A) They cannot walk
Their bones are too thin, and they have tiny little feet that are only good for perching, scratching or preening. This means if they wish to move, even a few inches, they must fly. Hummingbirds can fly up, down, right, left, backwards, upside down and even loop-de-loops! They will not generally nest in nesting boxes and commonly make their nests from plant fibers, plant down, moss and spider webs.
Sources: hightechscience.com and epinions.com
Source: http://www.pdphoto.org/PictureDetail.php?mat=pdef&pg=7917 (Wikipedia)