I have a couple bird feeders at the preserve, but I realize when I feed them it is for our benefit in viewing them, not strictly for their benefit. And I understand there are certain risks to feeding them: it's not that they will become dependent on me but rather that gathering them together at a high density to feed can cause the spread of disease—so feeders must be kept clean.
I don't recommend feeding deer or other mammals. They too are susceptible to diseases that spread at higher population densities, so feeding them could harm the species you intended to help. And there are often other consequences as well: trampled vegetation, artificially high populations of the species and perhaps their parasites, diseases, and predators, and species invasions due to the disturbance.
For example, a neighbor of the preserve has put salt blocks out in his backyard at the base of the steep hillside of the preserve. Over the last few years we have found that this otherwise undisturbed oak forest has developed soil erosion and a change of flora in the understory. Instead of oak seedlings and Ericacious plants (blueberry, huckleberry, deerberry, and pinxter azalea) the understory has become filled with signs of disturbance: pokeweed, wineberry, and mile-a-minute. I think the intense grazing of deer on the steep hillside has created the conditions that have fostered the vegetation changes. This was not an intended outcome of feeding deer, but everything is closely related.
The Game Commission prohibits feeding bear, even if unintentionally through having bird feeders that they will get into (people upstate only feed birds when the bears are hibernating). And some municipalities prohibit feeding deer precisely because they are grappling with the consequences of habitat change and high deer populations: increases in human Lyme disease, a complex of the populations of black-legged tick, white-footed mouse, white-tailed deer, and Borrelia burgdorferi , the spirochete that causes Lyme disease.
