Hummingbirds need large amounts of energy to flap their wings between 60 and 200 times per second. Their wings beat so rapidly that it is difficult to see them move. They often appear suspended in air for extended periods of time without changing their location. Hummingbirds have long bills and grooved tongues to reach into flowers to feed on flower nectar. They also feed on insects. The body system most directly interacting with the skeletal system to enable hummingbirds to beat their wings between 60 and 200 times per second is the-

F.
muscular system.

G.
integumentary system.

H.
endocrine system.

J.
circulatory system.