect and relaxes the body and inhibits or slows many high energy functions.

not necessarily. higher fitness requires the organism to be reproductive successful. for example, an organism at age 10 with 15 offspring is much more fit than an organism at age 15 with only 5 offspring.