Usually directly feeding animals is looked down upon because decreasing their fear of humans tends to be a negative for their survival, and it might impact their ability to forage for their own food. With birds specifically, though, putting up feeders is more mixed. Migration takes an enormous amount of energy and and human sprawl has removed a lot of natural food sources. And especially in wintertime, food can be quite scarce for birds. But at the same time bird feeders can actually be big spreaders of disease and I know that there was guidance that people should take down feeders at the height of bird flu.

Now when it comes to mallards, they’re honestly a species that is incredibly urbanized already, so I don’t think directly feeding them is doing a great deal of harm.

depends on if you want them to like you more than you care about ecology

There are signs by me that say not to feed migratory species in winter I believe. I presume the food makes them stick around when they would otherwise migrate.

Peas were always an option.

I believe, the problem is mainly white bread, which is what people typically have in mind for feeding ducks.

As opposed to wholegrain, it only retains the endosperm, which is mostly just carbohydrates without many nutrients:

I think, the lack of fiber is also particularly problematic. At least, I’ve heard that it gives them diarrhea, which probably means their guts don’t have time to extract the few remaining nutrients.