I found this on the Humane Society website:

"People" food is not formulated for animal consumption. Foods manufactured for humans are rarely nutritionally adequate for animals and may cause serious health problems. This is particularly the case when animals gather to feed continually on human handouts such as bread, french fries, and popcorn

When animals learn that humans can provide a cheap and easy food source, they often lose their natural fear of humans. The HSUS is particularly concerned about feeding that habituates large, potentially dangerous animals to human habitation and presence. Once wild animals learn that they can boldly forage for human food, conflicts, nuisance behavior, and risks to human safety are sure to occur.

Wildlife fed from or near vehicles may result in traffic hazards and costly property damage when animals are hit or attempt to enter vehicles in search of food. For example, in Yosemite National Park in 1998, over 1,100 vehicles were broken into by black bears—causing more than $630,000 in damages.

Animals who become dependent on human food sources may gather in abnormally large numbers, thereby aiding the spread of disease. Unnatural crowding and competition may result when wild animals gather together for food handouts from people. Such stressful conditions increase the incidence of fighting and injury among animals, as well as help the spread of diseases, some of which may be transmitted to pets and humans.

Does this mean backyard bird and squirrel feeding is wrong? No. The HSUS opposes the feeding of wildlife when the reasonable assumption can be made that animals may come to harm. While feeding birds and squirrels in your backyard is generally not an activity that fits into this category, some conditions may warrant curtailing the provision of such food sources. For example, during the warmer months, when natural food sources are more readily available, it's usually best to reduce the amount of feed you put out each day or suspend feeding altogether.

If you do provide feed for backyard wildlife—at any time of the year—remember that it's also important to maintain safe, clean feeding stations in order to prevent the spread of disease.

More Tips

If you feed your pets outside, take the bowls in at night to prevent midnight raids by your wild neighbors.
For hints on the safe disposal of food containers, check out our "Don't Trash Wildlife!" page below.
Don't touch wild animals. For information on identifying orphaned and injured wildlife, read "Found an Injured or Orphaned Animal?"
When visiting parks and similar settings, follow the rules regarding wildlife. Encourage others to do the same.

For more info:
http://www.hsus.org/wildlife/urban_w..._wildlife.html

Also you can do a google search on "What NOT to feed wild birds?" I found alot!