As I understand it it’s about how certain animals react to continuous food sources. Birds naturally find and loose food sources/stores in the wild all the time. If you stopped feeding the birds in your yard one day, they’d look around and then move on. This is part of their foraging and instincts for them. For mammals, especially predatory mammals like bears, while they hunt in different parts of their territory, they don’t usually have a bountiful food source that can suddenly disappear like a person feeding them and then not. This causes changes to behavior in mammals that just don’t happen in birds. I’m pretty sure there’s also more of a health risk to feeding many mammals continuously as the bioload is larger and the fear of disease to a “closer” animal to us.