Wow Madeleine, that's a BIG question! I don't really have an answer to that one! I know they are helpful in nature. They eat nectar from flowers and when they go from flower to flower the spread the pollen from one flower to the next. It's called "cross pollination" and it is a very necessary part of plant reproduction.
PowerKnowledge Life Science: Look under Animals (login info is on your password sheet)
Encyclopedia Britannica - Elementary:
http://school.eb.com/levels/elementary/article/353273
Ontario Hummingbird Project
http://www.ontariohummingbirds.ca/migrationrange_maps.php
Canadian Wildlife Federation - Hummingbirds
http://cwf-fcf.org/en/discover-wildlife/flora-fauna/fauna/birds/hummingbirds.html
World of Hummingbirds:
http://www.worldofhummingbirds.com/facts.php
Why do you think we have hummingbirds? If you find any cool information about hummingbirds - let me know!!
Have a great day,
Mrs. M