An American Black Duck (top) and a male Mallard CC 3.0 / GNU 1.2

A study conducted in Sheffield, England, found that the abundance of garden birds increased with levels of bird feeding. This effect was only apparent in those species that regularly take supplementary food, raising the possibility that bird feeding was having a direct effect on bird abundance. In contrast, the density of feeding stations had no effect on the number of different bird species present in a neighbourhood.

The Canadian ornithologist Jason Rogers wrote about the environmental problems associated with the use of bird feeders in the journal Alberta Naturalist. In this article, Rogers explains how the practice of feeding wild birds is inherently fraught with negative impacts and risks such as fostering dependency, altering natural distribution, density and migration patterns, interfering with ecological processes, causing malnutrition, facilitating the spread of disease and increasing the risk of death from cats, pesticides, hitting windows and other causes.

In a paper in the journal Oecologia, it was reported that feeding of blue tits and great tits with peanut cake over a long time period significantly reduced brood size. This was driven by smaller clutch sizes in both species and lower hatching success rates for blue tits. Studies by the University of Freiburg and Environment Canada found that Blackcaps migrating to Great Britain from Germany had become adapted to eating food supplied by humans. In contrast blackcaps migrating to Spain had bills adapted to feeding on fruit such as olives.

It's essential that you regularly clean, rinse and dry your bird feeding stations. This is especially important, because unclean bird feeders may contribute to the spread of salmonella, as well as viruses, amongst the wild bird population. It also important that bird food is kept dry, and is not allowed to go mouldy, as rotten food has the potential to poison wild birds.

In some cities or parts of cities (e.g. Trafalgar Square in London) feeding pigeons is forbidden, either because they compete with vulnerable native species, or because they abound and cause pollution and/or noise.

The Mallard (wild duck) is omnivorous and very flexible in its foods choice. Its diet may vary based on several factors, including the stage of the breeding cycle, short term variations in available food, nutrient availability, and inter- and intraspecific competition. The majority of the Mallard's diet seems to be made up of gastropods, invertebrates (including beetles, flies, lepidopterans, dragonflies, and caddisflies), crustaceans, worms, many varieties of seeds and plant matter, and roots and tubers. During the breeding season, male birds were recorded to have eaten 37.6% animal matter and 62.4% plant matter, most notably Echinochloa crus-galli, and nonlaying females ate 37.0% animal matter and 63.0% plant matter, while laying females ate 71.9% animal matter and only 28.1% plant matter. Plants generally make up a larger part of the bird's diet, especially during fall migration and in the winter.

It usually feeds by dabbling for plant food or grazing; there are reports of it eating frogs. It usually nests on a river bank, but not always near water. It is highly gregarious outside of the breeding season and forms large flocks, which are known as a sord.

See also:
https://en.wikipedia.org/wiki/Bird_feeding
https://en.wikipedia.org/wiki/Mallard

Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)