Many people like to go and feed the ducks at the ponds and lakes near where I live. It's lovely that they want to interact with and appreciate these wild animals. Unfortunately, they nearly always feed bread. I imagine that they are unaware it can in fact cause more harm than good throwing bread in these situations, particularly when in large quantities. It is unhealthy for the birds to eat, as well as the fact that leftover bread encourages rats and algal blooms. (I'll avoid a long discussion here as it's not crucial to the question, but if you are interested, there's more detail about the reasons in this article.)
I tend to take bird seed or similar to throw to the ducks. I'd like to encourage others to do the same, but would feed awkward about doing so. How can I politely convince people whom I see feeding the ducks to bring more suitable alternatives in the future?
I want to make sure that I do not come across as negative or confrontational - as I said, the motives behind feeding the ducks are good, and besides, if I come across as too evangelical I'm sure it is less likely to be effective in persuading anyone.
Note that some local spots now carry signs asking people not to throw bread, but they are often not very obvious, or else people ignore them.