Hummingbirds...

...can eat twelve times their body weight in nectar in a day. (Source 1)

The ruby-throated hummingbird, native to eastern
North America and Central America (Source 4).

Maybe they eat so much because of the high metabolic rate they have, which is necessary for the large amount of energy they spend to hover, since hummingbirds flap their wings 12-80 times/second, depending on the species.

More on their eating habits:

"Like the similar nectar-feeding sunbirds and unlike other birds, hummingbirds drink by using protrusible grooved or trough-like tongues.^[5] Hummingbirds do not spend all day flying, as the energy cost would be prohibitive; the majority of their activity consists simply of sitting or perching. Hummingbirds feed in many small meals, consuming many small invertebrates and up to twelve times their own body weight in nectar each day. They spend an average of 10–15% of their time feeding and 75–80% sitting and digesting."  (Source 1)

Video of hummingbird hovering, slowed down (Source 2).

Scientists did a study to find out how hummingbirds can feed on nectar during a heavy rainstorm.  In this video clip, it shows how the hummingbird deals with more and more rainfall.  They found that the bird changes the position of its body, so it's almost horizontal, which helped to diminish the impact of the raindrops on the wings (Source 3). VIDEO TIP: click on the small movie to enlarge.

The violet sabrewing hummingbird, native to Mexico and Central America (Source 5).

Bibliography
1. "Hummingbird" Wikipedia article.  Wikipedia, 2012.  WWW: http://en.wikipedia.org/wiki/Hummingbird.

2. "Time Warp: Hummingbird" YouTube video, uploaded by DiscoveryNetworks, 2009. WWW: http://www.youtube.com/watch?v=D8vjYTXgIJw.

3.  "Video: How Hummingbirds Weather the Storm" by Nicholas Fleur, 2012.  American Association for the Advancement of Science (AAAS) magazine.  WWW: http://news.sciencemag.org/sciencenow/2012/07/video-how-hummingbirds-weather-t.html?ref=hp.

4. "Ruby-throated Hummingbirds Return" blog post by Mary Holland, 2011.  WWW: http://naturallycuriouswithmaryholland.wordpress.com/2011/05/07/ruby-throated-hummingbirds-return/.

5. "Violet Sabrewing Hummingbird" photograph in a blog post in Flight Log by Corbett, 2009.  WWW: http://jrscience.wcp.muohio.edu/costa_rica_animals_04/monteverde/violet_sabrewing_male.jpg.