Birds descended form (#33) ____ that ran around on two legs some 160 million years ago. All birds have (#34) ______ that insulate and help get the bird aloft. Generally, birds have a greatly enlarged (#35) ____ to which flight muscles are attached. Bird bones contain (#36) ____ _____, and air flows through sacs, not into and out of them, for gas exchange in the lungs. Birds have a large, durable (#37) ____ - _____ ______ that pumps oxygen -poor blood to the lungs, and oxygen-rich blood back to the heart and to the rest of its body, just as you do. Many birds display (#38) ____ behavior: a recurring pattern of relocation between two places in response to environmental rhythms. Birds also lay hard-shelled (#39)_______ eggs, have complex courtship behaviors, and generally nurture their offspring.
There are three groups of existing mammals: those that lay eggs (examples are the (#40) _______ and the spiny anteater), those that are (#41) ________ (examples are the opossum and the kangaroo), and those that are (#42) ______ mammals (there are more than 4,500 species of these). Mammals regulate their body temperature, have a (#43) _____-chambered heart, and show a high degree of parental nurture. Most mammals have (#44)______ as a means of insulation and mammalian mothers generally suckle their young with milk.
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This question was answered on: Sep 16, 2020
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